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Abstract

This Chapter addresses the species-rich dry grasslands of Central Europe on calcareous to acid soils, including their flora, development, environmental conditions, and vegetation types and synsystematics. It describes plant adaptations to the environment, population biology and community ecology, productivity and water and nutrient cycling, vegetation dynamics, and the human influence together with conservation issues in these habitats.

The progressive clearance of forest in Central Europe over the last few millennia (see Chap. 3 in Vol. 1) has caused fundamental changes to the microclimate at the soil surface, affecting the plants and animals that live there. The removal of trees turned soils that were previously damp or moderately dry into dry habitats that were warmer in the summer and much colder in the winter than the damp and cool forest interior. The microclimate thus became more similar to the regional climate in continental steppes or Mediterranean karst landscapes.

Mostly south-facing, deforested slopes now support dry or periodically dry nutrient-poor grasslands that, due to their relatively extreme microclimate, are extrazonal habitats (see Fig. 7.1). These dry or xerothermic grasslands can, however, also form on open, loose sediments in flat areas. The periodic dryness of these habitats is not due to the influence of the climate, but rather due to physical conditions of the soil, as is the case in coastal dunes and some inland fluvioglacial sandy sediments particularly in northern Central Europe.
Fig. 7.1

A dry Xerobromion grassland (Teucrio-Seslerietum) on a steep south-facing limestone slope in the Unstrut valley (Thuringia, central Germany), which is characterised by strong soil desiccation in summer and topsoil erosion. Creeping dwarf shrubs (among others Helianthemum canum, Teucrium montanum and T. chamaedrys) and xerotolerant grasses (mainly Sesleria albicans and Festuca pallens) are the life forms best suited to this seasonally dry and mobile substrate. Although the community can still be assigned to the submediterranean Xerobromion alliance, the species composition clearly reflects the subcontinental climate

As many dry grasslands are, according to the available research, also nutrient-poor, we will use the term dry grassland to imply that not only periodic drought but also nutrient shortage likely are limiting productivity.

7.1 Flora and Development

7.1.1 Flora

Dry grasslands occupy climatically or edaphically atypical habitats in Central Europe that cover relatively small areas. Nevertheless, these are very species-rich and structurally diverse habitats with the largest numbers of submediterranean and pontic-sarmatic species of any Central European habitat. More than 10 % of the Central European vascular plants occur mainly in dry grasslands, equivalent to around 500–600 species. In Bavaria alone, around 300 species were identified as occurring mainly in nutrient-poor calcareous grasslands (Quinger et al. 1994), in addition to 300 calcicole bryophyte and lichen species. Many of these species are now threatened with extinction.

Some dry grasslands on basic soils can be particularly species-rich, with up to 80 species per 4 m2 (Dengler 2005) or 67 species per m2 (Klimeš et al. 2001). In general, the species richness increases with decreasing acidity until a pH of about 6 (Dengler et al. 2014). For example, Dengler (2004) found an almost linear increase in species density (vascular plants and cryptogams) from acid grasslands (pH 4–5, on average 15–20 species per 10 m2) to basic grasslands (pH 7–8, on average 25–27 species) in northeastern Germany (see also Schuster and Dieckmann 2003; Becker and Brändel 2007). In the Czech Republic, however, the flora of acid and basic dry grasslands was found to be similarly rich (Chytrý et al. 2003) and in the Festuco-Brometea in Valais (Switzerland), weakly acidic grasslands (pH 4.5–5.5) even had a slightly higher species density than basic grasslands (Schwabe and Kratochwil 2004). In the latter, high rainfall promotes the plant diversity (cf. Pausas and Austin 2001) and appears to cancel out the effect of the soil acidity.

In Switzerland, plot-level diversity in grasslands increases from the colline (around 28 species on 10 m2) to the subalpine and alpine belts (approx. 42 species on 10 m2) (Koordinationsstelle Biodiversitäts-Monitoring Schweiz 2009, in Dengler et al. 2014), which contrasts with the overall richness decrease with elevation of the vascular flora in the Alps (see Fig.  5.1).

The scattered distribution of the dry grasslands in the Central European cultural landscapes has led to the development of local subspecies or varieties, as well as endemic species in the dry grassland flora. For example, there are several Stipa taxa (e.g. Stipa bavaria) and subspecies of pasque flower (Pulsatilla vulgaris agg.) with small ranges, as well as Tephroseris integrifolia that is native to the region of Augsburg in southern Bavaria (Korneck and Sukopp 1988; Quinger et al. 1994).

7.1.2 The Role of the Climate and Humans in the Development of Dry Grasslands

Some dry grassland species must have colonised Central Europe as early as in the early postglacial or even in the late glacial around 14,000–10,000 years ago, when the region was dominated by continental climate conditions. This is particularly the case for the less cold-sensitive species from the Russian and Ukrainian steppes, such as the Stipa species. For example, macrofossils of Helianthemum canum, Dianthus carthusianorum, Euphorbia seguieriana and other dry grassland plants have been found from Weichselian sediments in Hesse and the UK (Godwin 1975; Huckriede 1982). The continental dry grasslands of Central Europe are therefore considered to have developed naturally with continuous presence of typical steppe species until the Boreal period, before man started to open the forests (Hejcman et al. 2013). Becker (2003) considers it possible that Astragalus exscapus was present in central Germany during the Warthe substage of the Saale glaciation 150,000–110,000 years ago, when this area was dominated by steppe tundra. However, particularly the species with more submediterranean ranges will have only established in Central Europe much later.

Gradmann (1950), Meusel (1940) and other early phytogeographers noted that the species composition of dry grasslands cannot be explained by their habitat conditions and use history alone, but is also influenced to a large degree by processes of dispersal and vegetation history. As there are no particularly large areas of dry grassland in Central Europe, dispersal routes play a large role in maintaining populations. Studies have provided good evidence for the migration of submediterranean species (e.g. Trinia glauca) along the Rhône and Swiss Jura through the Alsace region into southwestern Germany, as well as the movement of Pontic and Southeastern European species (e.g. Astragalus exscapus) along the Danube into the Czech Republic, Bavaria and further into Thuringia and Saxony-Anhalt (Gradmann 1950; Schönfelder 1971; Becker 2003). Species of the Eastern European and Ukrainian steppes such as those in the genus Stipa could also have migrated via pockets of dry grassland along the northern edge of the Carpathians and the Sudetes into southwestern Poland, the western Czech Republic, and Saxony and Thuringia in Germany. The species-rich psammophyte flora of Central European sandy grasslands originated partly from the continental east and partly from the oceanic west. At the edges of high mountain ranges, e.g. in the northern Prealps, dry grasslands support a mixture of lowland dry grassland species and light-demanding arctic-alpine species. These are carried to the north e.g. by the Alpine rivers Lech and Isar, where the sparsely vegetated gravel plains served as migration routes into southern Germany (Bresinsky 1983; Müller 1990).

The relatively dense forests of the Atlantic period must have pushed the dry grassland communities back into small refugia, and it was only with the expansion of crop cultivation and livestock grazing by settlers during the Neolithic around 7500 years ago that large clearings were created, as shown by the increase in non-tree pollen in the pollen profiles (cf. Baumann 2006). Fischer (2002) states that the sandy grasslands on the river dune systems along the Elbe must have spread as humans became sedentary in the Neolithic, as these sandy habitats were favoured for settlements (Behre 2000). In the Kyffhäuser and Saale-Unstrut region (central Germany), Neolithic settlements are often found under modern dry grasslands (Becker et al. 2007). The majority of more submediterranean calcareous dry grassland species must have been present at the latest by around 500 BC, as evidenced by the remains of around 85 plant species found in Celtic burial sites in southwestern Germany (Wilmanns 1997). Open, xerothermic habitats must have expanded rapidly in Central Europe during this phase (cf. Pott 1996). However, the majority of calcareous grasslands were created in the Middle Ages between around 1200 and 1500 (Baumann 2006). Numerous light-demanding and thermophilic plant and animal species that are also adapted to survive periods of drought and cold were able to (re)colonise the newly created open habitats from the Eastern and Southeastern European steppe and karst landscapes, producing new communities under the influence of human management. These included species that survived in small populations in gaps in the forest cover, either on cliff faces or stone runs, young sand dunes, eroding banks or on the gravel deposits of larger rivers. However, natural rocky outcrops were the original habitat of only a few dry grassland species, such as Festuca pallens or Gypsophila fastigiata.

The sandy dry grasslands on inland dunes are also relatively young habitats (Berger-Landefeldt and Sukopp 1965). Small sandy grassland patches may have survived the ice age e.g. on sandy river banks (Krausch 1968), as suggested by the presence of relict species such as Jurinea cyanoides, Poa badensis, Koeleria glauca or Alyssum montanum. However, the majority of stands present today must be younger than this, and their characteristic species probably mostly arrived between the Neolithic and the Middle Ages (Philippi 1981).

Without the moderating effect of forest on the climate, highly differentiated habitats can form at a very small scale. Differences in aspect, soil depth, grain size or chemical properties of the soil, which barely affect the species composition in the forest, can produce highly variable combinations of species. One forest community is therefore usually replaced by several different anthropogenic non-forest communities, especially if its soil has been affected by varying levels of trampling or erosion.

7.2 Environmental Conditions and Habitat Classification

The macroclimate promotes the formation of dry grasslands in areas that experience periods of drought in summer. This is particularly the case in regions of Central Europe with less than around 550 mm annual precipitation (see Map 3 in Chap. 1, Vol. 1). Large areas of dry grassland are found particularly in areas with continental climates, e.g. at the edge of the Hungarian Plain in southern Slovakia, in the Czech Republic and in the Pannonian region of Lower Austria (see Fig. 7.2). Somewhat smaller areas are also found along the Vistula and Oder and their tributaries, in the Saale-Unstrut region and the Kyffhäuser, in Mainfranken, in the Swabian and Franconian Jura, in the Upper Rhine Plain near the Mosel and in the continental lateral valleys of the central and southern Alps (e.g. in the Valais, the Aosta valley, Engadine and Vintschgau). The oceanic regions of northwestern Germany and the Netherlands only support semi-dry grasslands, and these only at small scales.
Fig. 7.2

The distribution of dry grassland regions in Central Europe. Modified from Litzelmann (1938). 1: Lower Austria-southeastern Czech Republic, 2: northwestern Czech Republic, 3: southwestern Slovakia and the western Hungarian Plain, 4: uplands of the upper Vistula and Oder, 5: the slopes of the Oder and Warte valleys, 6: lower Vistula valley, 7: Saale-Unstrut region, 8: Mainfranken, 9: Swabian and Franconian Jura, 10: northern Upper Rhine Plain and Nahe and Mosel valleys, 11: southern Upper Rhine Plain and Swiss Jura, 12: continental western Alps, 13: Vinschgau

7.2.1 Aspect and Microclimate

The microclimate at the soil surface is fundamentally changed by forest clearance. The majority of the light is no longer absorbed in the forest canopy, but rather at the soil surface or in the sward (Berger-Landefeldt and Sukopp 1965). Temperature, humidity and wind speed vary much more widely on daily and annual time scales than under forest cover (see Fig. 7.3). Not only slope and aspect (or exposition), but also the structure of the vegetation has a major influence on the microclimate of the dry grassland plants. Open grasslands tend to be warmer and drier than those with dense and tall vegetation.
Fig. 7.3

There are clear differences in the average monthly maximum temperatures at the soil surface between forest interior, forest edge, dry grassland and rocky cliffs throughout the year, but the minimum temperatures are largely the same (Modified from Baller 1974)

The forest supports a fairly balanced climate both in years with little difference between the temperature extremes (1966) and in those with widely varying minimum and maximum temperatures (1968). In contrast, the bare earth sheltered within a sparsely vegetated dry grassland can often heat up to over 40–50 °C, so that even the monthly average can reach a maximum of 55 °C (VI 1968). At the same time, the minimum temperatures during the early morning hours are usually under 10 °C. The bare rock with little vegetation cover usually cools down at night. The forest fringe vegetation is intermediate in its extreme temperatures between the open habitats and the forest interior.

7.2.1.1 Aspect

Most dry grasslands in colline to montane zones colonise slopes or slightly sloping plateaus, i.e. areas with local climates differing from the regional climate. Steep south-facing slopes receive on average up to 35 % more direct radiation per year than flat areas, and north-facing slopes receive 35 % less (Slavíková et al. 1983; McCune 2007; see Figs. 7.4 and  2.19). Higher temperatures and atmospheric saturation deficits, as well as higher wind speeds in sparse vegetation, cause the higher evaporation rates on south-facing compared to north-facing slopes (e.g. Reichhoff 1979, 1980; Leuschner 1989; see Fig. 7.5). For this reason, the dry ‘Xerobrometum’ grassland on the south-facing slopes of the Badberg in the Kaiserstuhl (southwest Germany) is well developed, and the vegetation is more xerothermic than in the semi-dry ‘Mesobrometum’ on the more mesic north-facing slope. Dry grasslands are also found mainly on the south- and southwest-facing slopes of the limestone mountains in the Swiss and Franconian Jura, whilst more mesic grasslands are present on north- to northeast-facing slopes with the same inclination. It is only at higher elevations that the Mesobrometum also occurs on south-facing slopes. Quantin (1935) compared the vegetation, microclimate and soils of these two calcareous dry grassland communities with sparse vegetation of the Anthylli-Teucrietum of very dry and warm rocky habitats. The results showed that the air temperature (15 cm above the soil surface) in the Mesobrometum was lower throughout the year than in the Xerobrometum or even in the Anthylli-Teucrietum (see Table 7.1: row 2). Also the daily, monthly and yearly variation in temperature increased in this sequence.
Fig. 7.4

Soil moisture, direct solar irradiance (relative to a horizontal plane) and vascular plant diversity in dry grasslands as a function of the aspect of the slope (N, W, S and E on the x axis) on a conical basalt hill (Raná hill) in the northwest Czech Republic (Modified from Slaviková 1983)

Fig. 7.5

Changes in air and soil temperature, evaporation and vapour pressure deficit on the 28th August 1981 in a Xerobrometum and a Mesobrometum on the Badberg in the Kaiserstuhl. Modified from Leuschner (1989). The Xerobrometum is on a south-facing slope, and the typical Mesobrometum on a northwest-facing slope. The habitat of the latter is consistently cooler with damper soil and more humid air

Table 7.1

Life forms, microclimate and soil factors in dry grasslands on slopes with shallow inclines in the southwest Jura

Open rock ledge community (Anthylli-Teucrietum, order Sedo-Scleranthetalia) on shallow soil, sun-exposed

Dry grassland (Xerobrometum, order Brometalia) on shallow sunny slopes

Semi-dry grassland (Mesobrometum, order Brometalia) on shallow shady slopes

Vegetation cover and climate (in 1932)

Anth. Teucr.

Xero-brom

Meso-brom.

Soil properties (in 1932)

Anth. Teucr.

Xero-brom.

Meso-brom

1 Life forms (%)

5 Grain size distribution (%)

Chamaephytes

35.0

18.6

10.6

Coarse gravel

23.6

28.3

24.3

Hemicryptophytes

25.0

49.3

76.3

Fine gravel

30.1

32.6

32.5

Geophytes

2.5

8.7

7.8

Coarse sand

19.0

14.0

19.5

Therophytes

37.5

21.9

5.3

Fine sand

19.0

14.6

13.5

Parasites

1.5

Silt and clay

8.3

10.5

10.3

2 Air temperature (°C), mean

6 Soil water content (vol. %)

Spring (April 3–23)

12.8

9.7

7.5

Spring

(April 25)

17.4

23.5

26.9

Summer (June 26–July 16)

23.4

20.3

15.3

(May 16)

20.2

27.7

33.6

Autumn (Sept. 18–Oct. 8)

17.6

16.8

12.0

Summer

(June 11)

18.6

24.9

27.9

Winter (Dec. 11–31)

6.1

5.8

0.7

(July 19)

10.9

20.2

23.3

Mean of the four periods (12 weeks)

15.0

13.1

8.9

(August 13)

9.3

13.2

14.9

Autumn

(Sept. 17)

10.6

10.9

16.0

3 Diurnal temperature variation (°C)

Summer (June 26–July 16)

21.5

13.0

9.2

(Nov. 11)

17.4

26.3

29.6

Winter

(Dec. 21)

24.8

28.4

32.3

Mean of four periods (12 weeks)

17.6

10.9

8.9

Maximum

(April 25, 1932–October 23, 1933)

25.8

30.0

33.6

Minimum

 

8.7

10.9

14.9

4 Evaporation (cm3 d−1, Piche atmometer)

7 Soil chemical parameters

Spring (April 3–23)

5.4

3.3

2.6

pH: maximum

7.9

7.9

7.3

Summer (June 26–July 16)

8.3

6.5

4.6

pH: minimum

7.5

7.1

6.9

Autumn (Sept. 18–Oct. 8)

6.5

4.3

2.4

Carbonate content (%):

maximum

51.5

67.1

28.2

Winter (Dec. 11–31)

4.4

3.2

1.5

minimum

12.7

12.0

4.4

Mean of four periods (12 weeks)

6.2

4.3

2.8

Humus content (%):

maximum

4.6

17.1

21.4

minimum

1.7

7.1

11.4

All measurements at 15 cm height above ground

All measurements in 0–10 cm soil depth

From data in Quantin (1935)

This considerable difference in temperature must also affect the water regime in the grassland. Indeed, the potential evaporation rate in the Mesobrometum is considerably lower than in the Xerobrometum (see Table 7.1: row 4). The soil water content in the main root zone (5–10 cm below the soil surface) clearly shows that evapotranspiration uses less of the soil water in the Mesobrometum than in the Xerobrometum (see Table 7.1: row 6). The more frequent periods of extreme drought in the shallow soils of the rocky Anthylli-Teucrietum and the Xerobrometum mean that the short-lived spring therophytes have much more space to develop than in the Mesobrometum (see Table 7.1: row 1). The vegetation also contains a large proportion of chamaephytes, as they are not shaded out by grasses, which usually grow faster and are denser, as is the case in the Mesobrometum. Hemicryptophytes – particularly graminoids – account for over three-quarters of all species in the Mesobrometum.

The differences between the Xerobrometum and Mesobrometum caused by the aspect of the slope are found throughout Central Europe, but modified by local climatic changes from the south to the north. For example, in southern Sweden, Mesobrometum communities occur only in the warmest and driest areas, e.g. on steep south-facing slopes on gravelly ridges (Pahlsson 1966) or on very shallow soils in the alvar grasslands on Öland (Dengler and Löbel 2006).

7.2.1.2 Microclimate

The small-scale variation in the species composition of dry grasslands caused by changes in microclimate, even within the same stand, has been described by many authors from numerous plant communities (e.g. Kraus 1911; Heilig 1930/31; Müller-Stoll 1935; Volk 1937; Preis 1939; Dörr 1941; Zoller 1954; Mahn 1957; Bornkamm 1958; Krausch 1961; Berger-Landefeldt and Sukopp 1965; Gigon 1968; Philippi 1973a; Rychnovská and Úlehlová 1975; Reichhoff 1979, 1980; Lötschert and Georg 1980; Etherington 1981; Slavíková et al. 1983; Kratochwil 1984; Zumbühl 1985; Leuschner 1989; Becker 1999).

Small-scale temperature measurements and soil analyses clearly show that the microclimate of almost every individual plant differs slightly. This is the case for dry grasslands on shallow, stony soil with heterogeneous relief as well as other open vegetation types. The co-occurrence of species with different habitat requirements can be largely explained by the presence of microhabitats with different water storage capacity, heat storage capacity, pH or nutrient levels. Bare granite and quartz rock warms up the fastest, followed by carbonate rock and damp sand or clay soil. Dry Ranker and Rendzina soils heat up strongly at the surface, but remain relatively cool in the lower horizons (see Table 7.2). In some grasslands, these microhabitats form a characteristic thermal mosaic following the seams of rock or other irregularities in soil formation. Digging animals may also often cause small-scale variation in environmental conditions, e.g. field mice and rabbits (see Gigon 1981; Leutert 1983; see Sect. 7.5.4).
Table 7.2

The thermal properties of various materials (ρC specific heat capacity per unit volume, K thermal conductivity, d damping depth as the depth at which the amplitude of variation in temperature has decreased to 1/e = 0.37 of that at the surface)

Material

ρC

K

d

(106 J m−3 K−1)

(W m−1 K−1)

(cm)

Limestone

2.8

3.42

21

Granite

1.34

4.94

31

Sand (dry)

1.17

0.27

8

Sand (30 % water content)

2.42

2.49

17

Clay (dry)

1.19

0.28

8

Clay (30 % water content)

2.45

1.74

14

Peat (dry)

0.25

0.03

5

Peat (80 % water content)

3.61

0.48

0.6

Water

4.19

0.59

0.6

Air

0.0012

0.03

78

Ice

1.93

2.24

18

Recently fallen snow (89 % air content)

0.19

0.06

9

Compressed snow (56 % air content)

0.77

0.49

13

From Stoutjesdijk and Barkman (1992)

Even the shade of a large plant can reduce the fierceness of the sun enough for heat- or drought-sensitive plants to establish. In this way, the mosaic of species in the upper vegetation layer can produce a series of microhabitats, allowing the coexistence of physiologically highly different organisms. Thus, whilst the temperature on the unshaded soil surface on the south face of the Badberg in the Kaiserstuhl on the 1st of May rose to 50–60 °C, only 20–30 cm northwards in a dense tussock of Bromus erectus it only reached 15–17 °C. At night, the bare surface of the soil cooled to below 5 °C and was covered in dew in the morning. In grass tussock the temperature remained above 10 °C, meaning that it only experienced an eighth of the variation in temperature of the adjacent bare earth.

Alongside horizontal variation in microclimatic conditions, there is also vertical variation, as e.g. shown in the daily variation in Fig. 7.6. In a sparse Stipa grassland at the edge of the Oder valley, the temperature close to the soil surface on a day in mid-summer varied between around 30 and 75 °C, but 5 cm above and below this it reached at most 40 °C, and 15 cm below the soil surface it was below 30 °C. The dry, dead leaves surrounding the young shoots of hemicryptophytes protect them from overheating by isolating them from the soil surface. The hot soil surface can often have a relative air humidity of as low as 20 % at midday, and in the continental steppe grasslands of the Vintschgau in the southern Alps, Florineth (1974) recorded a minimum of only 11 %. At temperatures of 35 °C, this corresponds to very high saturation deficits of around 4.5–5.0 kPa.
Fig. 7.6

Changes over the course of a day in global radiation (insolation), relative air humidity (RH), air temperature at 2 and 200 cm above the ground (dashed line) and soil temperature at 1, 3, 5, 10 and 15 cm below the soil surface (solid line) as well as the potential evapotranspiration from a Stipa capillata grassland on a south-facing slope in the Oder valley on the 12th of July. The amplitude of the temperature variation reduces rapidly within the first 10 cm of soil, and the daily minimum temperature occurs in the evening. The changes in relative humidity almost exactly reflect the changes in air temperature, while the absolute humidity changes little over the course of the day (From Brzoska in Ellenberg 1963)

7.2.2 Soil Moisture Regime

Many dry and rocky grasslands on mountain slopes grow on shallow soils. As the water storage capacity of the soil is mainly dependent on its depth, the profile depth is highly influential on the structure and composition of the colline and montane dry grasslands. Nutrient-rich meadows and semi-dry and dry grasslands in southwestern Germany and northern Switzerland can be easily classified according to their soil depth (from 80 to 10 cm) and corresponding plant-available water capacity (from 120 to 2 mm water) (see Table 7.3). The more mesophilic Colchico-Mesobrometum has a plant-available water capacity over 40 times greater than the extreme Xerobrometum, and twice as high as the xerophilic Teucrio-Mesobrometum. The time in which the soil water reserves are exhausted in periods of drought is 4 weeks for the Colchico-Mesobrometum, 2 weeks for the Teucrio-Mesobrometum and only a few days for the Xerobrometum (Gigon 1968; see also Fig.  8.6). This difference is well reflected in the decreasing height and density of the vegetation.
Table 7.3

The storage capacity of plant available water in soil profiles under various dry grassland communities compared to nutrient-rich meadows in Central Europe (in mm water)

Plant community

Locality and study plot

Soil depth (cm)

Capacity of plant-available water (mm) (−100 hPa to −1.5 MPa)

Capacity of plant-available water (mm) (−300 hPa to −1.5 MPa)

Sedo-Scleranthetalia (rock debris and rock ledge communities)

Allio stricti-Festucetum pannonicae

Bad Wildungen

7.7

1.9

 

(Hesse)

Diantho-Festucetum pallentis

Albungen

6.0

2.4

 

(Hesse)

Polytricho-Allietum montani

Maderstein

6.8

5.1

 

(Hesse)

Polytricho-Allietum montani

Sauerburg

8.2

7.0

 

(Hesse)

Brometalia erecti (calcareous dry grasslands)

Xerobrometum

Lützelberg

max. 10

2.9

2.1

(Alsace)

Xerobrometum

Strangenberg

max. 10

3.5

2.2

(Alsace)

Xerobrometum

Bollenberg

50

13.7

10.6

(Alsace)

Mesobrometum globularietosum

Bollenberg

50

24.1

18.2

(Alsace)

Teucrio-Mesobrometum

Swiss Jura

50–60

 

29

(B 3)

Teucrio-Mesobrometum

Swiss Jura

50–60

 

30

(A 1)

Mesobrometum typicum

Badberg/Kaiser-stuhl, SW Germ.

80

53.2

39.3

Colchico-Mesobrometum

Swiss Jura (B 7)

50–60

 

73

Colchico-Mesobrometum

Swiss Jura (A 3)

50–60

 

98

Arrhenatheretum salvietosum

Swiss Jura

55

48

 

Arrhenatheretum salvietosum

Swiss Jura

70

61

 

Arrhenatheretum typicum

Swiss Jura

70

80

 

Arrhenatheretum typicum

Swiss Jura

80

118

 

From data in Gigon (1968), Riemer (1984) and Leuschner (1989). Calculated as the difference between the water content at field capacity (determined at −100 or at −300 hPa) and at the permanent wilting point (−1.5 MPa).

