Variation in leaf functional traits of the Korean maple (Acer pseudosieboldianum) along an elevational gradient in a montane forest in Southern Korea
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Plant functional traits have been shown to be useful to understand how and why ecosystems and their components vary across environmental heterogeneity or gradients. This study investigated how plant functional (leaf) traits vary according to an elevation-associated environmental gradient. Environmental gradients (mean annual temperature and precipitation) were quantified, and leaf traits (leaf area, specific leaf area, leaf nitrogen, leaf phosphorus, leaf carbon, and leaf C/N ratio) of the understory woody plant species Acer pseudosieboldianum were examined across an elevational gradient ranging from 600 to 1200 m in a Baegunsan Mountain in Gwangyang-si, Jeollanam-do, South Korea. The results showed that mean annual temperature and precipitation decreased and increased along with elevation, respectively. Leaf area of the plant species decreased slightly with increasing elevation, while specific leaf area did not differ significantly. Leaf nutrients (nitrogen, phosphorus, and carbon concentrations) were higher at high elevations, but leaf C/N ratio decreased with elevation.
KeywordsAltitudinal gradient Environmental filtering Functional traits Leaf nitrogen Specific leaf area
Inductively coupled plasma-optical emission spectrometer
Specific leaf area
Plant “functional traits,” which are individual’s morphological or physiological features relevant to survival, growth, and reproduction, are considered primary drivers of species interaction, community assembly, and species diversity (Roscher et al. 2012; Kunstler et al. 2015). These traits can affect the way plant individual interact with other plants or organisms in other trophic levels, which determine patterns of species interactions in a community (Perez-Hargundegy et al. 2013). It is often considered that community assembly is determined by environmental filtering and the survival of a species through the filtering is largely related to functional characteristics or traits of the species (Hulshof and Swenson 2010).
Also, these functional traits are considered as useful proxies to understand how and why ecosystems and their components vary across environmental heterogeneity or gradients (Garnier and Navas 2011). For instance, key functional traits such as plant size or leaf traits (mass per unit area or leaf stoichiometry) appear to correlate strongly with whole-plant performance, so these traits can be utilized to understand variation in plant function and diversity (Reich et al. 1997; West et al. 1999; Westoby et al. 2002). Individual plasticity in a tolerance against abiotic stresses can greatly influence the responses of a plant community to environmental changes, so functional traits of individual species in a community may act as a useful indicator for assessing and predicting variation in community responses to environmental changes (Mouillet et al. 2010).
Elevational gradients in mountain systems have been occasionally utilized as an experimental setting to test the stress-gradient hypothesis (Schob et al. 2013). It is generally assumed that higher elevations are more stressful for plants and this physical stress at high elevations is thought to be primarily caused by low air temperature, low partial pressure of CO2, and high UV radiation, along with thin soils and low nutrient availability (Korner 2007; Huber et al. 2007). Altitudinal differences in environmental conditions can