Apart from the soil depth, aspect also has a large influence on the soil water regime. Steep southeast- to southwest-facing slopes are drier both in summer and in winter than west-, north- and east-facing slopes (see Fig. 7.7, see also Table 7.1). Müller-Stoll (1935) measured minimum topsoil moisture contents in summer on patches of bare soil of less than 1 %, corresponding to a soil matric potential of around −15 MPa (−150 bar). The strong drying of the south-facing slopes is mainly caused by the increased evaporation from the bare soil patches that quickly heat up in sparse vegetation, and less by increased plant transpiration on the south-facing slope. As measurements in a Czech Stipa grassland show, the soil moisture varies widely at small scales, whereby the highest moisture levels are always found under the isolated grass tussocks, particularly if they are large (Slavíková et al. 1983; see Fig. 7.8). Shading by the plants thus protects the topsoil from drying out (cf. also Reichhoff 1980a). The primary role of soil evaporation and the secondary role of transpiration in determining the moisture content of the topsoil in south-facing dry grasslands is also shown by the hydrological comparison between rocky, dry and semi-dry grasslands in the French Jura (Quantin 1935). The lowest soil moisture was found in the rocky Anthylli-Teucrietum and the highest in the dense Mesobrometum with a much greater transpiring leaf area (see Table 7.1).
Fig. 7.7

Annual average soil water content in dry grasslands as a function of aspect (N, W, S and E on the x axis) on a conical basalt hill (Raná hill) in the northwest Czech Republic (Dec. 1971–Nov. 1972) in a wet period (February–May 1972) and a dry period (July–September 1972) (Modified from Slaviková 1983)

Fig. 7.8

Soil moisture (in percent of weight) in mid-summer in the uppermost horizons below various plant species in a sparsely vegetated dry grassland in the northwest Czech Republic (Raná hill near Most). Under most (but not all) tussocks and rosettes, the moisture levels are higher in the centre than at the edges, showing that protection from evaporation is more important than increased transpiration (From Slaviková 1983)

Lack of water in dry grasslands may be caused by edaphic factors, as well as climatic factors. This is particularly the case in acid pioneer grasslands establishing on bare sandy soils (Berger-Landefeldt and Sukopp 1965). These occur not only on south-facing slopes with extreme microclimates, but also in flat or even north-facing areas. Sandy soils have a field capacity, i.e. the amount of water remaining 1–2 days after being saturated and free drainage has ceased, of only around 5–15 vol% water. This is equivalent to 25–75 mm water in the first 50 cm below the soil surface. In silty and particularly clayey soil, the field capacity is three to four times higher (Scheffer and Schachtschabel 2010; but cf. Sect. 4.3.2 in Vol. I). Dry grassland on dune sands therefore can have soil water contents of less than 1 vol% during dry summer periods, even in northern Germany, thereby dropping far below the permanent wilting point, which is conventionally set at −1.5 MPa (see Fig. 7.9; see also Lötschert and Georg 1980). The available soil water in the subcontinental steppe grasslands in the middle Elbe valley also sinks below 10 mm water during summer droughts (see Fig.  8.6).
Fig. 7.9

Seasonal changes in soil moisture at 0–5 and 5–15 cm below the soil surface in two sandy dry grasslands and a Lolio-Cynosuretum in the Elbe valley, showing maximum water capacity (MWC) and permanent wilting point (PWP), all given in percent by volume. Modified from Jeckel (1984). The Diantho-Armerietum sedetosum showed the highest water deficits in its relatively humus-poor dune sand. The soil matric potential dropped below −0.61 MPa several times in 1981. The subassociation D.-A. trifolietosum with several species of the Cynosuretum grew in a less extreme habitat (slightly lower on the dune slope) and showed a similar pattern, but less extreme. The Lolio-Cynosuretum on humus-rich alluvial loam never experienced water stress, despite being exposed to the same regional climatic conditions, because the soil was richer in humus and perhaps also in clay

In contrast, the sandy soil with large pore spaces in the lower horizons is protected from drying out, as the hydraulic conductivity of the topsoil quickly drops to almost zero under drought conditions. Species of dune plants with very deep roots such as Ammophila arenaria (see Sect.  2.4.2) can therefore exploit water reserves in the subsoil. However, these deep-rooting, productive species do not occur in the inland sandy and dune grasslands, presumably due to the lack of nutrients and higher acidity of these sandy soils in comparison to the calcareous white dunes of the coast.

The term dry grassland should not be misinterpreted to mean that these habitats are constantly dry. In fact, the soils of dry grasslands are as damp as normal meadow soils for most of the year, particularly in spring and late autumn. It is the summer droughts that are critical for this vegetation type, which in Central Europe can last from several days to several weeks. Particularly long droughts occur at irregular intervals, e.g. in 1911, 1919, 1933, 1947, 1949, 1952, 1959, 1964, 1970, 1973, 1975, 1976, 1980, 1982, 1983, 1991, 2003 and 2013, and these can be significant disturbances for dry grasslands as well as other plant communities. Kuhn (1973) calculated the frequency of extreme droughts for eleven areas with different types of climate in Switzerland. Within a period of 30 years, an area with a suboceanic climate (Rigi) can expect one 25-day drought, whilst a continental area (Sion) will experience a 75-day drought (see Fig. 7.10). Such extreme events are probably just as influential on the species composition of a dry grassland as the average precipitation regime. Even after extreme droughts, most true dry grassland species can regenerate relatively quickly (see Sect. 7.7.2).
Fig. 7.10

The frequency of droughts of up to 95 days in length in 14 areas of Switzerland, calculated from long-term climate records (From Kuhn 1973)

7.2.3 Soil Types and Their Nutrient Supply

The composition of dry grasslands varies within areas sharing the same regional climate, not only due to aspect and degree of slope, but also with the soil depth. Bare rock is only colonised by foliose and crustose lichens and cushion mosses. Wherever Lithic Leptosols have formed with several millimetres of humus-rich topsoil, the first therophytes and shallow-rooting grasses are able to colonise and form gappy vegetation (Korneck 1993; Becker 1998). It is only where the fine earth is thick enough above the solid rock to store rainwater that communities dominated by graminoids can form (see Figs. 7.11 and 7.12). These are the sites of dry or semi-dry grasslands. Protorendzinas develop on steep, eroded slopes on basic bedrock, often with a thin and frequently dry humus layer (‘xeromoder’), which typically support dry grasslands. Rocky grasslands on acid bedrock grow on shallow Protorankers or Rankers, which under favourable conditions will develop into acidic Cambisols.
Fig. 7.11

The distribution of dry grassland communities on Jura limestone in the Swabian Alb according to soil depth, semi-schematic. From Ellenberg (1952a). The deep soils (right) are the most productive. The potential natural vegetation would be a Hordelymo-Fagetum (instead of the Bromus form of semi-dry grassland on limestone), a typical Galio odorati-Fagetum (instead of the Brachypodium form or the degradation stage) and a Luzulo-Fagetum (instead of the Nardus grassland)

Fig. 7.12

Vegetation and soil zonation in a sunny sandstone outcrop in the southwest Czech Republic. (Modified from Moravec 1967). Constant erosion means that fine earth does not accumulate on the dry edge of the outcrop. The soil remains at the Protoranker stage, and the vegetation at the Asplenietum septentrionalis community. In places where more topsoil can accumulate, a brown Leptosol supports the Polytricho-Scleranthetum perennis. The dense Cerastio-Agrostietum strictae community requires relatively deep soil, i.e. a well developed sandy Cambisol. Woody plants could theoretically establish here, although this was prevented by sheep grazing

Less steep limestone, dolomite or gypsum slopes are covered with Mullrendzinas (Rendzic Leptosols) or even deep and superficially acidic Terra fusca soils, supporting semi-dry grasslands. These grasslands also grow on Pararendzinas (calcareous Regosols), which formed from unconsolidated substrates such as loess, glacial till or outwash gravel. These habitats could certainly support forest, and their natural vegetation would be a downy oak forest or a thermophilic and drought-tolerant sessile oak or beech forest, or a pine forest. High rates of erosion after deforestation can, however, cause these soils to become so shallow that the trees have difficulty establishing even after all human influence has ceased. Nevertheless, scattered trees survive in the rocky steppe habitats even in dry and continental areas such as the low mountain regions in the Czech Republic (Klika 1933) or in central Valais (Switzerland) (Braun-Blanquet 1961).

Continental steppe grasslands are often found on Chernozems or their degradation stages, which usually form from loess and have deep, humus-rich topsoils. Sandy dry grasslands grow on Regosols formed from sand with very low humus content, and in some places also on deeper podzolised Cambisols. The nutrient concentrations in sandy soils can vary widely depending on the origin of the material and how it was transported. Pure quartz sands have the lowest cation exchange capacity and therefore only low concentrations of exchangeable potassium, calcium and magnesium, particularly if the sand has been deposited by the wind.

Nutrient-poor Pleistocene sands often have a cation exchange capacity five to ten times lower than clay or silty soils (< 50 compared to 200–500 μmolc g−1). However, some Pleistocene, Tertiary or even older sand deposits contain more silicate, and sometimes also carbonate materials, or are even rich in calcium and magnesium such as the dolomite sands of the northern Franconian Jura (Hohenester 1960). The sands can be distinguished as poor, medium or rich depending on the pH, the base saturation and the N and P supply, each with a characteristic grassland vegetation type. The soil acidity may also vary from highly acidic (pH < 3.5) to neutral or even basic (pH > 7, cf. Lötschert and Georg 1980). Sandy grasslands in continental eastern Central Europe usually have higher pH values than those in the oceanic west due to the climatic water balance. Glacial sands here can also reach pH (KCl) values of 6–7 with high base saturation (Kozlowska and Wierzchowska 1985). This is also the case for the dry Corynephorus grasslands of the east, particularly if these grow on freshly deposited substrate.

The carbon and nitrogen contents of dune and aeolian sands below dry grasslands are very low, probably as a result of the lack of phosphorus in these highly acidic substrates. Lache (1976), Storm et al. (1998) and Jentsch and Beyschlag (2003) recorded only 0.1–0.6 % C and less than 0.04 % N in the Ah horizon of northwest German inland dunes, i.e. six to ten times lower than in nutrient-rich meadows. The C and N contents in the topsoil of the dry calcareous grasslands, however, are usually similar to those in neighbouring forests. This may at first glance be surprising, as high summer temperatures generally accelerate litter decomposition. However, summer drought decreases the activity of the mineralising soil organisms, so that some dry grasslands have very high C and N pools, such as in the Chernozem steppe grasslands of eastern Central Europe. On very steep slopes, the humus-rich topsoil can be lost through erosion, as suggested by the low humus contents measured by Quantin (1935) in a sloping Anthylli-Teucrietum in the French Jura (see Table 7.1).

Dry grasslands thus develop on a wide range of soils, which share the common characteristic that the biological activity in the soil is slowed for several weeks or months by drought. This climatic drought is compounded by the low or very low capacity for plant-available water in most dry grassland habitats, such as in the case of the rocky soils only a few centimetres thick as well as the deep sandy soils with large proportion of coarse sand that reduces the available water.

7.3 Vegetation

7.3.1 Classification of the Major Habitat Types

In order to give an overview of the variety of vegetation units within dry grasslands, we will first look at the major habitat factors without going into detail. The Ellenberg indicator values (EIV) of the numerous character species can be helpful in this respect, as is shown in the ecograms in Fig. 7.13 (see the explanation of the EIV scores in the Directions for use in this volume). This portrayal is somewhat unusual, as instead of showing the average indicator values it instead gives the ranges of the relevant character species (see Table 7.4). Only few studies analysed the relationships between measured abiotic site factors and dry grassland vegetation types. One example is the study of Jandt (1999) in a variety of xerothermic habitats in northern Thuringia.
Fig. 7.13

Ecograms of character species of major Central European dry grassland alliances in the classes Festuco-Brometea and Koelerio-Corynephoretea (see Table 7.4), according to temperature and continentality values or moisture and reaction values. All character species of drygrasslands are light-demanding (EIV-L 7–9) and adapted to extremely low nutrient levels (N < 3). See also Table 7.5. It should be emphasised that the alliances are placed in the ecograms according to their character species (from Ellenberg et al. 1992), and not to the average values for the whole stand. These show similar tendencies but less clear differences. The largest amplitudes in T and C values are seen in the Mesobromion character species on calcareous soils, and in those of the Sedo-Scleranthion on silicate bedrock, and of the Corynephorion on sandy soils. All alliances have extreme M and R values, but particularly the Festucion valesiacae and the Xerobromion. Further details are given in the text

Table 7.4

Character species of calcareous and sandy dry grasslands, particularly in the suboceanic regions of Central Europe, showing the ecological characteristics of the species

 

On lime-rich (calcareous) soils

On non-calcareous soils

Classes:

Calcareous dry grasslands

LTCMRN

Sandy dry grasslands

LTCMRN

Festuco-Brometea

Koelerio-Corynephoretea

Festuco-Brom. (FB)

Ajuga genevensis

8xx 372

Acinos arvensis

963 251

Koel.-Cor. (KC)

Allium carinatum

854 382

g Agrostis stricta

973 221

FB

KC

Allium oleraceum

764 374

Androsace septentrionalis

877 252

L 7–9

7–9

Allium sphaerocephalon

985 382

Artemisia campestris

965 252

T 5–8

5–8

Asperula cynanchica

7x5 383

B Brachythecium albicans →

935 2x–

C 3–7

2–7

Aster linosyris

875 282

B Ceratodon purpureus

8xx 2x–

M 2–4

2–4

g Bothriochloa ischaemum

976 383

Helichrysum arenarium

867 251

R 7–9

1–6

g Brachypodium pinnatum

655 474

Herniaria glabra

865 342

N 1–4

1–3

Campanula glomerata

7x7 47x

Hieracium echioides

866 261

= Species with partly deviating ecological behaviour

g Carex humilis

765 283

Holosteum umbellatum

865 3x2

Centaurea jacea subsp. subjacea

837 362

Lactuca perennis →

974 282

g = graminoid

Centaurea scabiosa

7x3 384

Minuartia viscosa

865 3x2

l = legume

Eryngium campestre

975 383

g Poa badensis →

874 381

B = bryophyte

Euphorbia cyparissias

8x4 3x3

g Poa bulbosa

877 352

o = orchid

g Festuca rupicola

977 382

B Polytrichum piliferum

917 21–

 

Filipendula vulgaris

765 382

Potentilla argentea

963 231

 

Galium verum

76x 473

Potentilla collina

974 221

 

Gentiana cruciata

764 383

Potentilla inclinata

975 261

 

Minuartia setacea

976 271

Potentilla recta

975 352

 

Odontites lutea

775 392

Potentilla rhenana

984 221

 

Ornithogalum kochii

985 281

B Racomitrium canescens →

936 16–

 

Orobanche caryophyllacea

865 392

Rumex tenuifolius

965 321

 

g Phleum phleoides

867 382

Scleranthus perennis

864 241

 

Pimpinella saxifraga

7x5 3x2

Sedum acre

863 2x1

 

g Poa pratensis subsp. angustifolia

76x xx3

Sedum forsteranum

871 341

 

Polygala comosa

866 382

Sedum rubens

772 3x3

 

Potentilla heptaphylla

754 392

Sedum sexangulare

754 261

 

B Rhytidium rugosum

9x6 37 –

Sempervivum tectorum

8x2 34x

 

Salvia pratensis

864 384

B Tortula ruralis

9x5 26−

 

Sanguisorba minor

765 382

Taraxacum laevigatum →

865 372

 

Scorzonera austriaca

777 382

l Trifolium arvense

863 321

 

Stachys recta

764 392

l Trifolium campestre

863 463

 

B Thuidium abietinum

8x6 27–

Valerianella carinata →

773 48x

 

Thymus praecox

856 381

Valerianella dentata →

762 47x

 

l Trifolium montanum

8x4 382

Veronica verna

875 241

 

Veronica austriaca

866 392

 

Veronica spicata

776 372

From Oberdorfer et al. and Ellenberg et al. up to 1992. Further information is given in the text, and the table continues on the following pages

The over 100 dry grassland communities so far identified for Central Europe can be divided roughly into two classes, the Festuco-Brometea (calcareous dry grasslands) and Koelerio-Corynephoretea (sandy and rocky dry grasslands).

The diversity of the Central European dry grassland communities means that although the two classes are floristically easy to distinguish, the lines are somewhat blurred regarding soil chemistry and vegetation structure. As we will see, sandy or rocky grasslands can also occur on carbonate substrates, and may produce dense swards. In turn, some Festuco-Brometea grasslands can be found on acidic soils or have particularly sparse vegetation. Nevertheless, the majority of communities in this class are found on calcareous soils, whether from carbonate rock, loess at an early stage of weathering, till or other substrates. The pH indicator values of their character species are therefore generally high (R 6–9), except for in one rather marginal alliance (Koelerio-Phleion phleoidis, see Fig. 7.13, left; EIV-R 3.5–5.5). The whole class is relatively thermophilic (T 6–8).

The class Koelerio-Corynephoretea of sandy and rocky grasslands (previously called the Sedo-Scleranthetea) is in some respects quite a contrast to the Festuco-Brometea. Two groups of communities can be clearly distinguished within this class in terms of their flora and habitat. The order Sedo-Scleranthetalia colonises shallow Protorankers or Protorendzinas (Lithic Leptosols) on both silicate and carbonate solid bedrock in rocky or cliff habitats, whilst numerous communities within four other orders (see the overview of communities in Chap.  14: no. 5.2) grow on sandy and usually deep unconsolidated substrates. The majority of the communities in the class Koelerio-Corynephoretea grow on weakly to highly acidic soils (EIV-R 6 to R 2), although some are found on carbonate rocks and basic sands (the alliances Alysso-Sedion: R 6–9, Koelerion albescentis: R 3–7, Koelerion glaucae: R 7–8 and Seslerio-Festucion pallentis: R 5–8; see Table 7.4). With the exception of the Koelerion glaucae (continentality values: C 6–8) most communities in the Koelerio-Corynephoretea have a generally suboceanic character (C 1–C 6) (Table 7.4, 7.4a and 7.4b).
Table 7.4a

Character species of calcareous and sandy dry grasslands

Orders:

Suboceanic calcar.dry grasslands

LTC MRN

Continental calcar.dry grasslands

LTC MRN

Brometaliaerecti

Festucetalia valesiacae

Bromet. er.

Anthyllis vulneraria

863 372

Achillea collina

966 272

L 7–9

Arabis hirsuta

753 48x

Achillea setacea

978 271

T 5–8

g Avenochloa pratensis

764 3x2

Allium flavum

– –

C 2–4

g Bromus erectus

852 383

l Astragalus onobrychis

976 291

M 1–4

g Carex caryophyllea

8x3 4x2

Centaurea stoebe

976 282

R 7–9

Dianthus carthusianorum

854 372

Dianthus gratianopol. →

974 271

N 1–3

Globularia punctata →

865 292

Euphorbia seguierana

976 281

Festuc. vales.

Helianthemum ovatum

854 392

Hieracium bauhinii →

974 371

L 7–9

Hieracium wiesbaurianum

763 381

Orobanche coerulescens

977 281

T 6–7

Hippocrepis comosa

752 372

l Oxytropis pilosa

977 171

C 5–8

Iris germanica

883 382

Pimpinella nigra

966 281

M 1–3

g Koeleria pyramidata

764 472

Potentilla arenaria

976 181

R 7–9

Linum leonii →

975 391

Potentilla pusilla →

964 281

N 1–2

Linum viscosum

7x4 48?

Pulsatilla grandis

96? 391

 

Ononis natrix

883 381

Pulsatilla montana →

845 281

Pulsatilla vulgaris →

765 272

Ranunculus illyricus →

876 474

Scabiosa columbaria

852 382

Scorzonera purpurea

876 282

Teucrium montanum

854 191

Silene otites

877 272

l Trifolium ochroleucon

774 482

g Stipa joannis

878 272

  

Verbascum phoeniceum

766 372

  

Veronica prostrata

876 281

Sedo-Scler.

Silicate rock debris and rock ledge communities Sedo-Scleranthetalia

LTC MRN

 

L 6–9

Allium montanum

9x5 262

T 6–8

Arabidopsis thaliana

663 444

C 2–5

Arenaria leptoclados

983 3x2

M 2–4

g Festuca pannonica →

976 2x1

R 3–6

Jasione montana

763 332

N 1–4

Petrorhagia saxifraga →

973 271

Sedum album

9x2 2x1

Teucrium botrys →

964 282

l Trifolium ornithopodioides

962 3x?

L light, T temperature, C continentality, M moisture, R acidity/alkalinity, N nitrogen, x indifferent, unknown

The two dry grassland classes are also similar in that their character species of class, orders and also alliances are reasonably drought-tolerant (M 2–3 or 4) and limited to moderately to very warm habitats (usually T 5–8 or 9). It is only the communities of the Sedo-Scleranthion (see Table 7.4b: bottom section) that also occur at high elevation, where they are dominated by succulent species.
Table 7.4b

Character species of calcareous and sandy dry grasslands

On lime-rich (calcareous) soils

Alliances:

Suboceanic calcar.dry grasslands

 

Continental calcar.dry grasslands

 

Alliances:

Xerobrom.

Xerobromion

LTC MRN

Festucion valesiacae

LTC MRN

Festucion v.

L 7–9

Allium pulchellum →

976 281

g Carex supina

777 272

L 7–9

T 6–8

Dorycnium germanicum

764 291

Erysimum odoratum

976 282

T 7–8

C 2–5

Fumana procumbens

973 291

g Festuca duvalii

987 181

C 6–8

M 1–2

Globularia punctata

865 292

g Festuca valesiaca

877 272

M 1–2

R 7–9

Gypsophila fastigiata →

885 261

Hieracium rothianum

986 281

R 7–9

N 1–2

Helianthemum apenninum

872 281

Nepeta pannonica

876 27?

N 1–2

 

Helianthemum canum

874 291

Seseli hippomarathrum

986 291

 
 

Helianthemum nummularium

764 371

g Stipa capillata

878 282

 
 

g Koeleria vallesiana

985 291

g Stipa pulcherrima

987 181

 
 

Linum tenuifolium

984 292

Thymus oenipontanus

– –

 
 

Orobanche amethystea →

785 3x1

(in Tyrol endemic)

  
 

Orobanche teucrii

862 291

   
 

Phyteuma tenerum

864 281

   
 

g Stipa bavarica

96? 171

   
 

Thymus froelichianus

874 281

   
 

Trinia glauca

985 181

   

Alliances:

Continental calcar.semi-dry grasslands

LTC MRN

Suboceanic calcar.semi-dry grasslands

LTC MRN

Alliances:

Mesobrom.

Mesobromion

 

Cirsio-Brachypodion

 

Cirsio-Brach.

L 7–9

o Aceras anthropophorum

772 483

Adonis vernalis

767 372

 

T 4–7

o Anacamptis pyramidalis

872 392

l Astragalus danicus

877 392

L 7–9

C 2–6

g Brachypodium rupestre

754 473

Danthonia alpina

96? 3?2

T 6–7

M 3–4

Carlina acaulissubsp. simplex

9x4 4x2

Inula spiraeifolia

876 392

C 5–8

R 7–9

Carlina vulgaris

753 473

Linum austriacum

976 382

M 2–4

N 2–3

Cirsium acaule

954 382

Linum flavum

876 483

R 6–9

Koelerio-P.

Equisetum ramosissimum →

877 481

Linum perenne

7x6 382

N 1–3

L 7–9

Erigeron acris →

957 482

l Onobrychis arenaria

777 291

 

T 6–8

Euphorbia verrucosa

862 393

l Ononis arvensis

867 472

 

C 2–6

Gentiana aspera →

834 492

Scabiosa ochroleuca

876 382

 

M 2–3

Gentianella ciliata

7x4 382

Senecio integrifolius

767 48?

 

R 4–6

Gentianella germanica

754 483

Seseli annuum →

875 392

 

N 1–2

o Herminium monorchis →

757 582

g Stipa tirsa

878 362

 
 

Hieracium cymosum

765 382

Thesium linophyllon →

875 281

 
 

o Himantoglossum hircinum

772 392

   
 

l Medicago lupulina

75x 48x

Acid semi-dry grasslands Koelerio−Phleion phleoides [in order Brometalia erecti]

  
 

Nonea pulla

766 392

Armeria elongata

763 362

 
 

l Onobrychis viciifolia

876 383

Armeria arenaria

882 362

 
 

l Ononis repens

852 472

g Festuca cinerea

974 2x1

 
 

l Ononis spinosa

865 473

g Festuca trachyphylla

866 3x2

 
 

o Ophrys apifera

762 492

g Koeleria macrantha →

767 372

 
 

o Ophrys holosericea

o Ophrys insectifera

874 492

754 493

Lychnis viscaria

etc. (not yet clear, especially for the oceanic communities)

764 342

 
 

o Ophrys sphecodes

884 493

   
 

o Orchis militaris

765 392

   
 

o Orchis morio

753 473

   
 

o Orchis simia

882 382

   
 

o Orchis tridentata

974 392

   
 

o Orchis ustulata

755 4x3

   
 

Polygala chamaebuxus →

644 383

   
 

Prunella laciniata

772 392

   
 

Ranunculus bulbosus

863 373

   
 

o Spiranthes spiralis →

862 552

   

On non-calcareous soils

 

Silicate rock debris and rock ledge communities

 

Alpine succulent communities on silicate rock

  

Alliances:

Sedo-Veronicion dillenii

LTC MRN

Sedo-Scleranthion

LTC MRN

Alliances:

Sedo-Vero.

Androsace elongata →

987 261

Cerastium arvensesubsp. strictum

936 3x2

Sedo-Scle.

L 8–9

Gagea bohemica

985 252

Plantago serpentina →

966 462

L 8–9

T 6–9

Scleranthus polycarpos →

943 231

Sedum annuum

933 341

T 2–6

C 2–6

Scleranthus verticillatus

974 261

Sedum vulgare

942 342

C 2–6

M 2

Spergula pentandra

964 261

Sempervivum arachnoideum

934 221

M 2–3

R 3–6

Tuberaria guttata

972 251

Sempervivum montanum

822 321

R 2–6

N 1–2

Veronica dillenii

876 252

Silene rupestris

932 331

N 1–2

In the following, we will discuss the major dry grassland communities of both classes in more detail, although the multitude of described associations means that we can only give a general overview. We will focus more on the large-scale relationships rather than specific regional types. Many phytosociologists have studied Central European dry grasslands from a regional perspective, leading to the creation of conflicting classifications (cf. Dierschke 1986a, 1997; Chytrý 2007). Here, we mainly follow Rennwald (2000), as well as Korneck (1993), Pott (1995) and – for the Alps – Mucina and Kolbek (1993a, b), who give more of an international perspective, even if they still come to contradictory conclusions in some cases.

7.3.2 Calcareous Dry Grasslands (Class Festuco-Brometea)

The class Festuco-Brometea (Chap.  14: no. 5.3) contains grasslands with topsoil of pH 6.0–7.5, and rarely any lower. This class differs from the Koelerio-Corynephoretea (acid sandy and rocky grasslands) in its unusually large number of character species, and the two are linked by only a few intermediate community types (see Table 7.4). As implied in the introduction to this section, this is mainly due to the fact that the chemically intermediate soils, i.e. the weakly calcareous to weakly acidic, are mainly deeper than the limestone or dolomite Rendzinas on the one hand and the silicate Rankers on the other. In addition, deep soils generally do not support dry grasslands, but rather arable fields, fertilised pastures or forest. The chemical differences between calcareous and silicate soils have thus been extended by centuries of human use and the resulting soil erosion.

7.3.2.1 Submediterranean Calcareous Dry Grasslands

The calcareous dry grasslands can be divided into those with more continental, and those with more oceanic or submediterranean species. At lower elevations, the climatic and floristic contrasts are more pronounced than higher up in the mountains. The overview in Table 7.5 is therefore limited to colline areas, but uses examples from across Central Europe to show how the extremes are linked by intermediate vegetation types. This is somewhat similar to the gradient of thermophilic broadleaved forests in Central Europe (see Table 6.3 in Vol. I), but even clearer in the case of dry grasslands. Only older vegetation records were used to avoid the distorting effect of eutrophication.
Table 7.5

Dry grasslands on base-rich bedrock in various areas of Central Europe. From tables in Braun-Blanquet, Preis, Oberdorfer, Quantin, Bornkamm and othersa

Order:

Festucetalia valesiacae

Brometalia erecti

Alliance:

Stipo-Poion

Festucion valesiacae

Xero-bromion

Meso- bromion

Association no.:

1

2

3

4

5

6

7

8

9

C

Carex liparocarpos

5

        

F

Potentilla pusilla

5

        

P

Poa alpina carniolica

4

        

P

Koeleria vallesiana

4

        

F

Pulsatilla montana

4

        

S

Petrorhagia saxifraga

4

        

P

Asperula cynanchica

3

        
 

Scabiosa gramuntia

3

        

K

Scorzonera austriaca

3

        

F

Festuca valesiaca

5

5

       
 

Elymus hispidus

1

3

       

C

Veronica praecox

1

3

       

C

Astragalus excapus

3

       

F

Achillea setacea

1

2

       

F

Centaurea stoebe

 

5

       

S

Myosotis stricta

 

5

       

C

Thymus glabrescens

 

4

       

F

Achillea collina

 

4

       

F

Pulsat. pratensis nigra

 

3

       

F

Stipa pennata & joannis

3

2

3

      
 

Tortula ruralis

2

3

4

      

S

Poa bulbosa

1

3

1

      

F

Oxytropis pilosa

1

2

1

      

S

Sedum rupestre

2

1

1

      
 

Melica transsilvanica

 

3

1

      

F

Carex supina

3

2

      

K

Veronica spicata

2

1

1

1

     

S

Erysimum crepidifolium

 

4

5

2

     

B

Festuca cinerea

 

2

1

2

     
 

Sesleria albicans

   

5

     
 

Buphthalmum salicifol.

   

3

     

F

Erysimum odoratum

   

2

     

K

Thymus praecox

 

1

5

1

5

    

K

Festuca rupicola

 

1

3

 

5

    

F

Thesium linophyllon

 

2

 

3

    

F

Stipa capillata

5

4

5

 

2

3

   

K

Allium sphaerocephalon

4

2

3

  

3

   

F

Potentilla arenaria

 

5

5

3

3

3

   
 

Galium glaucum

 

4

5

3

1

1

   

S

Alyssum montanum

 

1

5

1

1

2

   

F

Seseli hippomaratrum

 

3

1

  

1

   

F

Stipa pulcherrima

 

1

 

2

2

2

   

S

Artemisia campestris

5

4

5

3

3

4

1

  

K

Stachys recta

2

3

4

3

2

2

3

  

B

Koeleria macrantha

1

5

3

2

2

 

3

  
 

Medicago minima

2

2

 

1

 

2

3

  

K

Bothriochloa ischaemum

2

2

  

1

3

1

  

F

Silene otites

4

3

 

1

  

2

  

C

Globularia punctata

2

  

3

 

5

4

  

K

Lactuca perennis

1

 

1

2

1

 

2

  

S

Melica ciliata

1

     

3

  
 

Teucrium chamaedrys

3

3

5

4

4

5

1

1

 

K

Aster linosyris

3

4

5

2

5

5

 

1

 

K

Carex humilis

3

4

1

5

5

5

5

4

 

K

Phleum phleoides

3

3

4

4

 

3

4

3

 

K

Asperula cynanchica

1

2

5

5

4

4

 

4

 

K

Odontites lutea

4

1

 

1

  

1

1

 
 

Peucedanum oreoselinum

4

  

2

   

3

 

F

Euphorbia seguierana

4

     

1

1

 

K

Euphorbia cyparissias

1

5

5

5

4

5

4

5

1

S

Arenaria leptoclados

2

4

5

2

 

3

2

 

1

S

Acinos arvensis

1

2

1

2

  

4

 

1

S

Erophila verna

1

5

1

1

  

3

 

1

 

Hieracium pilosella

1

1

3

3

3

5

 

4

4

K

Centaurea scabiosa

3

1

 

2

4

1

2

5

2

B

Hippocrepis comosa

2

  

5

3

3

1

5

1

B

Helianthemum ovatum

5

 

3

4

5

1

B

Bromus erectus

4

 

5

5

5

3

K

Brachypodium pinnatum

1

1

2

3

3

5

5

K

Sanguisorba minor

1

1

2

3

1

3

5

B

Anthyllis vulneraria

1

  

2

4

1

3

5

3

B

Linum tenuifolium

1

1

  

5

3

4

 

1

S

Alyssum alyssoides

2

2

   

1

3

 

1

K

Eryngium campestre

 

5

  

3

2

3

1

 

B

Dianthus carthusianorum

 

5

5

5

2

1

3

4

 

K

Pimpinella saxifraga

 

1

2

4

 

2

4

5

S

Sedum album

 

2

4

2

   

3

 

S

Sedum acre

 

2

5

2

 

1

  

1

 

Medicago falcata

 

2

 

1

2

1

 

1

3

 

Scabiosa canescens

 

2

1

 

1

1

 

1

 

K

Salvia pratensis

 

1

3

4

5

3

4

1

B

Avenochloa pratensis

 

1

 

1

5

 

3

1

4

C

Cerastium pumilum

 

1

1

  

1

4

  

K

Galium verum

 

1

 

3

1

  

3

1

 

Thymus pulegioides

  

5

4

1

2

4

 

4

B

Festuca trachyphylla

  

5

3

3

4

5

5

2

B

Pulsatilla vulgaris

  

1

5

5

1

4

  

K

Potentilla verna

  

1

5

3

1

4

1

 

B

Teucrium montanum

   

3

3

4

1

1

 

B

Helianthemum nummul.

   

5

2

5

 

5

 

B

Fumana procumbens

   

1

 

5

1

  
 

Anthericum ramosum

   

5

1

  

3

 

B

Scabiosa columbaria

   

4

1

1

3

5

4

B

Arabis hirsuta

   

2

 

1

 

2

1

K

Prunella grandiflora

    

5

 

3

2

1

B

Koeleria pyramidata

    

3

1

 

3

 

M

Ranunculus bulbosus

    

1

 

2

5

3

M

Cirsium acaule

    

3

 

1

 

4

M

Carlina vulgaris

    

2

 

1

1

2

M

Ononis spinosa

    

2

  

2

2

B

Carex caryophyllea

    

 

5

4

M

Medicago lupulina

    

 

3

5

4

C

Trinia glauca

    

5

    

C

Onobrychis arenaria

    

4

    

D

Cladonia furcata L

    

4

   

2

D

C. convoluta L

    

4

    

C

Ononis natrix

 

1

    

4

  

S

Petrorhagia prolifera

2

     

4

  

C

Coronilla minima

      

3

  

C

Micropus erectus

      

3

  

S

Cerastium brachypetalum

      

3

  
 

Linum catharticum

      

4

5

4

 

Trifolium pratense

      

4

1

3

D

Briza media

      

3

5

4

D

Plantago media

      

2

5

4

M

Onobrychis viciifolia

      

1

5

1

M

Anacamptis pyramidalis

       

3

 

D

Potentilla heptaphylla

       

5

 

D

Silene nutans

       

5

 

D

Anthoxanthum odoratum

       

5

 

D

Polygala amara

       

4

 

D

Leucanthemum vulgare

       

4

1

D

Lotus corniculatus

       

5

5

D

Plantago lanceolata

       

5

4

D

Campanula rotundifolia

       

5

3

D

Daucus carota

       

5

3

D

Leontodon hispidus

       

4

3

D

Dactylis glomerata

       

5

2

D

Centaurea jacea

       

4

2

D

Carex flacca

       

2

3

D

Knautia arvensis

       

2

3

K

Poa angustifolia

        

4

D

Achillea millefolium

        

3

D

Agrimonia eupatoria

        

3

M

Gentianella ciliata

        

2

Several bryophytes:

         

K

Rhytidium rugosum

 

4

5

3

2

 

5

 

K

Thuidium abietinum

   

4

2

1

 

5

1

K

Homalothecium lutescens

    

2

  

3

1

K

Pleurochaete squarrosa

   

2

 

4

   

aSeveral less constant species were omitted, including the orchids Aceras anthropophorum, Himantoglossum hircinum, Ophrys apifera, holosericea und sphecodes, Orchis militaris, morio, simia and ustulata that occur occasionally in no. 8 and 9. The numbers after the plant names refer to constancy classes. Records prior to 1960 were intentionally used here, as the dry grasslands were largely still in active use then and not affected by eutrophication or abandonment

C character species of the community in which it occurs at high constancy, D differential species of a community or an alliance with few character species, P character species of the alliance Stipo-Poion xerophilae, F character species of the Festucetalia or Festucion valesiacae, B character species of the Brometalia, M character species of the Mesobromion or differential species separating it from the Xerobromion, K class character species of the Festuco-brometea, S character species of the class (or lower ranking unit) of the Koelerio-Corynephoretea, which in some cases also occur in K

No. 1: Inner Alpine Stipo-Koelerietum vallesianum in the lower Valais. From Braun-Blanquet (1961). Stipo-Poion = Stipo-Poion carniolicae

No. 2 to 4: Examples of ± continental Festucion valesiacae:

2: Festuco valesiacae-Erysimetum crepidifolii in the low mountains in western Czech Republic. From Preis (1939)

3: Erysimo-Stipetum in the Nahe valley and in Rhenish Hesse. From Oberdorfer (1957)

4: Seslerio-Festucetum rupicolae in the Franconian Jura. From Gauckler (1938) in Oberdorfer (1957). With similarities to the Brometalia!

No. 5 to 7: Examples of ± suboceanic Xerobromion:

5: Trinio-Caricetum humilis near Würzburg. From Volk (1937) in Oberdorfer (1957). With similarities to the Festucetalia!

6: ‘Xerobrometum rhenanum’ in the Kaiserstuhl. From von Rochow (1951) in Oberdorfer (1957).

7: ‘Xerobrometum lugdunense’ in the western Swiss Jura. From Quantin (1935)

No. 8 and 9: Examples of suboceanic Mesobromion:

8: ‘Mesobrometum collinum’ in Kraichgau. From Oberdorfer (1957)

9: Gentianello-Koelerietum close to Göttingen. From Bornkamm (1960)

The suboceanic-submediterranean calcareous grasslands are placed within the order Brometalia erecti (see Table 7.4). Their character species typically have low continentality values (EIV-C 2–5, or 6 at most). These communities are best developed in limestone mountain areas in the submediterranean region with warm summers and mild winters. In Central Europe, these communities are found in particularly species-rich forms in western Switzerland, southwestern Germany and in the Alsace region. There are only relatively species-poor forms in the Netherlands, the limestone region of the Eifel and in the Lower Saxony uplands, and those in Denmark and southwestern Sweden have little of the character of the Brometalia erecti left (Dierßen 1996a, Bruun and Ejrnaes 2000).

7.3.2.2 Continental Calcareous Dry Grasslands

In mid- and eastern Central Europe, a gradual transition occurs to the continental calcareous grasslands of the order Festucetalia valesiacae, which itself then transforms further east into the cold steppes of Ukraine and the Hungarian Plain. The character species of this order have a more continental range (EIV-C 6 or 5 to C 8, see Table 7.4). The westernmost occurrences of these communities are found in the dry valleys of the Valais Alps (see Table 7.5), in the Kaiserstuhl in southwest Germany and around the Nahe River west of Mainz, with many species of the Brometalia present in the latter two regions. There is a wide transitional zone between the Brometalia erecti and the Festucetalia valesiacae from the central German dry zone in Thuringia over Mainfranken and the Franconian Jura to Tyrol. This subcontinental-suboceanic transition zone includes the dry grasslands around Würzburg in northern Bavaria (see Table 7.6, no. 5). Both their species composition and the climate of their habitat could easily belong to the continental order of the Festucetalia valesiacae. The climate data in Table 7.6 and the vegetation table (Table 7.5) therefore clearly show how the two extremes are linked. Well-developed continental calcareous dry grasslands are found in the central German Saale-Unstrut region, on the slopes of the Oder valley, in the dry zone in the western Czech Republic, in the Pannonian area of Austria, at the eastern edge of the Alps, at the edge of the Hungarian Plain in western Slovakia, and in the dry areas of southern Poland with less than 550 mm, and in places with even less than 500 mm precipitation per year (Braun-Blanquet 1936; Klika 1939; Niklfeld 1964; Mahn 1965; Kolbek 1975; Toman 1992; Chytrý 2007). It is often the substrate that is the decisive factor in this transitional zone whether communities of the Festucetalia or of the Brometalia form. The vegetation on limestone soils contains more species of the Brometalia, whilst that on loess soils and base-rich silicate soils contains more Festucetalia species.
Table 7.6

Climatic data and indicator values for the dry grassland examples in Table 7.5. From data in Maurer et al. (1910), Reichsamt f. Wetterdienst (1939) and Quantin (1935), indicator values calculated from Ellenberg et al. (1992)a

Grassland alliance

 

Precipitation (mm)

Temperature mean (°C)

Quotientb July temp.: annual precip.

Mean EIV valuesa

Community no. (see Table 7.5)

Region, location

m a.s.l.

Year

IV–IX

 

Order

Year

 

July

   

mT

mC

mM

Stipo-Poion carniolicae

              

1. Valais: Sitten

540

638

318

Open image in new window

 

9.6

 

20.6

32.3

Open image in new window

 

6.5

4.9

2.2

 Siders

532

536

268

 

9.3

 

20.8

38.8

    

Festucion valesiacae

   

<350 Festu-cetalia

    

>30

   

2. Bohemia: Litomericec

177

502

318

8.6

 

18.6

37.0

6.5

5.3

2.6

3. Rhine Hesse: Mainz

94

512

290

10.0

 

19.2

37.5

 

6.2

4.7

2.5

 Oberlahnstein

77

590

332

10.3

 

19.0

32.2

    

4. Franconian Jura: Nuremberg

320

585

356

Open image in new window

tends to Brom.

8.7

 

18.3

31,3

  

5.8

4.6

2.8

 Amberg (Oberpf.)

525

677

399

6.9

 

16.5

24.5

     

Xerobromion

   

Open image in new window

tends to Fest.

         

5. Mainfranken: Würzburg

179

560

318

9.0

 

18.3

32.7

  

6.1

4.7

2.6

6. Kaiserstuhl: Oberrotweil

222

672

422

Open image in new window

 

9.7

 

18.7

28

Open image in new window

 

6.3

4.3

2.7

      

>9

      

7. Western Jura: Geneve

405

859

480

 

9.5

 

19.5

22.5

 

6.4

4.0

2.6

 St. Denis-Laval

398

747

443

>350 Brome-talia

10.0

 

18.5

25

<30

   

Mesobromion

          

8. Kraichgau: Bretten

214

734

417

8.8

 

18.1

24.5

5.5

3.9

3.4

 Pforzheim

258

728

431

8.6

 

17.5

24

   

9. Near Göttingen

155

607

378

8.5

<9

17.2

28.5

5.5

3.8

3.6

 Herzberg/Harz

242

802

436

 

7.6

 

17.3

21.6

    

aCalculated from the indicator values of the species shown in Table 7.5; extreme values are in bold

bQuotient calculated from 1000 times the average air temperature in the month of July and the average total annual precipitation (see Table 6.7 in Vol. I) as a measure for the continentality of the climate.

cFrom Preis

The growing season (IV–IX) has less rainfall in areas 1–3 than in areas 6–9, and the climate in 1–3 is more continental than in 6–9. Areas 4 and 5 are intermediate between the two groups

The Stipo-Koelerietum vallesianum in Valais (no. 1 in Table 7.5) and the semi-dry grasslands around Göttingen (no. 9) share only a few class character species and accompanying species. The flora of the Valais grassland has more in common with the submediterranean Xerobrometum of the southwestern Jura (no. 7), but is still surprisingly dissimilar considering how close regions 1 and 7 are to each other. These inner Alpine dry grasslands (no. 1) are much more similar to the, geographically very distant, steppe grasslands in the low mountains of the northern Czech Republic near Litoměřice (no. 2) and the rocky steppe grasslands at the edge of the Rhine valley near Mainz (no. 3). It is only in these three communities that feathergrasses (Stipa capillata and S. pennata with subsp. joannis) play an appreciable role. Other species of the order Festucetalia valesiacae (F in Table 7.5, see also Fig. 7.14) also only occur here, whilst the species of the Brometalia (B) become more important in the right hand side of the table.
Fig. 7.14

A cross-section through a Stipa meadow steppe on deep gypsum soils in the Kyffhäuser (From Meusel 1939)

From left to right:Scorzonera purpurea, Stipa pennata, Scabiosa canescens, Carex humilis, Festuca ovina cinerea, Fumana procumbens, Stipa p., Aster linosyris, Stipa capillata, C. h., Fest. o. c.; Festuca and Fumana occur in open patches created by livestock trampling. The steps in the cross-section are due to livestock paths along the contours of the slope (cf. Fig. 3.15 in Vol. I).

These floristic similarities and differences are partially explained when considering the indicator values describing the regional climates of the grassland examples (see Table 7.6). In regions 1, 2 and 3, the climate is relatively continental, with low rainfall during the growing season. The submediterranean summer depression in rainfall in Sion causes the average total for April to September to sink to that of Litoměřice, although the annual total precipitation is much higher in Sion. The temperatures are high in all three areas, particularly in July, as are those in areas 5–7.

The submediterranean-suboceanic dry grasslands are therefore not exposed to higher temperatures than the continental grasslands, at least in summer, but do receive more rainfall and have higher air humidity. This is shown by the quotient of July temperature to annual precipitation (see explanation in Table 6.7 in Vol. I). It is less than 30 for all areas with mainly Brometalia communities (no. 6–9), and higher in areas with Festucetalia valesiacae communities (see Table 7.6).

Inasmuch as normal weather stations are suited to characterise the climate in sunny dry grassland habitats, the climate data in Table 7.6 does appear to largely reflect the floristic differences in the communities in Table 7.5. Lacoste (1964) came to similar conclusions for the French western Alps, finding that the Brometalia and Festucetalia communities occur according to similar climatic patterns as in Central Europe.

7.3.2.3 Dry and Semi-dry Grasslands

Several well-differentiated alliances can be distinguished within the suboceanic and continental calcareous dry grasslands according to their habitat characteristics. The communities of the suboceanic alliance of the Xerobromion and the more continental alliance of the Festucion valesiacae are more drought-tolerant (EIV-M 1–2) and less productive than those in the semi-dry grassland alliances of the Mesobromion (suboceanic-submediterranean) and Cirsio-Brachypodion (continental; M 2–4). The former are also found in very dry habitats, and can be seen in summer as sparse, yellow-brown flecks in the wetter surrounding vegetation (Wilmanns 1998). However, both the Mesobromion and the Cirsio-Brachypodion are found in areas of potential forest, and it is only grazing or mowing that allow these dry grasslands to persist. This is to some extent also true for most Central European calcareous dry grasslands (Xerobromion and Festucion valesicae), of which many are now becoming overgrown with scrub and forest. It is only on the shallow soils of ridges and steep south-facing slopes that dry grassland fragments would remain open without human influence, i.e. be the potential natural vegetation (see Fig. 4.5 in Volume I).

7.3.2.4 Mesobromion and Xerobromion

A comparison of the examples in Table 7.6 shows that all the EIV temperature values are higher in the Xerobromion (no. 5–7) than the Mesobromion (no. 8 and 9). This is also visible in the ecograms in Fig. 7.13, left-hand side. The amount of precipitation, in contrast, differs very little. Corresponding to their lower temperatures, the Mesobromion communities have fewer drought-tolerant and thermophilic species of Mediterranean origin. Their richness in mesophilic meadow plants such as Dactylis glomerata, Lotus corniculatus, Plantago lanceolata and P. media, Daucus carota etc. is probably due to its lack of extreme drought periods. At lower temperatures, the vapour pressure deficit never becomes as high as on the sunny slopes of the Xerobrometum in the Kaiserstuhl or in the Swiss or French Jura (see Tables 7.6 and 7.1).

On warm, but not too dry calcareous soils in southwestern and western Central Europe, the Mesobromion (no. 8) supports numerous orchid species, e.g. Orchis morio, O. militaris, O. ustulata and O. simia, Ophrys holosericea, O. apifera and O. sphecodes, Herminium monorchis, Gymnadenia conopsea and Anacamptis pyramidalis (see Fig. 7.37). Their constancy is so low that they are hardly or not mentioned at all in Table 7.5. They are almost completely absent from the Mesobromion grasslands of northwestern Germany (no. 9) (Tüxen 1937), presumably because it is too cold for them here. Other species typical for the centre of the range of the Mesobromion, such as Bromus erectus, Brachypodium pinnatum, Hippocrepis comosa, Scabiosa columbaria, Cirsium acaule and Plantago media, also become rarer moving north.

The Onobrychido-Brometum or Mesobrometum is a widespread association of the Mesobromion alliance, which requires a single cut per year and is one of the most species-rich vegetation types in Central Europe. Another widespread association is the Gentiano-Koelerietum, which is formed through grazing. Onobrychis viciifolia (sainfoin) was introduced in the sixteenth century as a fodder crop and has since become characteristic for the Mesobrometum of southwestern Germany. The grazed Gentiano-Koelerietum differs from the mown Mesobrometum mainly in the large proportion of Brachypodium pinnatum (tor-grass) and Festuca ovina (particularly the subspecies Festuca guestphalica) at the cost of the grazing-sensitive Bromus erectus (upright brome). They also have different frequencies of species sensitive to cutting and unpalatable to livestock, such as Cirsium acaule, Carlina vulgaris and C. acaulis, Gentianella ciliata, G. germanica. The large geographical and ecological variation means that there are numerous regional associations, some of which are mentioned in the following sections.

Submediterranean dwarf shrubs are particularly numerous in the Xerobromion grasslands, such as in the Rhenish-Swabian Xerobrometum and in the Mainfranken-Thuringian Trinio-Caricetum humilis, including Fumana procumbens and Helianthemum nummularium agg., H. canum and H. appeninum, and the Lamiaceae Teucrium montanum and T. chamaedrys (Volk 1937; von Rochow 1951; Becker 1998) (see Fig. 7.1). Areas of bare earth resulting from the extreme dryness are sometimes colonised by the colourful and trampling-tolerant Toninio-Psoretum decipientis, with the sulphur-yellow Fulgensia fulgens, the red Psora decipiens, the grey-yellow Cladonia endiviaefolia, the russet Catopyrenium rufescens and the blue-grey Toninia caeruleo-nigricans, all of which are horizontally spreading lichens with Mediterranean origins (Günzl 2001). This cryptogam community is placed in its own class (Psoretea decipientis). Like the Xerobromion, it is considered to be a relict community in Central Europe from the Holocene climatic optimum, and stretches up to the grazed alvars on the Baltic island of Öland.

Disturbed, dry slopes e.g. on loess banks or railway embankments are sometimes colonised by semi-ruderal dry grasslands with large populations of Elymus. Müller and Görs (1969) placed these grasslands in their own class (Agropyretea intermedii-repentis), the communities of which contain more oceanic or more continental species, depending on the communities they are surrounded by.

7.3.2.5 Cirsio-Brachypodion and Festucion valesiacae

Some of the forb-rich Brachypodium grasslands in Brandenburg, the western Czech Republic, Poland, Lower Austria and other areas of eastern Central Europe are assigned to the continental semi-dry grassland alliance of the Cirsio-Brachypodion, which merges further east with the mesophilic meadow steppe of Eastern Europe in the forest steppe ecotone. This contains the colourful Adonido-Brachypodietum in Brandenburg, Saxony-Anhalt and Thuringia, characterised by Adonis vernalis (see Figs 7.15 and 7.16), which stretches with related communities into Poland, western Czech Republic, the Upper Rhine and the northern foothills of the Harz (Krausch 1961; Mahn 1965; Korneck 1974; Janssen 1992; Evers 1997; Jandt 1999; Becker 1998) and, among others, the Scabioso ochroleucae-Brachypodietum and the Brachypodio-Scorzoneretum in eastern Central Europe (Toman 1992; Chytrý 2007).
Fig. 7.15

A subcontinental Brachypodium grassland near Lebus on the Oder in spring with flowering Adonis vernalis. The scrub in the background points to the fact that the area would naturally support forest. The floodplain below still has many trees and is mostly flooded at this time of year. (Photo Krausch)

Fig. 7.16

A cross-section through a semi-dry grassland with Brachypodium pinnatum on deep loam over porphyry near Halle. From Mahn (1957). The soil contains no stones down to about 70 cm depth and is relatively lime-poor, but has a high water storage capacity. From left to right: Brachypodium pinnatum (sterile), Filipendula vulgaris, B. p., Euphorbia cyparissias, Festuca ovina rupicola, Salvia pratensis (with taproot), B. p. (flowering), Scabiosa canescens, B. p., Achillea millefolium, Centaurea scabiosa (very tall with taproot), Potentilla alba, B. p., Plantago lanceolata

The continental Festucion valesiacae is particularly characterised by the Stipa species with their tall tussocks and striking ‘feathers’. Stipa mainly occurs in Central Asia, but over ten species or subspecies are found in Central Europe. Within this alliance, the Potentillo-, Astragalo- and Festuco valesiacae-Stipetum capillatae are characteristic for the limestone mountains of Poland, Czech Republic, Slovakia and eastern Germany. In the west, it is represented by the Allio-Stipetum capillatae in Mainfranken and in the Upper Rhine (e.g. Korneck 1974, 1978; Kolbek 1978; Toman 1992; Oberdorfer 1993b; Hensen 1995). Like the dry grasslands of the Brometalia erecti, these continental types occur mainly on south-facing slopes with free-draining limestone or gypsum soils, i.e. they are extrazonal vegetation. However, towards the oceanic west and the continental east the habitat amplitude of these grasslands widens, and they will also grow on sandy-loamy substrates (e.g. till on the slopes of the Oder valley and loess and clay soils in the central German dry zone) as well as free-draining gravel terraces, such as in the southern Upper Rhine Plain.

Witschel (1987) found that Stipa species were mainly found in five areas of Baden-Württemberg: between Mannheim and Schwetzingen, in the Taubertal, in the Kaiserstuhl, on the Isteiner Klotz and in the upper Danube valley. Stipa capillata is the most widespread, as it is indirectly promoted by fire and grazing, as well as by abandonment. It grows particularly well in the Allio sphaerocephali-Stipetum capillatae (see Korneck 1974). The range of Stipa pulcherrima stretches from western Siberia to Central Europe, but it is only found in the Xerobrometum of the Kaiserstuhl in Central Europe. Stipa pennata subsp. pennata is also limited to the Xerobrometum, for example on the Isteiner Klotz. Its subsp. austriaca survives in the hottest habitats on cliffs and rocks in the upper Danube valley, together with several Alpine species. Also within the Pennatae section, S. joannis is found in both Xerobromion and Festucion valesiacae communities. This grass is less sensitive to the lime content of the soil than the other taxa. The most widespread Stipa grassland in the Czech Republic is the Festuco valesiacae-Stipetum capillatae which traditionally has been grazed (Chytrý 2007).

7.3.2.6 Inner Alpine Steppe Grasslands

The rocky steppes of the inner Alpine valleys have, despite being further west and closer to the sea, a more continental climate than the Czech or the Polish steppe grasslands. This is shown by the yearly variation in average monthly air temperature in Table 7.6. For Valais this is over 20 °C, as sharp frosts often occur, but lower in all other areas, including the Czech Republic. However, it is not just the climate, but also the geographic isolation of the inner Alps that is responsible for the floristic uniqueness of the steppe grasslands there. Braun-Blanquet (1961) thus assigned these communities not to the Festucion valesiacae, but rather created two new alliances, the Stipo-Poion carniolicae for the western Alps and the Stipo-Poion xerophilae for the eastern Alps. As he emphasizes, the climate of the eastern valleys is less continental than that of the western valleys. A much more detailed classification of the continental steppe grasslands of the eastern Alps can be found in Mucina et al. (1993), and of the western Alps in Schwabe and Kratochwil (2004).

7.3.2.7 Sesleria Grasslands in the Low Mountains

Communities dominated by Sesleria albicans and resembling the Seslerio-Caricetum sempervirentis of the alpine belt (see Sect.  5.3.5.1 and Fig. 7.11) form even in low mountain areas on steep calcareous slopes exposed to the sun or in deep shade, i.e. in areas with atypical edaphic or climatic conditions. This species is mainly found in the alpine belt of the Limestone Alps, and the naturally open grasslands there are placed within the class Seslerietea albicantis. Depending on the author, the colline to submontane Sesleria grasslands are placed within the Festuco-Brometea as their own alliance, suballiance or group of associations.

Phytogeographically speaking, the Sesleria grasslands are notable as they – like the Seslerio-Fagetum forests (see Sect. 5.3.5 in Vol. I) – contain several species that are mainly found in the Alps, e.g. Carduus defloratus, Thesium alpinum, Cardaminopsis petraea, Biscutella laevigata and Aster bellidiastrum. These species largely restricted to the Alps can be seen as relicts that were once widespread on base-rich substrata in the areas of Central Europe not covered by ice during the ice ages (Chytrý 2007). This is certainly true for Sesleria albicans itself, which is a poor disperser (Schubert 1963) and germinates poorly in low mountain habitats. It is rarely found in secondary habitats such as abandoned vineyards, even if these are found directly adjacent to a Sesleria grassland and have similar soil conditions. The areas in which Sesleria is now present must have persisted for long periods of time to allow it to colonise, or have climatic conditions that are conducive to its establishment.

Most of the rocky Sesleria grasslands in the low mountains are probably mostly naturally treeless and support high plant diversity. Some formed from Quercion pubescentis forests, although most replace the Hordelymo-Fagetum. Many Sesleria grasslands used to be grazed by sheep and goat, but this management has mostly now ceased.

Like in the Alps, Sesleria is not a pioneer species in low mountain areas, but only establishes once scree slopes have stabilised or on rocky terraces, as well as on damp calcareous soils. However, a relatively mobile substrate is beneficial for Sesleria communities in that it prevents more drought-tolerant dry grassland plants from taking over. Schubert (1963) provides an overview of several Sesleria-dominated communities for eastern Central Europe, noting their occurrence from sunny, shallow-soiled steep slopes to shady and humid scree slopes or cliff faces occasionally moistened with trickling water. Chytrý (2007) lists four Sesleria grassland communities in the Czech Republic, which he places in the Diantho lumnitzeri-Seslerion.

In the northwest German low mountain areas, Schmidt (2000) identified the Teucrio-Seslerietum (often occurring alongside the Xerobrometum), and the more mesophilic Polygalo-Seslerietum (Mesobromion alliance). The Sesleria grasslands at lower elevations in northern Germany lack many species that are unique to the southern German scree slopes with closer affinity to the alpine Sesleria-dominated communities, such as Buphthalmum salicifolium, Thesium bavarum, Rhinanthus aristatus or Phyteuma orbiculare (Oberdorfer 1993b).

7.3.2.8 Gypsum and Dolomite Dry Grasslands

Dry grasslands on gypsum closely resemble those on limestone in their species composition, although the substrate is chemically and physically quite different. Gypsum (CaSO4 ∙ 2 H2O) and anhydrite (CaSO4) usually occur in Central Europe together with rock salts and carbonate rock in Paleozoic Zechstein and Mesozoic Buntsandstein and Keuper formations. These soft rock formations quickly weather into karstic forms with jagged cliffs, supporting both species-rich dry grasslands and forests (e.g. gypsum beech forests) (e.g. Becker 1996; Jandt 1999). Gypsum Rendzinas often have a topsoil of loamy gypsum flour, which has a similar water-holding capacity to their carbonate or silicate counterparts (Heinze and Fiedler 1984a). However, when carbonate rock dissolves it leaves a clay-rich material with an exchange capacity much higher than that of the sandier gypsum residues. Gypsum Rendzinas are therefore poor in plant-available Mg and K, as well as having a very high Ca/Mg ratio that is unfavourable for plant growth (Heinze and Fiedler 1984b; Schmid and Leuschner 1998). Their high sulphate content means that gypsum Rendzinas acidify much more quickly than carbonate Rendzinas. Gypsum dry grasslands therefore often contain many acid-tolerant or calcifuge species, such as Artemisia campestris and Calluna vulgaris, whereby the latter accelerates the soil acidification (Meusel 1939; Groten and Bruelheide 1997).

In contrast to the gypsum soils of arid regions (cf. Duvigneaud and Denaeyer-de Smet 1968), there are no typical gypsum plants (‘thiophores’) in Central Europe that are largely restricted to these calcium- and sulphate-rich substrates. In her analysis of the gypsum dry grassland complex in the southern foothills of the Harz Mountains, which is the largest in Central Europe, Jandt (1999) found a group of species that occurred mainly on gypsum in the region (e.g. Gypsophila fastigiata, G. repens, Festuca pallens, Alyssum montanum, Koeleria macrantha). However, even the Gypsophila species occur on calcareous sands or gravels in other regions, so are in no way restricted to calcium sulphate soils (Meusel 1939).

The grassland communities on dolomite, a magnesium-rich carbonate rock, are also so similar to those on limestone under otherwise similar conditions, that they can only be distinguished at the level of the association or subassociation (cf. Mucina and Kolbek 1993a for Alpine dolomite dry grasslands). As dolomite (CaMg(CO3)2) weathers relatively slowly, the soils are usually shallower and can thus hold less water. As a result, the grassland communities are usually drier on dolomite Rendzinas, unless the climate is particularly humid.

7.3.2.9 Silicate Dry Grasslands

Acid rocks such as porphyry, gneiss, granite, shale or even quartz sands can also support grasslands with a species composition and structure more similar to the calcareous dry grasslands in the class Festuco-Brometea than to the acidic sandy and rocky grasslands of the class Koelerio-Corynephoretea. Importantly, these ‘silicate dry grasslands’ only develop in areas with dry summers, preventing major losses of basic cations and soil acidification. Several deeply weathered calcareous soils, such as the Terra fusca soils and pararendzinas on thick loess are also often superficially weakly to moderately acidic. Korneck (1974) and Mucina and Kolbek (1993b) found that these grasslands contain several species with high constancy, such as Phleum phleoides, Koeleria macrantha, Veronica spicata, Saxifraga granulata and Agrostis stricta. However, these potential character species also occur in other dry grassland orders, particularly the Festucetalia valesiacae and Sedo-Scleranthetalia of rocky and gravelly habitats. Alongside indicators of acid conditions such as Agrostis capillaris, Chamaespartium sagittalis and Luzula campestris (R 3–4), they also support species restricted to base-rich habitats, e.g. Stachys recta, Ranunculus bulbosus and Trifolium alpestre (R 6–8). These grasslands are therefore mainly characterised by their lack of unique character species, and thus to a certain extent represent an intermediate vegetation type between the acidic sandy Koelerio-Corynephoretea and the calcareous Festuco-Brometea. They probably also have intermediate soils, in that they are only moderately acidic or have a small-scale mosaic in base-richness. The vegetation of silicate dry grasslands is correspondingly sparse and interspersed with small patches of Sedo-Scleranthetalia swards. The silicate dry grassland communities are placed within the alliance of the Koelerio-Phleion phleoidis, including for example, the Genistello-Phleetum in the Rhine and Mosel valleys with the characteristic species Chamaespartium sagittale and Phleum phleoides (Korneck 1974), the Filipendulo vulgaris-Helictotrichetum in central Germany (Mahn 1965; Becker et al. 2007) and the Pulsatillo-Phleetum in northeastern Germany (Passarge 1964a). Chytrý (2007) lists three Koelerio-Phleion associations for the Czech Republic.

An ecologically similar vegetation type intermediate between calcareous semi-dry grasslands and acid Nardus grasslands is found in the limestone heaths of southwestern England, which contain a small-scale mosaic of calcicole and calcifuge species at pH values of 5–6 (Etherington 1981). Gypsum dry grasslands colonised by Calluna vulgaris quickly develop acidic topsoil, and therefore also produce a mosaic of communities with differing acid-tolerance (Groten and Bruelheide 1997; see Sect. 7.3.2.8).

7.3.3 Sandy Dry Grasslands and Vegetation of Cliffs and Rocky Debris (Class Koelerio-Corynephoretea)

Whilst the calcareous dry grasslands discussed above form largely closed swards dominated by graminoids, the grasslands in the class Koelerio-Corynephoretea are mainly made up of dwarf therophytes, low-growing grasses, succulent chamaephytes, bryophytes and lichens. The vegetation structure contains more gaps and is sparser than in the communities of the Festuco-Brometea, because the soil is usually shallower and more liable to dry out (see Fig. 7.18). In both classes the productivity is limited by periodic drought and lack of nutrients. The communities of the Koelerio-Corynephoretea colonise two types of habitat, rock cliff edges, ridges and ledges with shallow soils, and sandy or gravelly sediments with low clay and silt contents. The former support the order Sedo-Scleranthetalia, and the latter several orders of pioneer communities and sandy grasslands. The spectrum of colonised geological substrates is very wide: from silicate (granite, gneiss, and shale) to carbonate rock (limestone, dolomite) and sand and gravel deposited by wind and water. Roofs and asphalt surfaces can also support these communities. In contrast to the communities of the Festuco-Brometea, those of the Koelerio-Corynephoretea do not occur mainly on calcareous soils, and the majority occur on lime-poor or lime-free and often acid substrates (Passarge 1960, 1964a; Krausch 1961, 1968; Korneck 1974; Mucina and Kolbek 1993b). An overview of the particularly diverse grasslands of the Koelerio-Corynephoretea in the Czech Republic can be found in Chytrý (2007) who lists 11 associations.

7.3.3.1 The Order Sedo-Scleranthetalia

Bare rock is usually found on narrow ledges, ridges or peaks, and is thus spatially very limited (see Fig. 7.17, see also Fig. 7.12). However, it is soon covered in crustose and foliose lichens and cushion mosses that produce layers of humus, whilst cracks, fissures and small depressions accumulate enough mineral soil for vascular plants to develop directly. The often inconspicuous communities of the Sedo-Scleranthetalia formed here are extremely diverse and species rich. Korneck (1978, 1993) and Mucina and Kolbek (1993b) identified over 20 associations for southern Germany, the northern Alps and Austria, of which some are limited to very small ranges.
Fig. 7.17

Acid dry grasslands and other communities on a porphyry cliff near Münster am Stein (Rotenfels; Rhine valley). Modified from Haffner (1968). 1. Crustose and foliose lichen community (Parmelion saxatilis), 2. Asplenietum septentrionalis, 3. Cotoneastro-Amelanchieretum, 4. fragments of communities within the order Sedo-Scleranthetalia, 5. Erysimo-Stipetum, 6. Teucrio-Melicetum ciliatae, 7. Rumicetum scutati

The existence of the Sedo-Scleranthetalia is mainly due to the fact that their habitat is inhospitable for most of the Central European flora. Rock ledges and other areas with shallow soils or free-draining piles of fine debris cannot support the growth of trees, shrubs or even tall forbs, meaning that once they have established, the weakly-competitive small species can grow largely without competition. This allows many species to coexist at small scales, producing the large number of character species for these communities.

Although there is still no synsystematic classification for these communities for the whole of Central Europe, they can be roughly divided into four groups according to geological substrate, geography and elevation (each with one or several alliances):
  1. (I)
    At colline to submontane elevations:
    1. (a)

      therophyte-rich rock debris and rock ledge communities on silicate rock (alliance Sedo albi-Veronicion dillenii = Arabidopsion thalianae);

       
    2. (b)

      therophyte-rich rock debris and rock ledge communities on carbonate rock and gypsum (alliance Alysso alyssoidis-Sedion);

       
    3. (c)

      grassy rock debris and rock ledge communities mainly on silicate rock (alliance Seslerio-Festucion pallentis).

       
     
  2. (II)
    At montane to alpine elevations:
    1. (d)

      alpine succulent communities on silicate rock (alliance Sedo-Scleranthion biennis).

       
     

Thermophilic silicate rock debris communities, including the Gageo bohemicae-Veronicetum dillenii, are sparsely vegetated communities dominated by short-lived spring therophytes such as Veronica dillenii, V. verna or Arabidopsis thaliana alongside Sedum species and bryophytes and lichens.

Thermophilic carbonate rock debris communities colonise limestone rock ledges and the gaps in calcareous dry grasslands on shallow soils. These form not only on calcium carbonate substrates, but also on dolomite, gypsum, tephrite, melaphyre and other basic bedrocks. They are characterised by numerous submediterranean therophytes (e.g. Cerastium pumilum, Arabis auriculata and Hornungia petraea) and limestone bryophytes (Korneck 1978). Examples include the Cerastietum pumili e.g. in central Germany, the Alysso-Sedetum albi in southern Germany and the Saxifrago-Poetum compressae, whereby the latter is also a widespread pioneer community on gravel-covered roofs and around train stations (Bornkamm 1961c).

Festuca pallens-dominated communities grow on steep cliffs and ledges mainly of silicate, but also of limestone and gypsum rock. Their vegetation is sparse with many xeromorphic grasses and forbs, including succulents, dwarf therophytes and cryptogams, and has more of the character of a permanent community than the pioneer communities of the Sedo albi-Veronicion dillenii and the Alysso alyssoidis-Sedion mentioned above. The Festuca pallens-dominated communities can therefore seen as closely related to the calcareous grasslands of the Festuco-Brometea. These communities are mainly found in the continental southeast of Central Europe, and can be grouped into several alliances with numerous communities on silicate, carbonate and serpentine rock (Kolbek 1978; Mucina and Kolbek 1993b). These are present in central and southern Germany only as species-poor communities, e.g. the colourful Diantho gratianopolitani-Festucetum pallentis on limestone, the Thymo-Festucetum pallentis and Teucrio-Festucetum pallentis on central German silicate and carbonate soils and various types of the more acid-tolerant Melica ciliata-dominated grasslands.

Succulent-Rich Communities

In the montane to alpine belts of the silicate areas of the Alps and the silicate central Carpathians, mountain tops support conspicuous communities of cushion succulents dominated by Sempervivum, Sedum and Saxifraga species, together with e.g. Silene rupestris and Scleranthus polycarpos (Braun-Blanquet 1955a).

7.3.3.2 Pioneer Communities and Sandy Dry Grasslands

Community Types

The majority of calcareous dry grasslands within the class of the Festuco-Brometea grow on soils with topsoils rich in silt or clay, and therefore have a relatively high cation exchange capacity. There are, however, also dry grasslands on mainly acidic sandy soils up into the submontane belt, such as on the talsand and meltwater sand plains left by the glacial periods in the north of Central Europe and on the Pleistocene valley-edge inland dunes of the large rivers (Fijalkowski 1966; Krausch 1968; Celinski et al. 1978; Korneck 1978; Jeckel 1984; Pott and Hüppe 1991; Schröder 1989; Fischer 2003; Berg et al. 2004). Such sandy habitats are generally capable of supporting forest and were only transformed into grassland during the forest clearance of the Middle Ages or modern era. These pioneer communities are therefore almost always early stages in a secondary succession towards forest (see Fig. 7.18). In areas where grazing ceased and they were put under conservation protection, they visibly became choked with scrub, as was the case e.g. in the famous Mainz Sand Dunes and the Schwetzinger Hardt in the Upper Rhine valley, or the Morava floodplain in Lower Austria (Philippi 1971; Korneck 1974; Korneck and Pretscher 1984; Mucina and Kolbek 1993b; Fischer 2004). Like the rocky ledge and debris communities in the order Sedo-Scleranthetalia and the calcareous dry grasslands of the class Festuco-Brometea, the pioneer communities and sandy grasslands are characterised by a lack of nutrients and periodic drought.
Fig. 7.18

Initial phase of a subcontinental sandy dry grassland community with Corynephorus canescens, Chondrilla juncea, Filago minima, Silene conica, Scleranthus annua and perennis on acidified glacial moraine sand (Weichselian) on the Baltic Sea island of Hiddensee (eastern Germany)

Six well-differentiated sandy grassland alliances can be distinguished based on the base-richness of the substrate, the structure of the vegetation and the geographic range (the various orders to which they belong are shown in the overview in Chap.  14: no. 5.2):
  1. (I)
    Acid sandy pioneer communities and dry grasslands:
    1. (a)

      the short-lived, therophyte-rich alliance Thero-Airion,

       
    2. (b)

      the dune pioneer alliance Corynephorion canescentis,

       
    3. (c)

      the alliance Armerion elongatae on stabilised sandy soil.

       
     
  2. (II)
    Grasslands of generally base-rich sandy soils:
    1. (d)

      the subcontinental sandy steppe alliance Koelerion glaucae,

       
    2. (e)

      the submediterranean-suboceanic inland dune grassland alliance Sileno conicae-Cerastion semidecandris,

       
    3. (f)

      the oceanic coastal dune grassland alliance Koelerion albescentis.

       
     

The Thero-Airion has sparse, therophyte-rich vegetation and occurs on acidic and disturbed sandy or rocky soil with a suboceanic-submediterranean range. Low-growing, annual grasses (mainly Vulpia and Aira species) and forbs dominate on the soil, which can become very dry in summer. The Airetum praecocis and the Filagini-Vulpietum myuros are particularly widespread in the Pleistocene lowlands of Poland, Germany and the Netherlands on acidic sandy soils.

The Corynephorion canescentis contains pioneer communities on acidic coastal and inland dunes with few vascular plants but large numbers of lichen species (particularly Cladonia) (Biermann 1999; Fischer 2003). The drier and more nutrient-poor the habitat, the larger the diagnostic importance of the lichens. Inland dunes support the Spergulo-Corynephoretum (characterised by Spergula morisonii), whilst coastal grey dunes support the Violo-Corynephoretum (see Sect.  2.3.1.3).

All of these communities are highly light-demanding and disappear in the course of succession towards forest. Sheep herds once kept the aeolian sand mobile on most North Sea islands, at the edge of the Pleistocene river valleys in northern Central Europe, in the Austrian Morava plain and in the Upper Rhine Plain. Today, all inland dunes and almost all older coastal dunes have been stabilised and mostly afforested. Even the areas kept clear by conservation management have become less extreme habitats, as neighbouring woodland areas slow the wind speed, increase the humidity and provide nutrients in the form of blown litter. A further threat is the nutrient deposition from the atmosphere. Almost all the above-mentioned communities are therefore of major concern for nature conservation.

The Armerion elongatae is a low-growing and relatively densely-vegetated community studded in summer with the violet and red flowers of Armeria elongata and Dianthus deltoides, which share several characteristics with the Festuco-Brometea, and within this particularly the Koelerio-Phleion phleoidis. Within this alliance, the Diantho-Armerietum naturally occurs on terrace sands and dunes along the larger rivers of northeastern Germany and northern Poland (Pott 1995). This community becomes species-poorer towards the Elbe and Weser in the west (Walther 1977; Jeckel 1984). It consists mainly of perennial species, and now also colonises secondary habitats, such as the edges of paths, sand extraction sites and fallows in the sandy lower moraine landscapes of the lowlands (Heinken 1990). It used to be managed mainly by sheep and cattle grazing, and has now become very rare due to agricultural intensification, particularly with the use of synthetic fertilisers, and abandonment.

Jeckel (1984) studied the changes in flora and habitat in the river floodplain-dune ecotone in the middle Elbe valley near Dannenberg (Lower Saxony). Here, the vegetation changes from flooded meadows and damp pastures in the floodplain through flower-rich Diantho-Armerietum at the grazed edge of the dunes to extremely dry dune tops with the Corynephorion canescentis. This steep habitat gradient is clearly reflected in the ecological indicator values of the individual species as well as their stand averages, particularly in the case of nitrogen (see Table 7.7).
Table 7.7

Vegetation series on a grazed dune top near the Laascher lake, an oxbow of the Elbe in the Hanoverian Wendland (northern Germany). From Jeckel (1983/1984)a, ecological indicator values from Ellenberg et al. (1992)

    

Vegetation unit

I

II

 

III

 

IV

 

V

VI

 

VII

    

Relevé no.

1

2

4

10

12

14

16

20

21

26

27

C

M

R

N

Species

           

6

8~

6

?

Cnidium dubium

+

          

3

6

7

5

Potentilla reptans

+

          

×

6

×

×

Cardamine pratensis

+

          

3

7~

×

6

Rumex crispus

2

1

         

×

7~

×

7

Ranunculus repens

2

1

         

×

×

×

6

Rumex acetosa

1

1

+

        

3

5

×

6

Dactylis glomerata

1

1

1

        

×

5

×

6

Poa pratensis

2

2

1

+

       

3

5

5

5

Leontodon autumnalis

 

+

+

        

3

6

6

7

Alopecurus pratensis

3

2

  

+

      

3

5

7

7

Lolium perenne

2

2

2

 

1

+

     

×

5

6

6

Trifolium repens

3

3

 

2

1

+

     

7

×~

×

7

Elymus repens

+

+

1

1

1

1

+

    

×

5

×

8

Taraxacum officinale

+

+

1

+

 

+

 

+

   

3

×

×

×

Bromus hordeaceus

 

1

1

 

1

      

3

4

7

3

Lotus corniculatus

+

1

+

1

2

1

1

    

7

3~

7

4

Rumex thyrsiflorus

1

1

2

1

2

2

2

    

×

4

×

5

Achillea millefolium

2

2

1

2

1

1

2

1

1

  

×

×

×

×

Plantago lanceolata

1

2

2

1

2

1

2

1

2

  

3

×

4

4

Agrostis capillaris

1

2

4

4

3

3

3

4

3

  

×

4~

7

3

Galium verum

 

1

1

2

1

2

2

1

2

+

 

3

3

7

3

Ranunculus bulbosus

  

1

1

1

1

1

1

1

  

3

4

6

4

Trifolium dubium

   

+

+

      

3

4

6

3

Trifolium campestre

   

1

1

1

     

3

3

2

2

Rumex acetosella

   

1

1

1

1

1

1

1

 

×

×

×

×

Anthoxanthum odoratum

   

+

2

3

3

2

3

+

 

5

4

6

4

Cerastium arvense

   

2

+

2

2

2

2

+

 

3

3

6

2

Armeria elongata

   

+

2

 

1

1

+

  

5

3

8

3

Eryngium campestre

    

+

  

+

   

4

3

3

2

Dianthus deltoides

    

+

  

1

+

  

×

3

×

2

Pimpinella saxifraga

    

1

1

2

1

1

  

5

4

5

3

Festuca rubra ssp. arenaria

    

1

2

2

2

2

+

 

6

3

7

2

Veronica spicata

    

+

1

1

1

1

+

 

3

4

×

2

Hieracium pilosella

     

1

1

 

2

2

 

4

2

5

1

Sedum rupestre

     

+

 

1

+

2

 

3

4

×

2

Carex caryophyllea

     

+

     

4

3

7

2

Potentilla neumanniana

     

1

1

    

3

4

3

3

Luzula campestris

      

+

+

   

5

4~

7

3

Ononis spinosa

      

1

+

   

×

×

×

3

Poa angustifolia

      

+

 

+

  

4

2

6

1

Sedum sexangulare

      

+

 

+

  

2

×

3

2

Danthonia decumbens

      

+

+

1

  

3

3

2

1

Trifolium arvense

       

+

1

  

5

2

5

2

Artemisia campestris

       

+

1

1

 

4

3

7

2

Dianthus carthusianorum

       

+

 

+

 

6

3

×

2

Festuca trachyphylla

       

+

+

2

 

×

×

3

1

Festuca ovina agg.

       

+

+

+

 

5

2

5

1

Thymus serpyllum

        

+

2

 

3

3

3

2

Jasione montana

       

+

 

+

+

6

3

×

4

Carex praecox

        

+

 

+

3

×

×

4

Euphrasia rostkoviana

         

+

 

4

2

4

1

Scleranthus perennis

         

+

 

3

2

×

1

Sedum acre

         

1

 

2

4?

3

2

Festuca filiformis

         

1

 

7

2

7

2

Silene otites

         

+

 

3

2

2

1

Agrostis stricta

         

1

1

5

2

3

2

Corynephorus canescens

         

2

2

2

3

2

2

Carex arenaria

          

1

2

3

1

1

Teesdalia nudicaulis

          

+

  

mM

 

Mean EIV moisture

5.0

5.0

4.3

3.9

3.8

3.7

3.4

3.4

3.1

2.9

2.7

  

mR

 

Mean EIV soil reaction

6.3

6.1

6.3

5.8

5.9

5.9

5.5

4.6

4.7

4.6

2.2

  

mN

 

Mean EIV nitrogen (N)

5.8

5.6

5.1

4.2

3.8

3.7

3.0

2.7

2.4

2.0

2.0

aFrom Dierschke (1986). Vegetation units: I. Lolio-Cynosuretum, II. L.-C. ranunculetosum bulbosi, III. Diantho-Armerietum trifolietosum, IV. D.-A. sedetosum, V. typical, VI., Cladonia variety, VII. Spergulo-Corynephoretum. Bryophytes and lichens were omitted due to lack of space. The average indicator values are calculated from the occurrence of all species

On average, the EIV-mN sinks from above 5 in the moderately fertilised Lolium pastures to 4 and 3 in the dry grasslands and 2 on the tops of the dunes. The gradient in average moisture values is similar, but less steep, namely from mM 5.0 to 2.7. Within the Diantho-Armerietum, the mN ranges from 4.2 to 2.0, whilst the mM only ranges from 3.9 to 2.9. This clear but relatively small range is confirmed by the changes in soil water content (see Fig. 7.9). These indicator values support the impression that many sandy dry grassland communities are both nutrient-poor and dry (see Sect. 7.6.2). The average acidity shown in Table 7.7 appears to remain relatively homogenous in the grazed and occasionally fertilised grasslands. Its soil is moderately acidic to acidic, whilst the Corynephorion canescentis grows on highly acidic soil. The relatively strong ‘eastern’ character of the Diantho-Armerietum is shown in the continentality values. In addition to many suboceanic species (C3 and 4) and ones that are found across most of Central Europe (Cx), there are several species of steppe-like grasslands, e.g. Rumex thyrsiflorus and Silene otites (C7) and Veronica spicata and Festuca trachyphylla (C6).

Basiphilic dune dry grasslands are particularly frequent on the calcareous grey dunes of the West and East Frisian coast. Major communities of this type are described in Sect.  2.3.1.3. The corresponding inland communities (e.g. the Bromo tectorum-Phleetum arenarii) grow on calcareous drifted sand, e.g. in the northern Upper Rhine region between Mainz and Darmstadt and in the Pannonian region of Austria (Philippi 1973a; Korneck 1974; Mucina and Kolbek 1993b).

The alliance Koelerion glaucae contains communities of low-growing, sparse vegetation on stabilised neutral to basic sandy soils, which are widespread in subcontinental to continental Central Europe. The grasslands of this alliance stretching to the lower Elbe and the Upper Rhine should, according to Korneck (1978) be seen as part of the class of Eastern European sandy steppes (Festucetea vaginatae). They include the Jurineo cyanoides-Koelerietum glaucae of the calcareous drift sands in the northern Upper Rhine region (Volk 1931; Lötschert and Georg 1980), and the Festuco psammophilae-Koelerietum glaucae in northern Germany and Poland (Krausch 1968). The continental sandy grasslands of the class Festucetea vaginatae are characteristic for the Pannonian sand steppes in the Hungarian Plain and also occur in the adjacent lowlands. In Central Europe, they are most species-rich in the Morava floodplain in Lower Austria and southern Slovakia (Niklfeld 1964; Mucina and Kolbek 1993b).

7.4 Adaptations to the Environment

The dry grassland plants of the classes Festuco-Brometea and Koelerio-Corynephoretea are exposed to three main stress factors that limit their productivity and vitality: summer drought, heat and lack of nutrients.

7.4.1 Adaptations to Drought

7.4.1.1 Strategies of Different Plant Life Forms

The higher plant flora of most dry grasslands is dominated by slow-growing hemicryptophytes and chamaephytes, i.e. perennial herbs and grasses and partially lignified plants. Many of these have a scleromorphic structure with small, long-lived leaves with a high density of stomata, a low shoot-root ratio and a large amount of vascular and structural tissue. These characteristics are not only the result of the lack of water, but probably also of the lack of nutrients or a combination of both. If the stands contain many gaps, these can be colonised by therophytes, geophytes and succulents.

It is the dry summer periods that determine which species can survive in these extreme habitats and remain competitive. The plants of Central European calcareous and acid dry grasslands can be divided into about ten main strategy types to cope with water and nutrient scarcity, based on the growth form of their shoots and roots and their lifespan:
  1. 1.

    perennial graminoids with shallow to moderately deep roots (e.g. Carex flacca, Festuca ovina agg., Corynephorus canescens);

     
  2. 2.

    perennial graminoids with deep roots (e.g. Stipa species, Brachypodium pinnatum, Bromus erectus, Koeleria pyramidata);

     
  3. 3.

    perennial meso- to xerophytic forbs with shallow to moderately deep roots (e.g. Hieracium pilosella, Jasione montana, Geranium sanguineum);

     
  4. 4.

    perennial meso- to xerophytic forbs with deep (tap) roots (e.g. Scabiosa and Astragalus species, Stachys recta, Artemisia campestris, Securigera varia, Euphorbia cyparissias);

     
  5. 5.

    scleromorphic dwarf shrubs with deep roots (e.g. Thymus, Teucrium and Helianthemum species and Fumana procumbens);

     
  6. 6.

    pluvio-therophytes (e.g. Cerastium, Veronica, Aira, Erophila, Scleranthus and Vulpia species);

     
  7. 7.

    geophytes with storage rhizomes, taproots, bulbs or tubers (e.g. Allium, Muscari, Cirsium and Orchis species and Scilla autumnalis);

     
  8. 8.

    succulents (e.g. Sedum and Sempervivum species);

     
  9. 9.

    hemiparasites and parasites (e.g. Rhinanthus and Orobanche species);

     
  10. 10.

    poikilohydric cryptogams (bryophytes, lichens and algae).

     

The plants of these ten types differ in their drought tolerance, i.e. their ability to endure and recover from a certain degree of tissue desiccation, as well as their water turnover, i.e. the absorption, storage and loss of water.

Perennial, weakly scleromorphic to mesomorphic plants with moderately deep to shallow roots (no. 1 and 3) have a relatively high photosynthesis rate and develop faster in damp years than the remaining groups, as they invest less in their roots. In dry summers, however, their above-ground parts may die back. They survive dry periods in a minimally active or dormant state, either as rhizomes or seeds. Many character and differential species of the Mesobromion are examples of this drought escape strategy. Bromus erectus follows an intermediate strategy to the following group, just as many other species do not entirely fit in one category and may change their behaviour depending on the water supply.

Perennial and mainly scleromorphic, deep-rooted plants (no. 2, 4 and 5) survive dry periods in an active state and restrict their transpiration and thereby also their photosynthesis only after long periods of drought. However, severe drought reduces their flowering, and sometimes they also lose leaves and parts of their shoots. Such scleromorphic species dominate almost all Xerobromion communities and particularly the steppe-like grasslands of the Festucetalia valesiacae. As most of these plants use large quantities of assimilate for the growth of their root system and their vascular and structural tissues, their above-ground growth is relatively slow, so that they are outcompeted by the species of the previous group in habitats with better water supplies.

Pluvio-therophytes (no. 6) are mesomorphic or at most weakly scleromorphic, but usually complete their yearly growth and development before the start of the summer droughts (spring or winter therophytes that escape drought). They grow poorly when there is little rainfall in spring or in very dry habitats, and grow well with a better water supply and will remain green for longer in summer. The few summer ephemerals that germinate and become active later in the year behave similarly to the species of no. 1 and 3. Succulents (no. 8) remain active for longer thanks to their stored water, but grow more slowly than most of the species in the other groups. They therefore only occur in areas where the other species often suffer from water shortages and are therefore not highly competitive, particularly on rock ledges (see Sect. 7.3.3.1).

Among these ten types, only some of the bryophytes, lichens and algae (no. 10) have a high protoplasmatic desiccation tolerance and thus are drought-tolerant in the strict sense (Chaves et al. 2003). As poikilohydric plants, they can survive summer drought periods in an anhydrobiotic state with a low water content (<10 %) in their tissues. They only assimilate during wetter periods, particularly in early spring and in autumn and mild winters. In the remaining periods, growth rates remain low in mosses such as the Hypnum species (Kilbertus 1970). Many bryophytes and lichens are only active after rainfall, or in periods of high air humidity or dewfall (see Chap. 10 in Vol. I).

If dry grassland plants are sown in moist garden soil, then almost all species have high germination rates between 50 and 100 % (Krause 1950). Only a few (e.g. Oxytropis pilosa, Adonis vernalis, Poa badensis, Anthericum liliago) had low germination rates, and none of the species was xerophilic, i.e. growing less well in damp soil than in their natural, periodically dry habitat. However, the high water and nitrogen supply in the experiment of Krause (1950) did reduce the degree of scleromorphy of the tissue in most species, so that the plants lost turgor earlier when exposed to water shortage. Transplantation experiments by Smetánková (1959) with Carex humilis showed that the typical growth form of this species in dry grasslands is mainly formed in response to the specific habitat conditions and is not inherited. Watered plants grew better than drought-exposed ones, and assimilation and flowering were lowest in the driest rocky habitats.

7.4.1.2 Water Regimes of Dry Grassland Plants

Only few studies have compared the water status of different plant life forms in Central European dry grasslands, referring either to shoot water potential, the relative water deficit of the tissue, or alternatively to the absolute water content of the tissue. Differences in the amount of water per unit dry mass reflect differences in the degree of leaf hygromorphy or scleromorphy of xerophytes. The water content of fully turgid leaves of 18 species in dry grasslands in eastern Czech Republic differed by a factor of ten without even considering the succulents (see Fig. 7.19). The Stipa species had particularly low water contents (< 50 % on a dry weight or 33 % on a fresh weight basis), the lamina of which are rich in sclerenchyma (Rychnovská and Úlehlová 1975).
Fig. 7.19

Water content per dry mass of turgid leaves of various dry grassland plant species in July 1959 in eastern Slovakia after a dry spring (Modified from Rychnovská and Úlehlová 1975)

The symplastic water content of leaf tissue in dry grassland plants typically decreases with soil desiccation towards mid-summer, but at the same time, the relative water content threshold at which the plant will suffer lethal drought damage also sinks. Osmotic adjustment (the active enrichment of osmolytes in the cell, OA) and elastic adjustment help to maintain tissue hydration and turgor in periods of water shortage (Bartlett et al. 2012; Sanders and Arndt 2012), partly avoiding the negative consequences of tissue dehydration. The accumulation of protective proteins such as LEA proteins and detoxification of reactive oxygen species increase the protoplasmic drought tolerance upon tissue dehydration (Claeys and Inzé 2013). The xerophytes of dry grasslands usually combine elements of drought tolerance and avoidance strategies, or partly escape drought. However, it is not well known how the 10 physiognomic plant strategy types defined above differ in their drought adaptation at the physiological and molecular levels.

Dry grassland plants in temperate Central Europe can reach leaf water potentials during periods of drought that are just as low as those of xerophytes in Mediterranean and subtropical, summer-dry climates. Leuschner (1989), for example, measured minimum leaf water potentials in Bromus erectus in a Xerobrometum in the Kaiserstuhl (south-western Germany) of −5.2 MPa, which is similar to ψleaf values in Mediterranean macchia plants (−4 to −8 MPa) and desert shrubs (−5 to −8(to −16) MPa, Larcher 2001). However, when comparing species and plant functional types, water potential minima and maxima (predawn potentials) are not very informative unless they are related to the critical threshold values. Daily and seasonal changes in turgor are more useful measures, as well as quantitative information on drought impairment of photosynthesis and growth. Comparative measurements have, however, only been made for a few Central European dry grassland plants (see Fig. 7.23).

Changes in leaf water potential over the course of a day can demonstrate the drought stress exposure of a single plant species in different habitats, if the measurements are carried out in different locations in parallel. On the Badberg in the Kaiserstuhl, for example, Bromus erectus individuals in a south-facing Xerobrometum showed much lower daily leaf water potential minima (−5.20 MPa) than those in a neighbouring northeast-facing Mesobrometum (−3.05 MPa). In addition, the predawn potentials were much lower, because the Xerobrometum plants were less able to resaturate during the night (Leuschner 1989; see Fig. 7.20: top). On the Bollenberg in Alsace, however, Bromus erectus and Teucrium chamaedrys showed more similar diurnal leaf water potential fluctuations in Mesobrometum and Xerobrometum communities, because both stands were south-facing. The Mesobrometum and Xerobrometum plants of a species differed more with respect to leaf water content per dry mass than leaf water potential, which points to differences in leaf anatomy and cell wall elasticity with more rigid sclerophyllous cell walls in the latter (see Fig. 7.20: centre and bottom).
Fig. 7.20

Changes in xylem pressure potential (ψ in MPa) and leaf water content (θ, in % dry weight) of Bromus erectus and Teucrium chamaedrys in a Xerobrometum and a Mesobrometum over the course of a cloudless day (Modified from Leuschner 1989). Vertical lines represent the standard deviation. The measurements were carried out on the Badberg hill in the central Kaiserstuhl (top; southwestern Germany) and the Bollenberg hill near Roufach in Alsace (France). Both communities are on the southeast-facing slope, the Xerobrometum on a slipping and in places eroded, weakly rubified Protorendzina from stony loam, the Mesobrometum on a mull Rendzina formed from solid bedrock under forest cover.

In the Kaiserstuhl, the Xerobrometum is on a south-facing slope and the Mesobrometum on a north-facing slope. In the Mesobrometum, the water supply of both species is higher. This is reflected in the leaf water content and also the xylem pressure potential on slopes of different aspects

The osmotic potentials at turgor loss pointtlp) and at full hydration (π0) have been found to be closely correlated with water availability (Bartlett et al. 2012), but the concentration of osmotic substances in leaf tissue varies largely with plant functional type. Figure 7.21 shows that the Central European calcareous and sandy dry grassland plants accumulate more osmolytes (particularly carbohydrates and nitrogen compounds) in their protoplasts than plants of other habitats. Decreasing moisture in the habitat leads to a decrease in the average π and an increase in its range, both in terms of the seasonal variation in osmolyte content and the differences between species in the same habitat. Dry grasslands contain sclerophyllous dwarf shrubs such as Teucrium in close proximity to herbs as Hippocrepis, both with low π values, and geophytes such as Anthericum with relatively high (less negative) osmotic potentials (Müller-Stoll 1935; Reichhoff 1980b; see Fig. 7.22). Succulents have particularly low concentrations of osmolytes, and thus high potentials.
Fig. 7.21

Osmotic potential ranges in the liquid extract of shoots of various plant life forms and from different habitats in Central Europe. Dry grassland plants have not only the lowest potentials, but also the highest variability among the species (From Walter (1960) in Lösch (2001); with permission of Quelle & Meyer Verlag, Wiebelsheim)

Fig. 7.22

Seasonal changes in the osmotic potential of the liquid extracts of shoots of six plant species in dry grasslands in Kraichgau (northern Bavaria). The strong decline in π of dwarf shrubs and Brachypodium pinnatum suggests active osmotic adjustment (Modified from Müller-Stoll 1935)

Lösch and Franz (1974) and Müller-Stoll (1935) found a decrease in the osmotic potential in the shoots of Hippocrepis comosa (horseshoe vetch) in dry grasslands in Mainfranken of over 2.5 MPa towards mid-summer, pointing to considerable active osmotic adjustment that serves to maintain turgor and water absorption under drought condition (see Fig. 7.22). They observed particularly strong seasonal OA in dwarf shrubs, forbs and grasses with deep roots. In contrast, much of the π fluctuations observed over the course of the day by Lösch and Franz (1974) probably reflect passive concentration of solutes due to midday water loss.

7.4.1.3 Stomatal Conductance and Reduction of Transpiration

Among the numerous species and plant life forms that co-occur in dry grasslands, there are considerable differences in maximum stomatal conductance (g s ) and stomatal regulation strategies under drought. Porometer measurements by Kuhn (1984) on six herb species in a Swiss Mesobrometum showed maximum stomatal conductances of 300 mmol H2O m−2 s−1 during a dry period. The maximum daily gs values of the species differed by up to threefold, and the daily gs means by up to twenty-fold. There were also very large differences between different individuals of a species, even if these were grown under identical conditions in a climate chamber.

During wetter periods, the differences in stomatal conductance between the six species were lower: the maximum gs values varied between 150 and 500 mmol m−2 s−1, with extreme values of 900 mmol m−2 s−1, whereby the difference between the species was only 2–3 fold. Larcher (2001) gives typical maximum leaf conductances of steppe grasses of 350 mmol m−2 s−1. Meso- and xeromorphic forbs (such as Salvia pratensis) and deep-rooted grasses (e.g. Stipa pennata, Florineth 1974) have particularly high maximum leaf conductances, whilst many dwarf shrubs and particularly succulents have low gs (cf. Florineth 1974; Rychnovská 1975). The latter achieve maximum stomatal conductances of only 100–120 mmol m−2 s−1 (Larcher 2001).

Some dry grassland plants can therefore have very high transpiration rates when exposed to high vapour pressure deficits on south-facing slopes and the soil is moist. Under these conditions, Rychnovská and Úlehlová (1975) found that xerophytes transpire per hour four times the amount of water stored in the leaf tissue. On a hot south-southeast-facing slope in a dry grassland in the western Czech Republic, Teucrium chamaedrys plants under these conditions transpired twice as much as plants of the same species on the north-facing slope (Slavíková et al. 1983). The leaf age also influences the water turnover: for example, Mesobrometum plants generally have a higher stomatal conductance after mowing than before (Kuhn 1984). This is probably because there is relatively little transpiring leaf area compared to the root mass after mowing, and because new, physiologically more active leaves are formed.

Central European dry grassland plants also show clear signs of a midday depression in stomatal conductance, as is characteristic for many Mediterranean xerophytes (Tenhunen et al. 1987). The strongest midday depression was found by Kuhn (1984) in the mesophytic species Dactylis glomerata and Trifolium pratense, which severely and permanently reduced their conductance during summer drought periods in Mesobrometum communities in northern Switzerland. The same occurred in the character species Bromus erectus, which showed a highly sensitive regulation of stomatal conductance to air humidity. Salvia pratensis, in contrast, does not show a midday depression. These comparative measurements indicate that Central European dry grassland plants differ considerably in their sensitivity of stomata to air humidity.

The midday depression in stomatal conductance is a short-term reaction of the plant to prevent water loss in dry, warm air through a feedforward loop. Severe drought leads to a permanent leaf conductance reduction and finally to morphological drought-stress responses. Many dry grassland grasses roll their leaves during very dry conditions. Stipa pennata subsp. eriocaulis reduces its transpiration by almost half through leaf rolling (Florineth 1974). Festuca valesiaca, which occurs in the same habitat, always has rolled leaves, so does not morphologically regulate its transpiration. Many grassland plants reduce their transpiring leaf area through wilting of older leaves under drought and reduce the growth of new foliage. Plant hormones play a crucial role in this response, which is controlled by abscisic acid (ABA), the canonical stress hormone. Dry soil stimulates the synthesis of ABA in the roots, which not only causes stomatal closure (Schroeder et al. 2001) but can also directly inhibit shoot growth (Tardieu et al. 2010) and aquaporin expression and opening, thereby controlling hydraulic conductance in the flow path (Wilkinson and Davies 2010; Claeys and Inzé 2013).

Dry soils have a much lower hydraulic conductivity, reducing water uptake by the plant roots and thus reducing transpiration rates (Veen et al. 1992). The drier the soil, the more plants take up water from lower soil layers (Ehlers 1996). Plants growing on rocky substrates use mainly the water stored in rock fissures, into which their roots are able to penetrate (Eschbamer et al. 1983). Although there are few accurate measurements, it is unlikely that the root water uptake of dry grassland plants ceases at a soil water potential of −1.5 MPa, i.e. the accepted ‘permanent wilting point’ for cultivated plants at which water uptake practically ceases. Presumably there are large differences in wilting point between dry grassland species, as well as between different seasons. Transpiration measurements from steppe grasslands in the southern Alps show that Stipa species and Festuca valesiaca still take up water at water potentials of −2.5 to −3 MPa (Florineth 1974). Slavíková (1965) even found indication of water uptake at up to −5.3 MPa in dry grassland plants using a refractometric compensation method.

Under strong drought conditions, water loss only occurs over the cuticle. However, not all dry grassland plants have good cuticular protection against transpiration. Desiccation experiments by Bornkamm (1958; see Table 7.8) show the size of cuticular transpiration in ten dry grassland species. The mesomorphic species mostly found in the Mesobromion lose water more rapidly than those that are common in the Xerobromion. Helictotrichon pratense, for example, reaches its sub-lethal tissue water deficit after a transpiration time around eight times longer than Brachypodium pinnatum (cf. Fenner in Smith 1980).
Table 7.8

The length of time taken to dry out for cut leaves of different species of a semi-dry grassland (Gentianello-Koelerietum) near Göttingen. From Bornkamm (1958)a

Herbs and legumes

50 % loss (h)b

Sublethal deficit (h)c

Grasses

50 % loss (h)b

Sublethal deficit (h)b

Scabiosa columbaria

11.3

29.5

Avenochloa pratensis

7.9

20.8

Anthyllis vulneraria

15.3

20.1

Festuca valesiaca

5.2

16.0

Knautia arvensis

8.1

18.9

Bromus erectus

4.9

8.5

Hieracium pilosella

4.9

13.6

Brachypodium pinnatum

1.6

2.4

Pimpinella saxifraga

5.9

11.4

   

Lotus corniculatus

4.6

7.8

   

aAverage of 16 measurements, carried out at around 20 °C and at 0.09 cm3 h−1 Piche-evaporation

bHours until leaves had lost 50 % of their fresh weight

cHours until c. 10 % of leaf area is damaged due to desiccation

There are numerous attempts in the literature to find a classification for strategies of stomatal regulation and plant water use. Examples include the concepts of ‘stable and labile water use types’ from Stocker (1937), ‘conformers and regulators’ from Hickman (1970), ‘water savers and spenders’ from Passioura (1982), and the ‘isohydric/anisohydric’ dichotomy (e.g. Tardieu and Simmonneau 1998). Bornkamm (1958) grouped the species of a semi-dry grassland in northwestern Germany into those with high or low maximum transpiration, and also distinguished plants with high or low daily variation in relative water deficit. High water use levels during the growing season appear to be only possible when a plant has a deep root system, a sensitive stomatal regulation during dry periods, or enters dormancy during drought. Relatively high rates of water consumption may also only be possible if a plant with high transpiration is surrounded by other ‘water saving’ plants. None of the classifications mentioned above has proven itself to be universally applicable in the field (Kuhn 1984), because plant water consumption and leaf water status are largely dependent on the vertical extension of the roots and therefore the depth of the soil, as well as the specific water regime of the microhabitat of each individual plant. Moreover, it appears that the classification of plants into one of the mentioned categories is often not useful, as it is more likely that there is a continuum, rather than dichotomy, of water use strategies (Klein 2014).

7.4.1.4 Drought-Stress Effects on Photosynthesis and Growth

The photosynthetic capacity of dry grassland plants in Central Europe has been relatively little studied. Despite high summer temperatures, Stipa species have optimum temperatures for photosynthesis of 20 °C or less, and only S. pulcherrima has a maximum of 25 °C (Gloser 1967). This study showed that the dry grassland grasses Stipa pulcherrima and Melica transsilvanica have much lower light-saturated photosynthetic rates (Amax: 4.6 and 4.9 μmol CO2 m−2 s−1) than many damp meadow grasses (Alopecurus pratensis: 10.7, Phalaris arundinacea: 10.8 μmol m−2 s−1), linked to low maximum stomatal conductances (52–82 compared to 189–260 mmol H2O m−2 s−1). The relatively low leaf N content (1.4–1.7 % N) may potentially limit the photosynthetic capacity and stomatal conductance of these steppe grasses (cf. Schulze et al. 1994).

Summer drought stress and associated leaf water deficits have been found to greatly reduce the photosynthetic rate of dry grassland grasses in dry periods (Gloser 1967). Experimentally determined response curves oflight-saturated photosynthesis towater deficits in the leaf tissue showed that Bromus erectus was less sensitive to foliar water loss than five Stipa species, in which the photosynthetic rate halved at water losses of only 15–25 % of water saturation (see Fig. 7.23). Similar water losses were seen in all six studied grass species in the field, or were exceeded. Accordingly, summer drought frequently should reduce the CO2 assimilation of Central European dry grassland species. The causes of this reduction (stomatal closure or biochemical limitation) have in most cases not been identified for the different dry grassland species.
Fig. 7.23

Decline in light-saturated net photosynthesis rate (relative values) of five Stipa species and Bromus erectus under increasing foliar relative water deficit from 0 to 50 %. Bromus was less sensitive in its photosynthetic rate to tissue water loss than Stipa, although Bromus showed an earlier decline in leaf growth under drought conditions in the field than Stipa pulcherrima (cf. Fig. 7.24) (Modified from Gloser 1967)

It is likely that many of the Crassulaceae native to Central Europe, including the Sedum and Sempervivum species, use the CAM pathway of photosynthesis. However, Schuber and Kluge (1979, 1981) observed that Sedum acre and S. mite usually assimilate CO2 through the C3 pathway, and it is only during dry periods that the CAM pathway is utilized to a larger extent. Even under drought, CAM is only responsible for a small proportion of the carbon gain of the plants.

Above-Ground Growth Response

The extreme limitation of growth by summer drought in many dry grassland plants is shown by the measurements of Rychnovská and Úlehlová (1975) in steppe grasslands in the eastern Czech Republic. Bromus erectus showed leaf growth only in April and May at soil moistures over 10 %, whilst the leaves of the more drought-tolerant Stipa pulcherrima continued growing until the end of June (see Fig. 7.24). In contrast to the plants of damp meadows that have reduced, but still considerable, growth rates in mid and late summer (see Fig.  8.41), both of the dry grassland species largely ceased growing in around July. The leaves produced early in May were, as expected, less xeromorphic than those produced in the dry June or July (Rychnovská and Úlehlová 1975). Drought reduced not only the length of the leaf, but also the height of the stalks and the size of the inflorescence, as shown by a comparison of Bromus erectus in habitats with different soil moisture levels (see Table 7.9).
Fig. 7.24

Seasonal changes in leaf length and daily leaf growth (bars) in Stipa pulcherrima and Bromus erectus in response to changing soil moisture in a dry grassland in eastern Czech Republic in summer 1963. Whilst Stipa still showed leaf growth in a dry June and July, Bromus had reduced its leaf growth to almost zero by the end of May (Modified from Rychnovská and Úlehlová 1975)

Table 7.9

The vitality of Bromus erectus in habitats of different moisture levels

Soil moisture

Community

Lengtha of

Stalks (cm)

Panicle (mm)

Spikelet (mm)

Very dry

Rock ledge comm.

15–36–47

51–62–72

10–15–21

Dry

Xerobrometum

42–68–81

83–90–97

17–21–24

Fairly dry

Mesobrometum

102–113–121

105–125–140

20–23–25

From data in Quantin (1960)

aEach measured in more than 1000 individuals; bold = average

Most plants form smaller and thicker leaves under dry conditions, and thereby reduce their water loss over the long-term. Teucrium chamaedrys (wall germander) was found to have larger and thinner leaves, as well as a larger total leaf area, under higher soil water contents in a dry grassland on the Raná hill in northwestern Czech Republic (Slavíková et al. 1983; see Fig. 7.25). This strong relationship is mainly caused by the high sensitivity of cell elongation to tissue water loss, which is reduced even at small turgor reductions (Mullet and Whitsitt 1996; see Fig. 7.26). In many plants, leaf and shoot growth is also more sensitive to water shortage than root growth (Spollen et al. 1993), so that the area of water-absorbing fine roots increases relative to the leaf area with increasing drought. Root tips grow at much lower water potentials than shoot meristems, for example in maize at −1.4 MPa (Sharp et al. 1988). Taking only the relatively low shoot growth into account therefore gives a skewed impression of the reduction in productivity of dry grassland plants under drought conditions.
Fig. 7.25

The decrease in leaf size, specific leaf area and total leaf area of Teucrium chamaedrys plants in the dry grasslands around the mountain of Oblík in northwestern Czech Republic with decreasing soil water content (Modified from Slaviková 1983)

Fig. 7.26

Leaf water potential, stomatal conductance, photosynthetic rate and relative leaf growth rate of lupin in response to soil water content and soil matric potential (average of 4 and 10 plants, respectively) (Modified from Henson et al. 1989)

Below-Ground Growth Response

Plants that are capable of continuing root growth even in dry topsoil have a large competitive advantage in dry grassland habitats (Barth 1978). Root growth in dry conditions is supported by the synthesis of low molecular weight osmolytes (e.g. proline) in the root cells. This is stimulated by abscisic acid released under drought conditions, and helps to maintain turgor in the root meristems (Turner and Jones 1980, Ober and Sharp 1994). Roots formed in dry soils differ anatomically from those in moist soils (Bohner et al. 2003). The roots of Bromus erectus formed in dry soils have a multi-layered pericycle, a large central cylinder of vascular tissue, a thick-walled cortical parenchyma and a relatively large number of root hairs. In damp or wet soils, the cortex forms large intercellular lacunae and the cortex and endodermis adopt a meso- to helomorphic character (see Fig. 7.27).
Fig. 7.27

Cross-section through roots of the same age of Bromus erectus raised in soils of differing (but constant) moisture levels (From studies and drafts of Rehder in Ellenberg 1963)

In normal, relatively dry soil (upper section), the cortical parenchyma has thickened walls and no lacunae. In damp soil with little aeration (the two sections on the right), all roots form relatively large air-filled lacunae (shaded). In water-saturated soil, the cortical lacunae become larger (top left), whilst the roots in water have almost no intercellular spaces (bottom left). The number of root hairs decreases the less well aerated the soil is

The majority of Central European dry grassland plants have much larger root/shoot ratios than plants of wetter habitats (see Fig. 7.28, see Sect. 7.6.1.2). The dry grassland xerophytes adapt to drought not only with a large root system, but also with a better water transport in their shoots and roots. This is visible in the relative sapwood area (Huber value), i.e. the ratio of sapwood area to dependent leaf area. Among the dry grassland species, Müller-Stoll (1935) found that dwarf shrubs such as Thymus pulegioides or Teucrium montanum have particularly large relative sapwood areas (> 0.010 mm2 m−2), which have to supply their evergreen leaves with water even under extremely high insolation. Xerophytic forbs such as Potentilla verna, Hieracium pilosella and Euphorbia cyparissias have an intermediate strategy with fewer conducting elements (0.007–0.010 mm2 m−2), whilst fringe plants of semi-shade conditions such as Geranium sanguineum, Helleborus foetidus and Securigera varia have relatively low sapwood to leaf area ratios (0.002–0.007 mm2 m−2, Müller-Stoll 1935).
Fig. 7.28

Seasonal changes in the below-ground to above-ground biomass ratio in a Mesobrometum and an Arrhenatheretum. (Modified from Pilát 1969). After mowing and at the end of winter, the root:shoot ratio exceeds 15, because a large proportion of the above-ground plant parts have died off or been removed. The ratio is always lower in the Arrhenatheretum

Drought periods lead to the death of parts of the fine root system even in drought-tolerant species, as is visible in the high root necromasses in the soil. The plant prevents water losses to the soil by shedding root hairs, root cortex and often also the youngest root tips and fine lateral roots. Drought-tolerant plants are capable of rapidly regrowing the lost fine roots and extending new roots in the direction of moister soil horizons (Drew 1987). A reduction in water availability can even lead to an absolute increase in the fine root surface area by stimulating branching and root hair formation (Kutschera 1960). However, in most cases, only the root:shoot ratio is increased, i.e. more leaf than root area is lost. High potential root growth rates under variable soil moisture conditions and rapid root regeneration after drought damage are doubtless important prerequisites for successful colonisation of dry habitats.

The poikilohydric bryophytes and lichens use not only a tolerance strategy but also partly escape drought. They are able to photosynthesise even at low temperatures (and in some cases below freezing), so can grow even in winter or in the cool of the morning, when air humidity is high and there is water from dew available (Lösch et al. 1997; see Chap. 10 in Vol. I). Therophytes also escape drought stress (see Sect. 7.4.1.1 no. 6).

7.4.2 Adaptations to Nutrient Shortage

It is somewhat surprising that dry grassland habitats are linked with low nutrient levels, given that their geological substrates, such as limestone, dolomite or loess, are usually nutrient-rich. This shortage of nutrients is due to the following main factors:
  1. 1.

    Erosion, which is often increased in sloping areas by human activity, reduces the soil depth and therefore also the available root zone often to only a few centimetres (see Table 7.3). Loss of humus through erosion has a particularly large impact on the nutrient status of the soil.

     
  2. 2.

    Summer drought slows the mineralising activity of the soil microorganisms. The release of mineral nitrogen and other nutrients can therefore fall to very low levels during periods of drought (see Sect. 7.6.2).

     
  3. 3.

    Calcareous soils often have a poor availability particularly of phosphorus, but also of iron and zinc. P is bound as the mostly insoluble calcium phosphate and is also present as HPO42−, which is more difficult to take up than the H2PO4 formed in acid soils. Fe occurs at pH 6–7 almost only as Fe3+, which is difficult for plants to absorb. The lack of humus on steep slopes enhances the scarcity of P, Fe and Zn.

     
  4. 4.

    Dry soils can reduce nutrient uptake by the plants due to both physiological and hydraulic limitations. Low soil water contents reduce the transport of water and nutrients to the roots, particularly for slowly diffusing elements such as phosphorus. Misra and Tyler (1999) found lower phosphorus contents in the soil liquid phase of dry soils than in moist soils, which was to some extent reflected in the plant P contents. Dying fine roots and reduced nutrient uptake over the membranes of the root cells due to drought could also compound the nutrient shortage.

     
  5. 5.

    Grazing and mowing, which were once common on dry grasslands, have further decreased the nutrient levels, especially as these ‘wasteland’ areas were not fertilised.

     
Comparative studies on the nutrient contents in the biomass of grassland communities at different moisture levels have shown that calcareous dry grasslands can be limited in productivity by nitrogen as well as by phosphorus. The P contents in the biomass of dry grasslands were several times lower, and the N/P ratios much higher than in wet grassland communities such as Calthion and Magnocaricion communities (Kausch and Leuschner, unpublished; see Fig. 7.29). Fertilisation experiments in calcareous dry grasslands have shown a growth limitation by both N and P (Bobbink 1991; Willems et al. 1993; Wilson et al. 1995). In Mesobrometum communities in northern Switzerland, the P limitation was even greater than the N limitation (Köhler et al. 2001). This result supports the experience of farmers, i.e. that the productivity of grasslands with good supply of potassium increases just as much after fertilisation with P as it does after NPK fertilisation, especially as P particularly promotes the growth of legumes, which then fix N2 (Klapp 1971). Potassium is probably the most limiting nutrient in quartz sands poor in mica (Oomes and Mooi 1985), because non-exchangeable K is released only slowly from between the layers of silicate in these substrates and is quickly leached (Scheffer and Schachtschabel 2010). Growth limitation by P, N, K and also Fe was found in English calcareous grasslands (Smith 1980).
Fig. 7.29

N/P ratio in the above-ground and below-ground biomass of various grassland communities in central and southwestern Germany that differ in their water supply (dry: Xerobrometum, semi-dry: Mesobrometum, damp: Arrhenatheretum, moist: Calthion, wet: Magnocaricion). For each community, four unfertilised stands were studied and the biomass of graminoids and forbs was analysed separately (From Kausch and Leuschner, unpublished)

Like all of the studied grasses, and indeed almost all higher plants, the dry grassland species Bromus and Brachypodium are promoted by nitrogen fertilisation up to an optimum that is far higher than natural levels. The degree to which growth rates are increased is, however, lower in dry grassland species than in more nutrient-demanding grasses such as Arrhenatherum and Dactylis (Sharifi 1983). High growth potential of grasses is linked to two key characteristics: (1) the ability to form roots able to take up large amounts of N, P and other nutrients when nutrient supply is high, and (2) the ability to form delicate, large leaves with high specific leaf area and high N contents per mass that have a high photosynthetic capacity but a short life-span.

Species of nutrient-poor habitats do not have a high nutrient-uptake efficiency under high nutrient supply, so that they are less able to profit from fertilisation and are quickly outcompeted by other more nutrient-demanding species. However, these plants show a more economical use of nutrients in growth than the nutrient-demanding, fast-growing species. This can be seen particularly in the low amounts of N, P and other nutrients used to form new leaf and shoot biomass, i.e. a greater nutrient use efficiency (NUE, here defined as g biomass formed per g nutrients contained in the plant, Ingestad 1979). The leaves of these species contain less nitrogen and have longer life-spans, meaning also that they are thicker and have a smaller relative surface area and have a lower photosynthetic capacity per unit leaf mass. These differences are shown by a comparison of the leaves of the fast-growing meadow grass Alopecurus pratensis and of the slow-growing calcareous dry grassland grass Koeleria pyramidata.

Plants of nutrient-poor habitats also increase their nutrient use efficiency in that they have lower nutrient losses through litter production. They achieve this by efficient nutrient recovery from the dying leaves, which are transported to storage organs (roots, rhizomes or leaf meristems at the base of the plant) from which the nutrients will be remobilised for new growth in spring. Measurements of plant internal N relocation prior to leaf abscission show annual flows of between 10 and 60 % of the annual N demand of the plant (Hirose 1971; Werner 1983; Bobbink et al. 1989). Nutrient shortages increase the relative importance of this relocation, although it was found that the N relocation of species of nutrient-poor habitats (e.g. Brachypodium pinnatum and Molinia caerulea) was not more effective than that of species of nutrient-rich habitats (e.g. Solidago, Elymus and Epilobium species, Werner 1983).

Our understanding of the role of mycorrhizae in the growth and vitality of dry grassland plants is still incomplete. Almost all have mycorrhizal symbionts, whether arbuscular or orchid. Streitwolf-Engel et al. (1997) found that the productivity of Prunella species depended on the mycorrhizal species infecting their roots. The diversity of mycorrhizae can therefore influence the growth of different species and thereby probably the species composition of the vegetation (van der Heijden et al. 1998). Experiments in calcareous dry grasslands have shown that a single application of a large amount of fertiliser is sufficient to change a Bromus-dominated dry grassland into a Arrhenatherum-dominated productive meadow (Ellenberg 1952a; Zoller 1954). These mesophytic species outcompete the less nutrient-demanding Bromus erectus by using the additional nitrogen to form a larger leaf area and thus more productive swards. Corynephorus canescens also becomes overgrown by Calamagrostis epigejos or other productive grasses after the addition of N (Weigelt 2001). Thus it is not only the dry conditions, but also often the lack of nutrients in the soil that is the main cause of the dominance of Bromus or Corynephorus in dry grasslands.

Because nutrient-rich grasslands have a higher above-ground productivity than Mesobromion grasslands, even though the water supply is the same, the results of fertilisation experiments have often been interpreted to show that nitrogen can partly replace the demand for water. N fertilisation indeed decreases the water needed per unit produced biomass (the transpiration coefficient of productivity), thereby compensating for the potentially higher water losses from a larger leaf area (see Sect. 7.5.3). It has not, however, been conclusively shown whether a deeper or larger root system produced by the productive meadow grasses leads to a better exploitation of the soil water when a dry grassland is fertilised.

7.4.3 Adaptation to Heat Stress

South-facing dry grasslands are exposed to the highest temperatures of all Central European habitats, with maximum temperatures at the vegetation surface of 50 °C and at the soil surface of 70 °C and above. In steppic grasslands in the southern Alps, Larcher et al. (1989) determined heat resistance limits in mid-summer at the base of the shoots of the C3 grasses Stipa capillata, S. eriocaulis and Carex humilis of 65 °C, and in the C4 grass Bothriochloa ischaemum even almost 70 °C. C3, but not C4, grasses can therefore be expected to experience heat limitation of photosynthesis in this environment. Germination and growth experiments with the two dry grassland species Festuca valesiaca and F. rupicola showed that the seedlings were more sensitive to heat than the mature plants, and that this factor played an important role in the successful colonisation of extreme habitats (Hroudová-Pucelíková 1972). The dwarf shrub Teucrium chamaedrys showed heat-induced reduction of the photosynthetic activity at 45 °C in a dry grassland in northern Bavaria (Burghardt et al. 2008).

7.4.4 Adaptations to Basic and Acidic Soils

Dry grasslands can be found across the whole pH spectrum of soils occurring in Central Europe from very acidic to alkaline. In no other vegetation type is this gradient so clearly expressed in the species composition as in dry grasslands, whereby calcareous grasslands are dominated by calcicole species (or ecotypes) and acid grasslands by calcifuge plants.

Highly calcifuge (or acidophilic) species such as Deschampsia flexuosa, Danthonia decumbens and Silene viscaria mainly occur in acidic grasslands and grow poorly in calcareous grasslands. They are generally tolerant of high Al3+ and Mn2+ concentrations as well as of other potentially toxic elements in the soil solution (Foy and Fleming 1978; Runge 1983b). These plants suffer from lime-induced chlorosis on calcareous soil, caused by iron or phosphorus deficiency together with sensitivity to HCO3 (Kinzel 1982). In contrast to calcicole species, they are largely unable to release organic acids to dissolve P and Fe (cf. Sect. 4.5.2.6 in Vol. I). The availability of P and Fe, as well as of Mn and Zn, is low in calcareous soils, as they are mostly bound in compounds with low solubility.

Fertilisation experiments by Tyler and Olsson (1993) showed that the calcifuge species Silene viscaria is mainly limited by low P in calcareous soil, and not by high Ca2+ or HCO3 concentrations. Tyler (1994) therefore proposed that many calcifuge species are mainly excluded from calcareous grasslands by their inability to mobilise P and Fe. In contrast, Fühner (2005) found that lack of iron was more important than lack of phosphorus in various communities of the Gentiano-Koelerietum. The calcicole plants of calcareous grasslands are superior to the calcifuge plants in their mobilisation of P and Fe, because they excrete much larger amounts of oxalic and citric acids that occupy sites that bind phosphate (ligand exchange), thereby solubilising P sorbed to soil particles. The former is very effective at solubising phosphate in calcareous soils, while citric acid forms complexes with Fe ions, so that both become available for plant uptake (Ström et al. 1994). It is mostly the basiphilic grasses that mobilise iron through the excretion of citric acid or other organic chelators (phytometallophores). Dicots such as some Fabaceae and Brassicaceae instead release protons from their roots, which reduce iron to its more mobile Fe2+ form. Calcicole plants also have efficient mechanisms to mobilise and take up other nutrients that are less available in calcareous soils (e.g. Mn or Zn) (Marschner 1993). They counteract the high concentrations of calcium in their cells, which is unavoidable on calcareous soils, with the precipitation of calcium oxalate in the vacuole. On acid soils, however, calcicole plants are damaged by high Al3+, Fe2+ and Mn2+ concentrations, and some species are also sensitive to NH4+ (cf. Sect.  6.4.2).

The contrast between calcicole and calcifuge species is, even more than in other ecophysiological phenomena, the result of multiple factors (Kinzel 1982). These include particularly the type of mycorrhiza and the form of N nutrition (see Sect. 4.5.2.6 in Volume I and Schlenker 1968; Gigon and Rorison 1972; Falkengren-Grerup 1995a; Lambers et al. 2008).

Like many plants on limestone, the ‘dolomite plants’ also have a very wide amplitude regarding the acidity of the soil when grown in monoculture. Many of them even germinate less well on dolomite than on soils with low magnesium (Boukhris 1967). The opposite is true for several species of calcareous grasslands that do not occur on dolomite, because they cannot tolerate the high Mg concentrations. Cooper and Etherington (1974) and Cooper (1975) showed this experimentally in Helianthemum nummularium, Koeleria pyramidata and Plantago media, whilst Lotus corniculatus was tolerant of dolomite. Typical dolomite plants are probably insensitive to high Mg/Ca ratios mainly because they only need very low levels of Ca, and mechanisms to avoid Mg toxicity are probably only of secondary importance (Kinzel 1982; cf. Sect.  9.4.2). The Mg content of different types of dolomite can vary considerably, although the Ca/Mg ratio is usually above 1.

7.5 Population Biology and Community Ecology

7.5.1 Phenology

In early spring, the sunlight falling on the damp bare earth in the gaps in dry grasslands causes the first therophytes to germinate (e.g. Erophila and Veronica species, Holosteum umbellatum and Saxifraga tridactylites), which quickly mature to produce ripe seeds before disappearing again. These plants are particularly common in the areas that are the driest in summer, because this is where the soil is most open. The majority of sedges and grasses also start to shoot long before the broadleaved forests turn green, and even the later-flowering species start root and shoot growth early. Nevertheless, the dry grasslands are never deep green, but always somewhat yellow, brown or grey. To make up for this, the many different flowers are particularly colourful, changing from glowing yellow to blue, violet, red and white throughout the spring and summer. Fewer plants flower after mid-summer, and their scent is replaced by the smell of the oils released from the leaves of thyme and other herbs in the heat of the sun. As the water supply decreases, the hemicryptophytes relocate nutrients to the bases of their leaves, and the geophytes begin to transfer their carbohydrates and nutrients into their below-ground storage organs. Late summer is therefore a period of relative dormancy for most species. Some plants begin to shoot and flower in autumn, particularly in grazed dry grasslands, in which the flowering period continues for longer than in the mown grasslands with an early summer peak (Zoller 1954). These late flowering plants include e.g. Gentianella germanica and G. ciliata. Winter is hard for all perennial plants. The snow cover is often thin (see Fig. 6.8 in Vol. 1) and can be absent for weeks or months, so does not provide protection from the cold and from evaporative demand. Only a few evergreen plants with deep roots such as Teucrium montanum and Artemisia campestris or succulents such as Sempervivum and Sedum species are well adapted to survive the dry winter period. It should nevertheless be noted that these species are more common in the relatively mild submediterranean climate of the southwest than in the more continental Eastern Europe, where the likelihood of freezing is greater.

7.5.2 Seed Bank, Germination and Dispersal

The majority of calcareous and sandy dry grassland species have only short-lived seeds that often remain viable only for 2–3 years (Bekker et al. 1998; Krolupper and Schwabe-Kratochwil 1998; Poschlod and Bonn 1998; Sendtko 1999; Partzsch 2005). Some species are completely absent from the diaspore bank, which also includes vegetative propagules, e.g. bulbils and cryptogam parts (cf. Bernhardt 1991). The lack of diaspores often affects the course of secondary succession in dry grassland, reducing the success of restoration measures. This is initially quite surprising, given the very high seed densities (in dry grasslands 4000–83,000 m−2, Poschlod et al. 1991; Jentsch and Beyschlag 2003; Partzsch 2005). On the other hand, the diaspore bank often contains several species that are no longer, or were never, present in the vegetation, e.g. Chenopodium album and Pinus sylvestris, whilst typical dry grassland species are absent (Thompson 1986). The diaspore bank therefore never entirely reflects the current vegetation (Thompson and Grime 1979; see Table 7.10).
Table 7.10

The vegetation and seed bank in calcareous dry grassland in Strohgäu (southern Germany). Modified from Poschlod and Jordan (1992)a

 

Current vegetation

Diaspore bank

Plot no.

Soil depth (cm)

1

2

0–2

2–6.5

6.5–13

Brachypodium pinnatum

2

5

   

Avenochloa pratensis

2

    

Koeleria cristata

1

    

Trisetum flavescens

1

    

Agrimonia eupatoria

2

+

   

Securigera varia

+

1

   

Dianthus carthusianorum

1

    

Geranium pratense

1

    

Knautia arvensis

1

    

Sanguisorba minor

1

    

Stachys recta

1

    

Ononis repens

+

    

Cirsium arvense

1

1

   

Cirsium eriophorum

 

r

   

Lamiastrum galeobdolon

r

 

forest plant

Galium aparine

 

r

   

Torilis japonica

 

+

   

Campanula rotundifolia

1

    

Prunus spinosa

1

+

shrub

Prunus avium

+

 

tree

Carex flacca

+

 

48

48

16

Achillea millefolium

2

+

8

  

Euphorbia cyparissias

2

+

24

48

40

Lotus corniculatus

1

+

24

16

8

Origanum vulgare

2

+

264

280

168

Festuca ovina

2

+

16

8

 

Bromus erectus

2

r

8

  

Galium verum

1

1

16

72

8

Centaurea jacea

+

r

  

8

Thymus pulegioides

2

 

24

16

16

Potentilla neumanniana

1

 

64

56

 

Linum catharticum

 

+

8

8

 

Poa pratensis

  

872

336

48

Medicago lupulina

  

240

248

104

Hypericum perforatum

  

8

  

Trifolium repens

    

8

Polygonum aviculare agg.

  

8

  

Chenopodium album

   

16

8

Pimpinella saxifraga

   

8

 

Chenopodium polyspermum

   

8

 

Pinus sylvestris

tree

  

8

Sambucus nigra

shrub

 

16

 

Fewer than half of the species occurring in the grassland were present in the seed bank. Even the dominant grass (Brachypodium) was absent, but reproduces via rhizomes

aTwo facies of an abandoned Mesobromion and numbers of germinable diaspores in the seed bank per m2 in the first 0–13 cm of soil close to sampling plot 1 in 1991

Many diaspores reach depths of over 10 cm through the activity of soil organisms, where they are rarely disturbed e.g. during afforestation. Under a dense spruce forest, in which all of the species of the former Gentiano-Koelerietum had disappeared due to the lack of light, Poschlod and Jordan (1992) found 2832 germinable seeds per m2, i.e. barely less than in the adjacent abandoned calcareous grassland (3256 m−2). Calcareous dry grasslands that have been transformed into other vegetation types can, therefore, be reactivated from the seed bank, although to a lesser extent than is promised by these seed bank investigations (Kollmann and Staub 1995). Whilst many seeds will germinate after the soil has been worked, most of the seedlings soon die, mainly due to drought (Ryser 1990). This is also the case for legumes such as Medicago lupulina, the seeds of which remain viable for long periods of time. In addition, some characteristic species are not contained in the seed bank (see also Bakker et al. 1990).

The germination of some species of calcareous dry grasslands is hindered by bryophyte and lichen layers, such as Carlina vulgaris and Euphrasia officinalis in a Gentiano-Koelerietum in the Netherlands (Keizer et al. 1985). In sandy dry grasslands, Polytrichum piliferum is very successful in suppressing the growth of higher plants. Van Tooren (1990) found that the germination of C. vulgaris and Origanum vulgare was reduced not only by lack of light or drought, but also by the allelopathic effects of mosses. On the other hand, damp moss cushions can provide good germination conditions for some species, as long as they are not too dense. It may also be possible that the germination of sandy dry grassland plants is hindered by biological soil crusts, i.e. often species-rich communities of cyanobacteria, lichens, algae and bryophytes living on the soil surface (Hach et al. 2005); similar processes have also been observed in desert habitats (Prasse 1999; Büdel 2002).

Many vascular plants of dry grasslands are therefore dependent on regular disturbance for their germination. The activity of rabbits and ants in Corynephorus grasslands in northern Bavaria causes 1–15 % of the area to be disturbed at any one time, in which Corynephorus canescens, Spergula morisonii and Teesdalia nudicaulis preferentially germinate (Jentsch et al. 2002). Small-scale disturbances by rabbits, mice, wild boar (Treiber 1997), ants (e.g. Smith 1980) and other animals are therefore an important driver of biodiversity in sandy and calcareous grasslands, as they allow less competitive species to regenerate.

Although the majority of pioneer species in dry grasslands are wind-dispersed, most of the species studied to date have very small dispersal distances of tens of centimetres to a metre (e.g. Teesdalia nudicaulis, Frey et al. 1999, Corynephorus canescens, Jentsch 2001). Some of the species dispersed by ants, such as Carex humilis, are therefore highly restricted to their habitat and can be used as indicators for ancient dry grasslands with long continuity (Krause 1940; Schuhwerk 1990). The long-distance transport of these species between sandy dry grasslands probably occurs mainly via other vectors or extreme events, such as wandering animals and occasional storms. The importance of transhumant shepherding for the species composition of Central European grasslands is discussed in Sect. 3.4.1 in Volume I and Sect.  8.5 in this volume (cf. Fischer et al. 1996; Poschlod et al. 1998). However, Maurer et al. (2003) state that the identity of the dominant species is less dependent on the dispersal strategy and more on their ability to spread clonally.

Despite these limitations, some dry grassland plants can still spread surprisingly quickly on bare soil, such as on rock slides, the edges of paths, burnt areas, fallow fields or in limestone quarries (Smith 1980). Grazing livestock often also play a role in these secondary succession processes. They accelerate the dispersal of endo- or epizoochorous grassland plants, such as Trifolium repens and Plantago media, as well as of those that can attach to fur or hooves. However, most dry grassland species, and especially the pioneers of secondary communities, are anemochorous (wind-dispersed), for example the Stipa and many Asteraceae species. Stipa pulcherrima seeds flew up to 34 m (Sendtko 1999). In a Xerobrometum in the Western Swiss Jura, for example, 51 % of the species were anemochorous, 16 % zoochorous, and the rest had no special dispersal strategy (Quantin 1935). In the shallow soils of dry grasslands on rock ledges (see Table 7.1), which only occur as small islands, wind-dispersed species made up 68 % of the community.

7.5.3 The Influence of Competition on the Species Composition

Many dry grassland species are not hindered by the shade of trees and shrubs in the dry grasslands, but are still at the edge of existence. Their ecological optimum is therefore also their physiological minimum, as was clearly observable in many regions of Central Europe in the extremely dry years of 1947, 1949 and 1952. It was not the fertilised meadows, but the steppe-like dry grasslands that incurred the most damage during the drought periods. Bromus erectus and other graminoids died out over large areas, whilst deep-rooted plants such as Salvia pratensis and Centaurea scabiosa remained green. This did not, however, cause a shift in the species composition of the community, as the damp summer of 1953 allowed Bromus erectus, Koeleria pyramidata and other species that had died back in the previous summers to regenerate surprisingly quickly from old seeds. Bromus therefore has a decisive competitive advantage in its ability to build up a deep root system quickly after germination (Cerletti 1997). Bromus was less affected by the drought on less silty soils (see Fig. 7.30), and regenerated faster.
Fig. 7.30

The effect of an extremely dry year (1947) and an unusually wet year (1948) on the vegetation pattern in a 1 × 1 m sampling plot in a Swiss Mesobrometum salvietosum (Modified from Lüdi and Zoller 1949)

1 = tussock grasses, mostly Bromus erectus, 2 = Picris hieracioides, 3 = Salvia pratensis, 4 = Daucus carota, 5 = Centaurea jacea, 6 = Lotus corniculatus, 7 = Medicago sativa. Mapping was carried out in August, near Villnachern in the canton of Aargau (cf. Fig. 7.35).

Despite the often harsh environment, the presence or absence of dry grassland species in their natural habitat is highly dependent on competitive interactions, as various experiments and comparisons of habitats have shown (Hall 1971; Marti 1994b). Based on their observations in the Rhine, Rhône and Po valleys, Wilczek et al. (1928) came to the conclusion that Bromus erectus is a largely light-demanding and relatively thermophilic plant. It is wrong to consider it to be a particularly xerophilic plant, as in particularly dry regions it is found in the relatively moist areas, following Walter’s ‘law’ of relative constancy of a species’ habitat across environmental gradients. In Valais, it therefore dominates on shady slopes (Braun-Blanquet 1961) and in submediterranean regions it is even restricted to the semi-shade of Buxus bushes. In the climatically less dry Swabian Jura, it also rarely occurs in the driest zones (Kuhn 1937).

Without competitors, i.e. in monoculture, Bromus grows best on moderately damp to damp soils. The optimum groundwater depth in sandy soil for this species is 35 cm, as long as the water is not too lime-poor (Ellenberg 1953; see Fig. 7.31). In this respect, Bromus behaves like Arrhenatherum and Dactylis glomerata, which are highly mesophilic meadow grasses (see Sect.  8.4.2.2). Similar to these species, Bromus will also grow on wet soils. In dry habitats, as mentioned in Sect. 7.4.1.4, Bromus adopts a scleromorphic stem and leaf structure, causing it to grow more slowly. However, it is on these dry soils that it performs the best against the tall meadow grasses. In mixed cultures with Arrhenatherum, Alopecurus pratensis and Poa palustris, Bromus is outcompeted from its physiological optimum, and only makes up a notable proportion of the stand at groundwater levels of 75 cm or more (see Fig. 7.31). It can also dominate on constantly wet soils. In mixed culture on loam soils it has two relative optima for water levels: one close to its physiological minimum water level and one close to its maximum water level (see also Austin 2005).
Fig. 7.31

In monoculture, indicators of dry (like Bromus erectus), mesophilic (like Arrhenatherum) and wet conditions (like Alopecurus pratensis and Poa palustris) have roughly the same optimum moisture level, namely of groundwater at 25–35 cm below the soil surface in sandy soil. The mutual competition means that none of the four grass species produce their highest yield here when in mixed culture (Modified from Ellenberg 1953)

Dotted = physiological optimum (monoculture), hatched = optimum when exposed to interspecific competition (mixed culture). The arrows show the shift in optimum.

Sharifi (1983) found similar results in experiments with Bromus, Arrhenatherum and Alopecurus in loamy soil, although the strong capillary rise meant that the optimal distance of the groundwater from the soil surface was larger

In the competition experiments carried out in loam soils by Sharifi (1983), Alopecurus was the most competitive species, as it filled a similar niche in mixed cultures as it did in monoculture. As in the experiment above, Arrhenatherum was outcompeted by Alopecurus in moist, nutrient-rich soil, whilst Bromus was competitively inferior to both species at high and low water levels. Ködderitzsch, Zielke and Leuschner (unpublished) recorded the productivity of three dry grassland grasses (Bromus erectus, Brachypodium pinnatum and Koeleria pyramidata) in monoculture and mixed culture with five fast-growing damp meadow grasses under varying soil moisture conditions. All of the species had their physiological optimum at moderate soil moisture (15 vol%) and their productivity decreased under both drier and wetter conditions (see Fig. 7.32). Alopecurus was, as expected, particularly competitive. These results confirm that Bromus erectus is outcompeted from its physiological optimum at 15 % moisture into drier areas by the more competitive grasses Alopecurus pratensis and Dactylis. They did not, however, find any difference in the behaviour of Brachypodium pinnatum, Koeleria pyramidata and Alopecurus aequalis in monoculture and mixed culture. Interspecific competition therefore appears to not always result in niche-shifting, but rather it is dependent on how asymmetric the competitive relation is between the partners. Competition in dry grasslands may even be stronger below ground than above ground, as shown by Marti (1994b) and Grubb et al. (1997) for calcareous dry grasslands and by Weigelt (2001) for acid sandy grasslands. These interactions are also largely affected by the arbuscular mycorrhizae of the grasses (Zobel and Moora 1995; West 1996), which connect multiple species and individuals and can lead to interspecific exchange of carbohydrates (Grime et al. 1987).
Fig. 7.32

Shoot mass production in eight grass species of damp to dry habitats in monoculture and mixed culture as a function of soil moisture

The shoot mass yield is shown relative to the highest yield of each species (=100 %). The thick line is the growth in monoculture of each species, and the thin lines show the productivity in the various mixed cultures. There was no clear shift in the ecological optimum (growth in mixed cultures) from the physiological optimum (growth in monoculture) of any of the eight species, although a small shift occurred in the dry grassland species Bromus erectus and Koeleria pyramidata (From Zielke et al., unpublished)

The results of the experiments on the behaviour of Bromus are confirmed by its natural distribution: alongside its ecological optimum in calcareous dry grasslands, the species also sometimes occurs as a swamp plant. In the Upper Rhine Plain and the wetlands west of Lake Constance, for example, Bromus erectus grows together with species of small-sedge or Molinia meadows on periodically very wet and calcareous soils. In the southern Russian steppes, Bromus erectus only occurs in wet depressions, i.e. as a helophyte.

The experiments carried out on Bromus erectus show how important competition is for the creation of certain species combinations, and how careful we must be when interpreting the physiological ‘needs’ of a species based on its natural distribution. Experimental testing has shown Bromus erectus to be a relatively thermophilic species with a moderate response to nitrogen that rarely occurs in highly acidic soils and that is highly palatable to livestock (and thus quickly weakened under grazing). Physiologically, it is neither xerophilic nor does it avoid nitrogen-rich habitats, and cannot be described as particularly calcicole. It is only due to its competitors that it is seen in Central Europe as an indicator for dry, nutrient-poor and calcareous soils.

7.5.4 The Causes of Species Richness in Dry Grasslands

With up to 40 higher plant species per m2, and up to 80 species per 4 m2, calcareous dry grasslands are among the most species-rich habitats in Central Europe. Moreover, certain semi-dry basiphilous grasslands belong to the most species-rich communities worldwide with respect to vascular plant numbers in plots <100 m2 (Wilson et al. 2012). One of the hotspots is located in the White Carpathians (Czech Republic), where species richness is about 25–30 % higher than in similar grasslands of the surroundings (Merunková et al. 2012) and in most other regions of Central Europe. But why are dry grasslands in general so much more diverse than e.g. nutrient-rich meadows, salt marshes or small-sedge communities? The shortage of both water and nutrients probably plays a large role, as resource limitation prevents the domination of the highly competitive meadow species, thus allowing numerous slower-growing, less competitive dry grassland species to coexist (Grime 1981; Keel 1995). This is certainly the case for the lichens, the diversity of which is highest in grasslands with a lower herb layer coverage. In a comparison of dry grasslands in northern Germany, Dengler (2004) found a linear increase in vascular plant diversity with increasing coverage of the stand. This suggests that, in these stands, the productivity increases with species richness, as long as the highly competitive meadow grasses are not present.

Braakhekke and Hooftman (1999) found the simultaneous effect of several growth-limiting nutrients (N, P and K) to be particularly important for the maintenance of species richness in dry grasslands, as species with different nutrient demands can then coexist without intensive competition (‘resource balance hypothesis’ of diversity).

Other authors emphasise the importance of regular intermediate disturbance to maintain species richness (e.g. Weigelt 2001; Jentsch and Beyschlag 2003), which creates regeneration niches for the many less competitive species. Many sandy and some calcareous dry grasslands are indeed characterised by a very high turnover of individuals and species in space and time, i.e. a high mobility of species within the grassland (Sykes et al. 1994). In a detailed study of a dry grassland in southeastern Sweden, an average of three species disappeared from a 10 × 10 cm permanent plot per year, and three new species appeared. Herben et al. (1994), Frey and Hensen (1995) and Klinkhamer et al. (1996) state that this small-scale species turnover, termed the carousel model by van der Maarel and Sykes (1993), is particularly dependent on the ability of the species to spread clonally. The better the nutrient supply, the higher the turnover of individuals (Marti 1994b). Several species can therefore coexist in a small area if they differ in their mobility and life-span.

The assumption of e.g. van der Maarel and Sykes (1993) and Mahdi et al. (1989) that different dry grassland species have similar habitat preferences and therefore overlapping niches is, however, certainly incorrect (Grubb 1986). Water and nutrient levels vary considerably over small scales (Ozinga et al. 1997) as well as over time, as was shown by the study of Gigon and Leutert (1996) in a Mesobrometum in northern Switzerland. This heterogeneity is further increased by the activity of animals (e.g. mice, ants, rabbits and root herbivores), mycorrhizae (Ozinga et al. 1997) and soil pathogens (Blomqvist et al. 2000), as well as the seasonal and interannual variation in weather. The small gaps in vegetation, which are vital for the germination of seeds, are also highly heterogeneous in their conditions (Cerletti 1997). Because the different species have adopted different strategies to use this resource heterogeneity, many different functional niches exist within initially homogeneous appearing dry grasslands (key-keyhole principle).

The high diversity in dry grasslands is also caused by the many hundreds of species that are present in the species pool (Dengler et al. 2014; see Sect. 7.1.1), the adaptations of which mean that gaps are quickly colonised (Zobel et al. 2000; Pärtel 2002). In addition, most semi-dry grasslands, like those in the White Carpathians, are embedded in a mosaic of other habitats such as forest islands and arable land, where spill-over effects may occur. Biotic factors can also contribute to biodiversity. According to van der Heijden (2002), a greater diversity of arbuscular mycorrhizae can promote the plant diversity in grasslands. In macrocosm experiments, stands with a species-rich arbuscular mycorrhizal flora exploited the phosphorus content of the soil more efficiently than stands with few AM species (van der Heijden et al. 1998). The vegetation pattern and the species richness of grassland communities is also influenced by the soil fauna, for example through the effect of nematodes and pathogenic fungi on the vitality of certain species (Olff et al. 2000).

7.6 Productivity and Cycling of Water and Nutrients

7.6.1 Productivity

7.6.1.1 Above-Ground Productivity

Shortage of nutrients and periodic drought reduce plant productivity to such an extent that the grassland communities discussed here have always been noted for their low yields and were seen in much of Central Europe as ‘wasteland’. Mesobrometum communities in western Germany only have 100–120 g m−2 of standing above-ground phytomass (Hundt 1954; Klapp 1965), which is a fraction of the amount produced by damp meadows. Values of 111–163 g m−2 have been measured for Stipa and Festuca grasslands in southeastern Czech Republic, and 178–270 g for Stipa grasslands in the southern Alps (Rychnovská et al. 1972; Florineth 1974). The sparse vegetation of rocky communities and therophyte-rich sandy pioneer communities often has less than 100 g m−2 above-ground biomass. Semi-dry grasslands on north-facing slopes can, however, contain a considerable biomass of bryophytes, which can reach 70–80 g m−2 (During 1990).

The above-ground productivity is correspondingly low. Klapp (1965) recorded hay yields of 81–143 g m−2 year−1 for Mesobromion communities, and Hundt (1954) and Dierschke and Briemle (2002) yields of 180–400 g m−2 year−1. The latter was a stand bordering on a dry Arrhenatheretum, in which around 200–500 g m−2 year−1 was recorded. Damp meadows usually produce at least 500–700 g m−2 year−1 (see Fig.  8.46). Gluch (1973) measured the above-ground net primary productivity in unused and sparsely vegetated Sesleria grasslands and semi-dry grasslands near Jena and found very low values (in g m−2 year−1):

Teucrio-Seslerietum

28

Onobrychido-Brometum

88

Arrhenatheretum salvietosum, Bromus variety

108

However, the above-ground productivity does not give a true picture of the total productivity of the stand, as xeromorphic plants have a larger below-ground biomass than above-ground.

7.6.1.2 Below-Ground Productivity

Dry grasslands have large root masses (see Fig. 7.33). Kmoch (1952) measured 1000–2190 g m−2 in western German calcareous grasslands (on average 1630 g m−2), and Florineth (1974) 1778 and 879 g m−2 for two Stipa steppe grasslands in the southern Alps. These values are similar to the root masses in unfertilised but moist grasslands (see Table  8.15). The root/shoot ratios are usually greater than 5 (4.9–6.6 in the steppe grasslands in the southern Alps, 8.5 in the Festuca sulcata grassland in southeastern Czech Republic, Florineth 1974; Rychnovská et al. 1972) and thus much higher than those of damp meadows (Polomski and Kuhn 1998; Bohner et al. 2003; see Fig. 7.28). Deep-rooting herbs usually reach to 1 or 2 m below the soil surface or even deeper, whilst many herbs in English calcareous dry grasslands only reach 10–20 cm below the surface (Anderson 1927). Around 90 % of the root mass in the Stipa grasslands in the southern Alps was found at 0–40 cm below the surface, and 50 % at 0–10 cm. In sandy grasslands with a low water storage capacity, in contrast, the uppermost 5 cm that frequently dries out may be largely root-free. Volk (1931) found that species with shallow or moderately deep roots tend to flower early, and species with deep roots usually flower in autumn.
Fig. 7.33

(a) The often small and xeromorphic dry grassland plants often have a very large root system, which greatly outweighs the above-ground phytomass. The root/shoot biomass ratio is smaller in more mesic soils. (a) Roots of Festuca rubra, Arrhenatherum elatius, Knautia arvensis and Plantago lanceolata in a moderately fertilised Arrhenatheretum alchemilletosum. (b) Below-ground runners of Teucrium chamaedrys in a Mesobrometum on limestone. The roots are only partially shown (Modified from Schubert 1960). The rhizomes and roots of Brachypodium pinnatum and the roots of Festuca rubra in an Origano-Brachypodietum (Modified from Kotanska 1970)

Pilát (1969) estimated the below-ground productivity of a semi-dry grassland in the western Czech Republic (Mesobrometum stipetosum) to be at least 1010 g m−2 year−1, whilst in a damp Arrhenatheretum it was only 373 g m−2 year−1. Wagner (1972) found similar results for a Mesobrometum near Göttingen. As the finest roots are generally short-lived and often at least partially excluded from the measurements, the true below-ground productivity of these plant communities must be higher than is generally assumed. The dry grasslands are certainly more productive than they appear at first glance, but invest most of their assimilates underground, out of reach of grazing livestock and mowers and invisible to most observers.

7.6.2 Water and Nutrient Cycling

Water Fluxes

The sparse vegetation of dry grasslands has a much lower evapotranspiration rate than damp meadows. Pisek and Cartellieri (1941) determined a water consumption of around 2.6 mm day−1 on a clear summer day, compared to 4.3 mm day−1 in a nutrient-rich meadow shortly before mowing. Extrapolations by Florineth (1974) and Rychnovská and Úlehlová (1975) show that Stipa and Festuca ovina steppe grasslands in the southern Alps and southeastern Czech Republic, respectively, have daily evapotranspiration rates in mid-summer of between 0.7 and 2.6 mm. This corresponds to a cumulative evapotranspiration of the Stipa grasslands in the Vintschgau during the growing season (April to September) of 157–352 mm. In contrast, Corynephorus grasslands in Brandenburg only lost 0.4–0.8 mm day−1 (Berger-Landefeldt and Sukopp 1965).

Nitrogen Mineralisation

In-situ measurements of the net N mineralisation rate show that many dry and semi-dry calcareous grasslands have relatively high levels of soil biological activity, as long as these are not hindered by drought. Xerobromion grasslands on shallow calcareous soils in Alsace and Alysso-Sedion communities on basalt or diabase rock debris in northern Hesse released 20–30 kg mineral N per ha and growing season in the uppermost 5 cm of the soil (range: 11.1–36.4, Riemer 1984; Leuschner 1989), which is equivalent to 15 μg N per g per week. This is just as high as in the topsoil of damp limestone beech forests. These communities can be seen as true dry grasslands, because the productivity is primarily limited by drought. This is shown in the extremely low soil moisture levels and the elevated summer nitrate contents of the soil in the data of Leuschner (1989).

Most calcareous grasslands and sandy grasslands, however, are not only dry, but also nutrient-poor. Despite the deeper soil profile, they have N mineralisation rates that are half or less than those of damp Arrhenatherum meadows (on average around 140 kg N per ha and growing season in Arrhenatherum meadows) (see Fig.  8.50). In a semi-dry grassland near Jena, Reichhoff (1980b) found no relationship between productivity and summer drought, but evidence of nutrient limitation.

Many dry grasslands show relatively low rates of N mineralisation. A Xerobrometum on an unconsolidated, rubified carbonate substrate on the Bollenberg in Alsace released only 11.1 kg N per ha and growing season (0–5 cm), and a neighbouring Mesobrometum only 4.8 kg N (Leuschner 1989). The low mineralisation rate of this Terra rossa-like soil may be due to the relatively low total N content, as well as shortage of P in the haematite-rich soil (Billès et al. 1971). Other semi-dry and dry grasslands on calcareous substrates (e.g. those on Calcaric Regosols on loess) also release only moderate to low amounts of mineral N, such as various Mesobromion communities in northern Switzerland, in southern Lower Saxony and in the Netherlands (2–21 kg N per ha and growing season, Gigon 1968; Dierschke 1974a; van Dam 1990). The total N and P concentrations in the soil of these sites tended to be lower than in the previously-mentioned dry grassland and Alysso-Sedion communities.

In sandy dry grasslands on acid or calcareous inland dunes, annual mineralisation rates of 15–30(−38) kg N ha−1 were recorded in the uppermost 5 cm of the soil (Lache 1976; Jeckel 1984; Storm et al. 1998). Including the lower soil layers in the sandy dry grasslands and calcareous semi-dry grasslands, which doubtless contribute to the N supply in these relatively deep soils, the former is estimated to release around 20–50 kg N, and the latter around 50–70 kg N per ha and growing season. This means that, despite low mass-specific mineralisation rates, the plants in this community can profit from much higher annual N supply rates than the above-mentioned Xerobromion and Alysso-Sedion communities on shallow soils.

Summer drought is an important cause of the low mineralisation rates in many dry grasslands. Riemer (1984) and Leuschner (1989) found a strong relationship between N mineralisation rate and soil moisture, which is expressed in some grasslands in a summer depression in mineralisation, for example in the Alysso-Sedion on basalt in northern Hesse (see Fig. 7.34). Schaffers (2000b) found that maximum mineralisation occurs at soil water potentials of −100 to −550 hPa. The higher nitrate concentrations in the soil after summer drought observed may be due to NH4+ release as a result of heat-related physical processes in the soil (van Schreven 1967), increased bacterial activity after rewetting (Ladd et al. 1977) or lower plant uptake.
Fig. 7.34

Net nitrogen mineralisation rate in the topsoil (per unit area down to 5 cm soil depth or per unit soil mass) and nitrate and ammonium concentrations in the soil in a sparsely vegetated stand of Polytricho-Allietum montani (Alysso-Sedion) on a south-facing basalt slope in northern Hesse (central Germany) in 1983. During a dry July, the mineralisation rate sank to zero, but increased considerably again after rainfall in August (Modified from Riemer 1984)

Not only in calcareous dry grassland soils, but also in the acidic sandy soils of Corynephorus pioneer communities, the majority of mineral nitrogen is released in the form of nitrate. It was only in the lichen-rich Lithic Leptosols of the Corynephoretum cladonietosum that Lache (1974) found very low nitrification rates.

Neither denitrification nor nitrate leaching play particularly large roles in dry grasslands (Bobbink et al. 1998). Even under high atmospheric N deposition such as occurs in the Netherlands, only a few percent of the nitrogen is leached (van Dam 1990), because the plant uptake is very efficient and the soil organisms immobilise the mineral N. The N supply of dry grasslands would be even lower if it weren’t for the various legume species, which fix atmospheric N with their symbiotic root nodule bacteria. These include particularly species of the genera Onobrychis, Hippocrepis, Anthyllis, Lotus, Trifolium, Ornithopus and Astragalus.

7.7 Vegetation Dynamics

7.7.1 Primary Succession on Rock Debris and Opencast Mine Areas

Communities on rocky outcrops are often seen as an initial stage in primary succession, leading to grasslands on stony Rankers or Rendzinas, followed by stone-free Cambisols or Terra fusca on deep scree and loose rock (e.g. Kubiena 1948; see Fig. 7.12). This succession is usually interpreted from a spatial series of different communities and soil conditions, not from direct observations. Moravec (1967, 1979) describes in this way the successional series in rocky grasslands on silicate rock in western Czech Republic, which are driven by the slow weathering of the rock.

In many cases, however, the small-scale mosaic of different communities and soils on rocky slopes is not a time series, but generally stable zones that show a very slow succession (see Fig. 7.11). Clear changes in the vegetation are usually regressive, as grazing animals have damaged the soil, leading to the erosion of the topsoil. Such soil loss or degradation occurs much faster than soil formation by weathering.

The large areas of sand and gravel produced by opencast mines, e.g. in eastern Germany or along the lower Rhine, are also interesting examples of primary succession, as they usually contain very little humus, nutrients and diaspores. Despite unfavourable environmental conditions, the succession can in some cases proceed very rapidly. For example, Wolf (1985) found the first tree seeds 1 year after the deposition of coarse sand in a brown coal mining, and a sparse birch-poplar-goat willow scrub after 10 years. Whilst therophytes such as Senecio viscosus, Vulpia myuros and Aira caryophyllea dominated in the first 3 years, rhizome geophytes and tussock hemicryptophytes dominated in the following 7 years. A well-developed sandy dry grassland did not form, but the herb layer was instead dominated by individual species that spread invasively, including many neophytes, until they were shaded out by the increasing density of the canopy. Dry acidic sands in an open cast mine near Halle were, in contrast quickly colonised by a sandy dry grassland community rich in Corynephorus, and only dominated by woody species in places (Fromm et al. 2002). Berger-Landefeldt and Sukopp (1965) state that the majority of Corynephoretum species are well-adapted pioneer plants. The course of the succession is largely determined by the arrival of diaspores from the surrounding areas and the local climatic and edaphic conditions.

7.7.2 Short- and Mid-term Changes in Dry Grasslands

Detailed studies at small scales of permanent dry grassland vegetation plots show that the species composition and coverage of species change considerably from year to year. Such vegetation fluctuations must be distinguished from the directional changes that characterise succession (see below and Sect. 7.7.3). Fluctuations in species domination can be caused by the population dynamics of the species present in the community, because different species have different life-spans (Gigon 1997). However, the annual variation in weather is probably more important. In the extremely dry year of 1947, Lüdi and Zoller (1949) observed that Bromus, as well as the rarely occurring meadow grasses Arrhenatherum and Dactylis, only covered a quarter of the area that they usually occupied in a Mesobrometum in northwestern Switzerland (see Fig. 7.30). On the other hand, the coverage of the deep-rooted forbs Salvia pratensis and Daucus carota had considerably increased. Stampfli (1995) and Stampfli and Zeiter (2004) followed the changes in frequency of numerous species in relation to the weather in a Mesobrometum in Ticino (southern Switzerland) over 13 years. Notably, the average relative air humidity was the most influential factor, ahead of the amount of rainfall and the temperature. Fluctuations in air humidity reflect temporal changes in evaporation rate and also in soil moisture. Using correlation analyses, they distinguished three types of relationship between the frequency of a species and air humidity: (a) plant species in which their growth was directly correlated to the humidity in the first part of the growing season (April to June) (e.g. Bromus erectus, Carex caryophyllea, Salvia pratensis, Brachypodium pinnatum, Trifolium montanum); (b) drought-tolerant species that are not influenced by water availability, survive dry periods and quickly colonise the gaps left from the effects of the drought (e.g. Helianthemum nummularium, Thymus pulegioides, Scabiosa columbaria, Prunella vulgaris); (c) drought-sensitive species that decline during dry periods and only recover after several years (e.g. Lotus corniculatus, Ajuga reptans, Trifolium pratense, Dactylis glomerata). The last group also includes some short-lived species such as Linum catharticum and Gentianella germanica (Runge 1968). Other species do not show any clear influence of changes in the weather, and their shifts in frequency are best explained by the changing competitive pressure from neighbouring plants (Stampfli 1995). Interannual fluctuations in dry grasslands can also have other causes, such as changes in population sizes and activity of field mice and ants, which influence the plant species composition through their digging, or small changes in mowing times (Dean et al. 1997; Gigon 1997).

Bornkamm (1961a, 2006) followed the changing dominance relationships of the major semi-dry grassland grasses Bromus erectus and Brachypodium pinnatum over 50 years in a Gentiano-Koelerietum near Göttingen (see Fig. 7.35). The coverage of Brachypodium increased at the cost of Bromus if the previous summer had been damp. In contrast, gaps in the vegetation promoted Bromus (Bornkamm 2006). Although there have been no detailed experimental studies, it is likely that interspecific competition plays an important role in this interaction, and particularly root competition (Marti 1994b). The constantly changing dry grassland vegetation shown in Fig. 7.35 and in the study of Runge (1963) is partly caused by the short life-spans of several semi-dry grassland species. Of the 27 Bromus tussocks recorded in 1953 in a semi-dry grassland in Göttingen, only 10 were still present in 1959, and of 18 Brachypodium plants only 2 remained. The other tussocks present had all formed in the previous 6 years. Both species were therefore able to rapidly establish in gaps via seeds or vegetative propagation.
Fig. 7.35

The spread of Bromus erectus and Brachypodium pinnatum on 2 × 2 m sampling plots in a calcareous semi-dry grassland near Göttingen. All species apart from these two grasses were removed from the left half in 1953, whilst all species were removed from the right half (Modified from Bornkamm 1961a)

After 4 years, this disturbance was barely visible any more, although there were clear changes in the dominance relationships of the two grass species from year to year. After a damp spring (1955, 1958), Bromus dominated and suppressed Brachypodium. This was reversed after a dry spring (1959). Both species covered large areas by 1954 even on the site that was bare earth in 1953.

The small-scale mobility of the two grass species is also shown by a disturbance experiment by Bornkamm (1961a) in the same grassland. In 1953, he removed all species apart from Bromus and Brachypodium from a 1 × 2 m area (the left hand panels in Fig. 7.35) and in an adjacent 1 × 2 m area removed all the vegetation. After only 3 years, both grasses had colonised the gaps with vegetation almost as dense as in the areas where they had existed for much longer.

Dry grasslands also show longer-term changes in the vegetation, not caused by weather or management changes (e.g. Dodd et al. 1994; Berlin et al. 2000; Köhler 2001). These changes are probably the result of directional climate change or atmospheric nitrogen deposition. In contrast to the sandy and rocky grasslands, the plant species in calcareous dry grasslands with naturally dense vegetation are almost all perennial, and some can reach very old ages, according to Grubb (1990) up to 100 years. Marti (1994b) found a half-life of 44 years for Salvia cohorts, and of 22 years for Bromus (cf. also Schweingruber and Poschlod 2005). Alongside the clonal plants such as Brachypodium pinnatum, other very long-lived plants include the dwarf shrubs such as Teucrium and Helianthemum (cf. also Zoller and Stäger 1949). These results suggest that all changes in the vegetation of calcareous dry grasslands occur with a certain delay, as old individuals can survive for a long time even under changing environmental conditions (Köhler 2001). Habitat loss and fragmentation may therefore lead to an extinction debt, i.e. extinctions to happen in future because of deterioration in habitat quality and connectivity in the recent past (Cousins 2009). For example, Helm et al. (2006) estimated that the extinction debt for fragmented calcareous grasslands in Estonia was around 40 % of the current species number. However, the empirical evidence for an extinction debt in Central and Northern European dry and wet grassland fragments is equivocal. While several studies (e.g. Rusterholz and Baur 2010; Gustavsson et al. 2007; Krauss et al. 2010) reported results indicating an extinction debt for plant and animal groups, others did not (e.g. Adriaens et al. 2006; Krause et al. 2015).

7.7.3 Secondary Succession in Abandoned Dry Grasslands

Traditional management of semi-dry and dry grasslands has long been abandoned in most of Central Europe, because neither mowing nor grazing of these areas is now economically viable. However, not all areas start to turn into forest after abandonment. Woody plants only rapidly encroach in places where they were already present, e.g. in the case of pasture weeds such as hawthorn, blackthorn and other thorny shrubs or groups of trees that dotted the pasture (Reichhoff and Böhnert 1978; Reichhoff 1985). Treeless dry grasslands, or those that were only colonised by juniper, can remain open for decades. For example, the Xerobrometum on the Rebberge near Hohentwiel (southwestern Germany) remained in much the same state for 30 years after being first studied in 1930 by Braun-Blanquet and colleagues (Müller 1966). Similarly, some semi-dry grasslands on deep and less sloping soils near Göttingen remained treeless for over 60 years, although scrub did establish here and there. Similar situations in the Kaiserstuhl have been reported by Wilmanns (1989b).

Apart from the relatively low seed input, the main barrier to the encroachment of woody plants is the large mass of dead grass leaves that, when compacted by snow, can cover the soil like a dense mat. Woody species require open ground to germinate. The occasional burning of the dead phytomass would therefore facilitate their establishment, as would trampling by grazing livestock. Intermediate disturbance by humans and livestock thus increases the growth of the same woody plants that decline in pastures due to strong grazing pressure and fire.

Species that reproduce via below-ground suckers are particularly successful at establishing in abandoned grasslands, e.g. aspen, privet, blackthorn and some other Prunus species (including P. domestica and P. cerasifera), in some regions Robinia, as well as Cotinus coggygria (smoke tree) as studied by Jakucs (1969) and Schlüter (1993). Such suckers are literal forerunners of the forest (see Fig. 7.36), even in relatively dry habitats. Grazed dry grasslands are generally more affected by scrub encroachment than mown grasslands, as the bare earth produced by trampling improves the germination of woody plants (Beinlich et al. 1995) and mowing removes all woody saplings. Blackthorn and privet bushes can encroach into grassland at rates of 0.25–0.33 m year−1 (Reichhoff 1985; Wilmanns 1975; Keel 1995). The mapping by Stephan and Stephan (1971) of the vegetation changes in the Stolzenburg nature reserve (Eifel, western Germany) clearly shows the succession towards forest. Schreiber (1995) found that all species capable of sucker formation were successful colonisers of abandoned dry grasslands, including species with above-ground runners. Oak and hazel spread into abandoned grasslands mainly via seed dispersal by mice and jays (Kollmann and Schill 1996). Birch and pine seeds carried by the wind also play a role, particularly in sandy dry grasslands (Fischer 2003).
Fig. 7.36

The vegetative spread of shrubs often leads to the succession to forest, particularly on formerly grazed grasslands. Modified based on Jakucs (1969), who studied this ‘sucker-led succession’ in detail in northern Hungary. Cotinus coggygria plays an important role here

Among the forb and grass species, those that can spread below-ground with rhizomes are also successful colonisers. This is the case in calcareous dry grasslands for Brachypodium pinnatum and Bromus erectus (Möseler 1989), as well as Festuca rupicola and Poa angustifolia (Partzsch 2000). Brachypodiumis more efficient at taking up nitrogen from the soil than other dry grassland plants, and therefore profits particularly from atmospheric nitrogen deposition. Mineral N release via mineralisation also increases in abandoned grasslands over time, independently of eutrophication due to N deposition, which also promotes the growth of Brachypodium (Schiefer 1981; Hartmann and Oertli 1984). In contrast, its litter is very poor in nitrogen and phosphorus, as it starts transferring these nutrients into its rhizomes from early summer onwards (Bobbink 1989, 1991). Brachypodium is therefore relatively sensitive to early mowing or grazing. Because it is able to grow in low light conditions, it can spread particularly quickly in scrubby dry grassland. Brachypodium patches can quickly reduce the species density in abandoned dry grasslands. In the orchid-rich semi-dry grasslands near Jena (eastern Germany), the number of species declined after abandonment from 16.8 to 11.7 per 0.25 m2, whilst the above-ground phytomass increased from 55.8 to 76.4 g dry weight and the mass of dead leaves remained roughly the same (67.3–76.9 g dry weight; Reichhoff 1974). Increasing dominance of grasses is also observed in many sandy dry grasslands when management ceases (see also Sect. 7.8.2).

The rapid decline in light-demanding forbs caused by the shading from ever-taller woody plants, or even tall grasses like Brachypodium, was shown by the observations of Tamm (1972) in southern Sweden. Primula veris populations have a half-life of around 50 years in open habitats in the region, but only 6.2 years under light shading, and 2.9 years in deep shade.

The eventual succession to forest is unavoidable in all Central European semi-dry grasslands, although the time taken to reach this climax stage can, depending on the habitat, take several decades or even centuries. It is impossible to predict accurately how long this process will take, and repeat surveys of permanent plots is the only reliable method to measure the progress. This has been carried out e.g. in southwestern Germany by Schreiber, in order to compare different management methods with the undisturbed development of abandoned grassland (see e.g. Schiefer 1981; Neitzke 1991; Schreiber 1993). It soon became obvious that the survey areas were almost all too small to cover the mosaic of vegetation that developed, caused by the establishment of pioneer plants with different dispersal and growth strategies (Schreiber, pers. comm.).

One change that always occurs in dry grasslands bordering on forests or dense scrub is the encroachment of tall herbs from the fringe communities (see Fig.  4.4 and Sect. 11.3.2 in Vol. 1). Once mowing or intensive grazing has ceased, these herbs, which are sensitive to mechanical damage, spread rapidly. They gradually encroach via rhizomes or stolons, and less via seeds, and shade out the low-growing and light-demanding dry grassland plants. Dierschke (2006) distinguished trailing and climbing species from the forest fringe with long above-ground shoots such as Astragalus glycyphyllos and Galium album, large semi-rosette plants (e.g. Agrimonia eupatoria and Inula conyza) and suckering plants (e.g. Hypericum perforatum). Although this process is usually termed encroachment by tall-forb fringe plants, it can also occur quite distant from the forest edge. The development of tall forbs hinders the establishment of trees and shrubs to a certain extent, but cannot prevent it in the long-term.

Kienzle (1984) described several ‘communities’ based on a study of calcareous dry grasslands in the Swiss Jura, which had formed through the encroachment of tall herbs into the Salvio-Mesobrometum described in detail by Zoller (1954). This resulted in e.g. an ‘Origano-Brachypodietum’, noted by Kienzle as a new ‘association’. Such mixtures of species from the orders Origanetalia (see Sect. 11.3.2 in Vol. I) and Brometalia (see Sect. 7.3.2.4 in this volume) should instead be termed ‘stages’ or ‘varieties’, because they do not have any unique character species, and generally continue to develop and change.

7.7.4 The Development of Steppe-Like Grasslands on Abandoned Fields

Prior to the widespread influence of humans, the treeless rocky heaths and some Sesleria scree communities were the last refugia of the light-demanding steppe plants that had spread over much of Europe during the early postglacial (see Frenzel 1968). Since the Neolithic, their populations have spread again considerably. Secondary dry grasslands, i.e. those created from forests, scrub or arable land, often differ from these primary grasslands in the presence or absence of certain species (Krause 1940, and further literature within).

In the colonisation of new areas, the random establishment and concentric spread of different species initially produces patterns, which were studied and analysed by Kershaw (1963). Based on their main cause, he distinguished morphological (caused by species-specific growth patterns), ecological (caused by small-scale variation in habitat conditions) and sociological (caused by interactions between individuals) patterns. These patterns are found mainly in the pioneer stage, and are rare in mature stands.

In the warm and dry climate of eastern Central Europe, abandoned vineyards and arable fields are (or were) colonised by Stipa capillata, Stipa pennata, Festuca valesiaca or other grass species within a few years (Sendtko 1999). In Hungary, for example, Stipa species can rapidly establish in temporarily sparsely vegetated areas, even, or especially, if these areas were formerly ploughed. Although the Stipa tussocks produce the appearance of steppe vegetation, they are not necessarily natural steppe communities. Abandoned calcareous arable fields have become important refugia for some now rare dry grassland species restricted to more ruderal habitats (Quinger et al. 1994).

7.8 Human Influence

7.8.1 Mown and Grazed Dry Grasslands

In previous centuries, all dry and nutrient-poor grasslands were grazed. Around 150–180 years ago, the transition to largely indoor cattle rearing began in southern Germany and northern Switzerland, and since the early twentieth century, sheep grazing has been in decline. As a result, productive grassland suitable for mowing, i.e. mainly damp meadows and semi-dry grasslands, was no longer grazed. In the eastern Swiss Jura, the Swiss Plateau, the Kaiserstuhl and in other parts of the Upper Rhine Plain, not even the Xerobrometum grasslands are grazed, because sheep farming is no longer financially viable in these unproductive areas.

The dry grasslands were traditionally mown once a year, and the semi-dry grasslands sometimes twice. This regular cutting at long intervals promoted relatively tall species, provided that they regenerate rapidly. This was the case e.g. for Bromus erectus, which dominated most stands.

In places were sheep or other livestock still graze, or grazed until recently, e.g. in parts of the Swabian and Franconian Albs and in western and northwestern Germany, Bromus erectus is much rarer (see Fig. 7.37). Bromus does not occur in the names of the common associations there, for example the Gentiano-Koelerietum. However, it soon began to dominate after grazing ceased, e.g. on the abandoned pastures around Göttingen (Bornkamm 2006).
Fig. 7.37

Sheep-grazed semi-dry Mesobromion grassland on limestone (Gentiano-Koelerietum) in a suboceanic climate (Dörnberg, north Hesse, central Germany). Orchids such as Gymnadenia conopsea and other unpalatable species are favoured by extensive grazing while Bromus erectus generally is rare

The main cause for the absence of Bromus erectus from pastures is the selective grazing by livestock (see Sect. 3.2.2 in Vol. I). This grass is highly palatable and is preferentially eaten, until it largely disappears from areas with suboptimal climates. With the decline in grazing, it began to re-establish in the region surrounding the river Weser (Lohmeyer 1953), although initially mainly in stony areas that were rarely visited by livestock. In the Swabian Jura where Bromus erectus had plenty of refugia, it only took a few years for it to dominate in the abandoned semi-dry grasslands (Kuhn 1937).

Some other species that are more frequent in mown grasslands are absent from grazed areas, e.g. the tall orchids such as Himantoglossum hircinum or Orchis militaris. These are eaten by sheep and easily trampled as soon as the tender flower spike develops. Only low-intensity grazing is therefore suitable to maintain orchid-rich grasslands (see Fig. 7.37), as the populations will gradually decline without regeneration from seed. The most orchid-rich semi-dry grasslands in northwestern Germany, e.g. the protected Ith meadows near Koppenbrügge (southern Lower Saxony), were mown, similarly to the famous orchid meadows on the hilly edge of the Upper Rhine Plain, e.g. in the Kaiserstuhl. Mowing is, of course, only one of several factors that promote the development of the light-demanding but trampling-sensitive grassland orchids (see Zimmermann 1979). Many species also need moister conditions than are found in dry grasslands, and are therefore used as character species of the Mesobromion (see Table 7.4). Marls that receive water from springs or surface water early in the year are the main habitats of the Ophrys species in the semi-dry grasslands studied by Studer (1962) on the Irchel (north of Zurich). The Colchico-Mesobrometum described by Zoller (1954) is found on deep but calcareous Cambisols and often merges with marshes, and is described as having the greatest constancy and number of orchids of all grassland associations in the Swiss Jura, with e.g. Orchis mascula, O. militaris, O. ustulata and O. pallens, Dactylorhiza maculata, Gymnadenia conopsea, Plantanthera chlorantha and Listera ovata. A further important habitat factor, at least for submediterranean genera like Ophrys, is sufficient warmth (or lack of very low temperatures in winter?). In the Swiss Jura, they mainly occur e.g. in the Teucrio-Mesobrometum of south-facing slopes as the Ophrys-Globularia punctata subassociation, which does not occur into the upper montane belt. Some orchids are also promoted by the occasional application of fertiliser, as long as this does not also strengthen their competitors. The above-mentioned Ith meadows were also traditionally fertilised.

In contrast to the mown dry grasslands, trailing species, rosette plants and poisonous, unpalatable or spiny plants spread in the grazed nutrient-poor pastures (Gradmann 1950; see Fig. 7.37). Sheep grazing selects for particularly low-growing forms of Pulsatilla vulgaris (pasque flower), for example in the Nördlinger Ries (southern Germany) (Gotthard 1965). Apart from juniper (Juniperus communis), also thistles (such as Carlina acaulis, C. vulgaris and Cirsium acaule), gentians (e.g. Gentiana verna, Gentianella ciliata and G. germanica) and spurges (particularly Euphorbia cyparissias) are the most obvious plants in grazed semi-dry grasslands. These are usually identified as the Gentiano-Koelerietum (see Bornkamm 1960; Dierschke and Knoop 1986). These calcareous dry grasslands are generally species-poorer than mown Mesobrometum communities in similar habitats, at least in southern Germany. T. Müller (1983b) found an average of 30–40 species per (large) plot in grazed stands, compared to 70–90 in mown stands (cf. also Gradmann 1950). In mown fen grassland, species richness in 25 m2 was 17 % higher than in grazed grassland (Stammel et al. 2003). This may be different in subalpine meadows, where change of the traditional mowing regime to extensive grazing did not reduce plot-level species richness (Fischer and Wipf 2002).

Grazing also promotes Brachypodium pinnatum, which sheep only graze when the plants are young. It is to some extent the opposite of the highly palatable Bromus erectus, especially as it can spread vegetatively via its long rhizome, and is not reliant like Bromus on producing ripe seeds for generative reproduction. Early mowing damages Brachypodium stands much more than Bromus stands, as the latter completes its developmental cycle much earlier. Where grazing has ceased, Brachypodium is outcompeted by Bromus, especially if the grassland is regularly mown (Krüsi 1992). As the management of an area of grassland can change over time, several intermediate stages can be seen, e.g. in the Swabian Jura, between the Brachypodium-rich grazed Mesobrometum and the Bromus-rich mown Mesobrometum (Kuhn 1937). These cannot be distinguished phytosociologically. In some places, the floristic difference between mown and grazed dry grasslands is very low, and there is only a structural difference (Wilmanns 1997).

All of the examples presented so far have belonged to the order Brometalia. This is not a coincidence, as the continental dry grasslands have almost never been mown either as stands belonging to the Cirsio-Brachypodion alliance or to the inner alpine alliances. They are therefore generally grazed, especially the communities on shallow soils (see the two middle columns in Table 7.5). If a single species dominates, then it will be sheep fescue (subspecies of Festuca ovina, see Fig. 7.14), which can withstand grazing with its dense tussocks, or Stipa species (see Fig. 7.38), of which only the young shoots are eaten. Species in the genera Teucrium, Thymus, Artemisia, Helianthemum, Asperula, Allium, Anthericum and Sedum also rarely or never show evidence of grazing damage.
Fig. 7.38

Extensively grazed subcontinental dry grassland (Festucion vallesiacae) on a south-facing limestone slope with Stipa pennata and Scorzonera purpurea in northern Thuringia (central-eastern Germany)

Sesleria is also largely avoided by grazing livestock, allowing it to play its role in stabilising steep slopes of scree and rock debris without the risk of it being ripped out by grazing animals (see Fig. 7.1). Species of Molinia (particularly M. arundinacea) play a similar stabilising role on the steep, periodically wet marl slopes of the Jura mountains. In the Swabian Jura, it dominates in a Mesobromion community described as the Tetragonolobo-Mesobrometum by Kuhn (1937) after the legume Tetragonolobus maritimus. This community corresponds to the Tetragonolobo-Molinietum described by Zoller (1954) from the Swiss Jura. Its low fodder value and poor accessibility means that the Sesleria and Molinia slopes are rarely grazed.

The majority of dry and nutrient-poor grasslands of Central Europe therefore depend on livestock, mowing and/or fire for their existence (see Sect. 7.8.4), and these disturbances also determine some of the peculiarities of their species composition. This was shown by a grazing ‘experiment’ that occurred as a result of the explosion and collapse of the rabbit populations in the nutrient-poor grasslands of southern England and parts of Central Europe (see e.g. Myers and Poole 1963; Thomas 1963). In a northwestern German semi-dry grassland, Runge (1963) followed the changes in numbers of species in permanent plots, finding that rabbits preferentially grazed Brachypodium pinnatum, i.e. they behaved differently to the grazing livestock. Kiffe (1989) compared the flora of four East Frisian islands without rabbits and four islands with rabbits. High grazing pressure caused bryophytes and lichens to spread, such as Brachythecium albicans, Ceratodon purpureus, Cladonia chlorophaea agg. and Cornicularia aculeata, as they profited from the additional light. Rubus caesius and Cerastium semidecandrum were also not eaten by the rabbits. In contrast, the sandy dry grasslands produced larger numbers of flowers in the absence of rabbits. Lotus corniculatus, Hypochaeris radicata, Hieracium umbellatum, Jasione montana, Plantago lanceolata and Trifolium arvense increased in number, as well as Anthoxanthum odoratum.

Herbivory by mice and their transportation of seeds does not have a large influence on the species composition of grasslands, but their creation of bare earth does (Leutert 1983; Ryser and Gigon 1985). They thereby promote less competitive species such as therophytes (e.g. Erigeron acris or Erophila verna) or chamaephytes and thus increase the species diversity at small scales (Dean et al. 1997).

7.8.2 Eutrophication

Not only abandonment, but also atmospheric nutrient deposition has caused changes in the species composition of dry grasslands in recent years (Hagen 1996). The most obvious change in dry grasslands affected by eutrophication is often the spread ofBrachypodium pinnatum, which is very efficient at taking up the additional nitrogen and avoiding losses by relocating nutrients to the rhizome before the death of their leaves (Bobbink 1991). The increase in Brachypodium in Dutch mown semi-dry grasslands since the late 1970s is largely attributed to the N deposition, which has risen from 10–15 to 30–35 kg N ha−1 year−1 (Willems et al. 1993). Calamagrostis epigejos takes over this role in sandy dry grasslands and steppe grasslands. Promoted by the N input and increased phosphorus availability which may result from a stimulation of organic matter decomposition, this clonal, productive grass suppresses the typical dry grassland species, including Stipa. Stipa capillata is only able to compete against Calamagrostis at low P levels (Süß et al. 2004). Fischer (2003) described the increasing dominance of grass species with the accumulation of litter in an abandoned sandy dry grassland, caused e.g. by Arrhenatherum elatius, Agrostis capillaris, Alopecurus pratensis and Elymus repens.

Briemle (1997) found that fertilisation rates below 20 kg N, 20 kg P and 32 kg K ha−1 year−1 caused no notable change in the species composition of a Mesobrometum. In Central European calcareous dry grasslands, Bobbink et al. (1996), Neitzke (2001) and Bobbink and Hettelingh (2011) found that 15–25 kg N ha−1 year−1 is the threshold for N deposition (critical load) after which nitrogen-demanding plants begin to spread. Fertilisation experiments by Morecroft et al. (1994) at 35–140 kg N ha−1 year−1 showed that the N mineralisation rate rose together with the N inputs up to values that are usually found in fertile meadows.

Lateral nutrient inputs can also cause the eutrophication of semi-dry grasslands. The edges of a Gentiano-Koelerietum that bordered on a fertilised meadow had up to three times higher N mineralisation rates (38–55 compared to 6–20 kg N ha−1 year−1) than the central area. As a result, fertile meadow species such as Arrhenatherum, Dactylis, Heracleum sphondyleum and semi-shade species (Medicago falcata, Clinopodium vulgare, Origanum vulgare and Viola hirta) established in the grassland. Regular mowing can remove around 17–22 kg N ha−1 year−1 from a grassland, and is an effective measure to combat eutrophication (Bobbink et al. 1998).

The more the grassland is influenced by drought, the less likely it is to be affected by eutrophication. This was shown by vegetation surveys repeated after 60 and 40 years in Xerobrometum communities in the Kaiserstuhl (southwestern Germany), and in dry grasslands on the porphyry hills north of Halle (Saale) (eastern Germany). Despite N deposition since decades, these had experienced relatively small changes in their species composition (Wilmanns 1988; Partzsch 2000), although the number of ruderal species, such as Echium vulgare, Isatis tinctoria and Lactuca serriola, had increased. In suboceanic Festuco-Brometea grasslands in central-western Germany, no decline in species richness was observed in the last 40–70 years, but a marked change in species composition was detected (Diekmann et al. 2014). Certain small, light-demanding specialist species of dry calcareous grasslands such as Acinos arvensis, Arenaria serpyllifolia and Gentianella ciliata declined, while taller, more productive species characteristic of thermophilic fringe communities (e.g. Clinopodium vulgare and Helictotrichon pubescens) increased. The increase in Bromus erectus, but not of the N-demanding Brachypodium pinnatum, is interpreted as an indication that the most important driver of change was not N deposition, but rather decreasing management intensity or a shift from grazing to mowing.

The development in the vegetation and nutrient availability in fertilised grassland after the cessation of fertiliser application and regular mowing was studied by Olff and Bakker (1991) and Olff (1992). Over the course of 45 years of succession, the productivity, nitrogen release and nitrification decreased, and the initial N limitation of growth was replaced by P limitation.

7.8.3 Habitat Fragmentation

Many Central European dry grassland plant species now exist in a few, strongly isolated and often very small populations. Stöcklin et al. (2000) studied 14 threatened Mesobromion species in 44 dry grassland fragments in northern Switzerland, and found that 30 % of the populations had fewer than ten flowering individuals, and 66 % had fewer than 100 individuals. These populations are thus not only isolated in space, but are also genetically isolated and therefore are often affected by genetic drift, as was shown e.g. by Becker (2003) for Astragalus exscapus. Genetic drift leads to genetic impoverishment in small populations, which can lead to their extinction due to their reduced ability to adapt to changes in habitat conditions. Chance effects are a great threat to small populations, including the stochasticity of genetic drift, and random changes in demographics and environmental conditions (Gilpin and Soulé 1986; Menges 1991). Small population sizes also often lead to inbreeding, which reduces the degree of heterozygosity of the individuals and thus increases the likelihood that detrimental recessive mutations are expressed, as was shown e.g. by Ouborg and van Treuren (1995) in Salvia pratensis. This results in lower fitness of the individuals, and therefore a higher extinction risk for the population (Jennersten and Nilsson 1993; Kéry et al. 2000; Leimu et al. 2006). On the other hand, small population sizes can also be beneficial in reducing the likelihood of seed predation (Jennersten and Nilsson 1993). Stöcklin and Fischer (1999) state that dry grassland species with short life-cycles, short-lived seeds and high habitat specificity are at particular risk of extinction when in small populations in fragmented habitats, whilst long-lived species such as Arnica montana or Astragalus exscapus can survive for a surprisingly long time in small remnant populations (Kahmen and Poschlod 2000; Becker 2003).

7.8.4 Conservation and Restoration of Dry Grasslands

In the last 50 years, dry grasslands have declined in area across almost all of Central Europe, through scrub encroachment, the invasion of ruderal species or management intensification (Wolkinger and Plank 1981). In the region of Stuttgart, for example, which includes large parts of the Swabian Jura, the 7000 ha of dry grassland that existed in 1900 had shrunk to 2000 ha by 1980 (Mattern et al. 1980). The maintenance or restoration of dry grasslands requires the use of traditional agricultural management, or at least certain interventions to prevent the establishment of tall herbs or scrub. For nature conservation efforts, it is particularly important that these interventions are the least labour-intensive but the most effective in the long-term (see Briemle et al. 1991).

It is useful in this context to consider the comparison between different management methods over a period of 15 years in a formerly grazed Mesobromion grassland close to Göttingen (Dierschke and Engels 1991; Dierschke 2006). In a Gentiano-Koelerietum on a limestone slope that had not been grazed for 30 years, the once rare Bromus erectus had increased to completely dominate the community. This had occurred mainly because its leaves that had died in autumn formed a thick litter layer around each tussock, which prevented all its competitors, including shrub species, from establishing. In the untreated control plot, six shrub species established during the study period, including blackthorn and hawthorn, which after 15 years had covered on average 13 and 18 % of the area, respectively. In the mulched treatment (i.e. mowing without removal of the biomass), these shrubs had only reached up to 3 %. Even the plots that were mown 1–3 times per year with biomass removal did not remain completely free of shrubs.

The forb and grass species reacted very variably to the treatments. Brachypodium pinnatum was the only species that was at its maximum coverage on the scrubby plots before the experiment began (even if this was only 10 %). Bromus erectus dominated the mulched plots after 15 years with on average 69 % coverage. Its coverage and height decreased the more frequently the parcels were cut, but still covered 39 % after 15 years even under three cuts per year. Characteristic species for forest fringes (see Sect.  11.3.2 in Vol. I), e.g. Medicago falcata and Astragalus glycyphyllos, achieved their highest coverage on the plots that were cut every 2 years or once per year. Trisetum flavescens, the only other tall tussock grass apart from Bromus, also reached its (quite modest) maximum coverage after mowing once per year. Low-growing graminoids, such as Festuca ovina, Briza media, Carex caryophyllea and C. flacca, as well as many low-growing forbs, especially of dry grasslands (e.g. Hieracium pilosella), increased in coverage with increasing mowing frequency. Overall, the authors conclude from the results that mulching is not effective at maintaining dry grasslands.

Regular grazing with sheep has been shown to be an effective management measure to maintain species-rich calcareous dry grasslands; however, it is labour-intensive and requires considerable subsidising (Hampicke 2013). It has the added benefit of transporting diaspores in the wool and hooves of the sheep, including those of species that are rarely or never preserved in the seed bank (Hennekens et al. 1983; Hakes 1988; Wilmanns and Sendtko 1995; Schumacher 2007). This is particularly the case for calcareous nutrient-poor pastures, which are best grazed at the beginning and end of the growing season. However, if the aim is to reduce the dominance of Brachypodium and Bromus in the stand, Rein and Otte (2001) recommend high grazing pressure during the main growing season over several years. Nutrient-poor meadows that were once cut once a year can undergo undesirable changes in the species composition if transferred to grazing management, and scrub often establishes faster, because they can germinate in the bare soil caused by livestock trampling. These stands are better managed with late mowing that is not carried out every year (Wilmanns and Sendtko 1995). The matted litter and biomass layer that forms over several years of abandonment can only be removed by high sheep or goat densities. Fire may also be an option, but it has a considerable impact on the plant and animal communities (see below). Köhler (2001) and Köhler et al. (2005) recorded the effects of a 22-year experiment with different mowing regimes in Mesobrometum communities in northern Switzerland, concluding that the highest phytodiversity is achieved via a mosaic of different management types, in which strips that are mown yearly in mid-summer are interspersed with strips that are only mown every second year.

Scrub encroachment, i.e. the natural succession to forest, must be prevented to preserve dry grassland habitats. Occasional grazing with goats has been shown to be effective in this respect (Eckert 1992), because these preferentially browse woody species, including stripping their bark, which eventually leads to the death of the plant above the point where the bark has been removed. Goats are therefore particularly suited for the initial management of scrubby dry grasslands, especially if they are helped by cutting some of the scrub. Even Brachypodium pinnatum is eaten by goats more than it is by other livestock.

In order to protect rare orchids (e.g. Ophrys sphecodes), Hutchings (1987) states that sheep (and goats) should mainly graze in winter to keep down the vegetation, but should be removed during flowering, seed ripening and dispersal. Tall orchids (such as Himantoglossum hircinum or Orchis militaris) are particularly damaged by heavy grazing. The main cause of the widespread decline in many orchid species in Central European dry grasslands is, however, eutrophication (Dijk et al. 1997). This promotes not only strong competition, but also directly reduces the vitality of some orchid species with very specific nutrient-demands, as has been shown by fertilisation experiments. Potential damage to the mycorrhiza probably only plays a small role.

Fire can also contribute to the heterogeneity of the vegetation. It was traditionally used in many dry grasslands to remove the dry grass at the beginning of the growing season, similarly to the African savannahs. The burning of grassland or heathland was also a method used by shepherds to prevent the spread of woody pasture weeds. Fires that move rapidly over the grassland driven by a light wind barely heat the soil, so do not damage the below-ground parts of the plants. However, uncontrolled fires can have catastrophic effects. Mahn (1966) recorded the effects on a plot in a dry grassland destroyed by fire (see Fig. 7.39). Regeneration was slow to start, because all plants, and most of the seeds, had been destroyed. After 5 years, most of the perennial species of the original grassland had re-established, with the exception of e.g. Calluna vulgaris. In a semi-dry grassland (Onobrychido-Brometum) in the Kaiserstuhl, Zimmermann (1979) observed highly selective effects of repeated fires. Species with below-ground runners and rhizomes (e.g. Brachypodium pinnatum) profited, including tall herbs and shrubs. Fires can also cause unacceptable damage to some animal populations. Species-rich calcareous dry grassland can thus only be maintained long-term by regular mowing or grazing and not by fire (Ryser et al. 1995), although this is possible in some Calluna heaths (see Sect.  6.8.3).
Fig. 7.39

Succession in a sandy dry grassland in central Germany after a severe fire. The distribution of the grass species was recorded within a 1 × 1 m permanent plot in spring and summer 1962 and in spring 1963 and 1964. Modified from Mahn (1966). Festuca cinerea and F. rupicola, as well as other grass species, regenerate relatively quickly

Sparsely-vegetated sandy dry grasslands often suffer from the encroachment of ruderal species, i.e. tall nitrogen-demanding species such as Calamagrostis epigejos, Echium vulgare, Oenothera biennis, Tanacetum vulgare or Erigeron annuus, which can lead to a succession towards Artemisietea communities (Stroh et al. 2002; Fischer 2003). The characteristic sandy grassland species of high conservation value are rarely or never present in the seed bank, and are no longer introduced via seed rain to many stands due to their isolation (Krolupper and Schwabe 1998). The lack of disturbance reduces the germination success of the characteristic psammophyte species (Jentsch and Beyschlag 2003).

According to Zehm (2004), the restoration of sandy dry grasslands cannot be achieved by sheep grazing alone. He recommends mixed grazing, e.g. with sheep and donkeys, together with oversowing with appropriate species (see also Bank et al. 2002; Kirmer et al. 2002). The sheep in the experiments carried out on the calcareous sands near Darmstadt (southwestern Germany) preferentially ate ruderal plants with nitrogen contents over 2 %, and largely ignored the less nitrogen-rich sandy dry grassland species. The animals also provide appropriate disturbance. Cattle are also suited for restoration projects on sandy dry grasslands, but cause damage to the lichen layer (Fischer 2003). The restoration of calcareous dry grasslands on former arable fields with nutrient-rich topsoil first requires the removal of the upper layer of soil (Pfadenhauer and Kiehl 2003). The mineral soil below should then be inoculated with hay containing diaspores (Schiefer 1984; Jeschke and Kiehl 2006) and grazed once in spring and once in autumn with sheep. The area can also be mown if further nutrient removal is needed. Even if the original species composition of a dry grassland is successfully restored, the current rates of atmospheric nitrogen deposition make it difficult to ensure its long-term survival (see Sect. 7.8.2). Furthermore, ancient dry grasslands with long continuity are richer in rare and protected species than younger grasslands, as it often takes a long time for these to reach and establish in new stands (Partzsch et al. 2003; Kiehl and Jeschke 2005).

In conclusion, it should be emphasised once again that the long-term conservation of dry grasslands (as well as the heathlands discussed in Chap.  6) in a state close to the typical condition before atmospheric N deposition is best achieved by extensive grazing, whilst the worst approach is to ‘protect’ the area by excluding any type of disturbance. Michalik (1992) provides good examples for both of these statements in his records and ecological interpretation of the changes in the flora of the Ojców National Park in southern Poland (see also Table 8.19).

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© Springer International Publishing Switzerland 2017

Authors and Affiliations

  • Christoph Leuschner
    • 1
  • Heinz Ellenberg
    • 2
  1. 1.Plant EcologyUniversity of GöttingenGöttingenGermany
  2. 2.Universität GöttingenGöttingenGermany

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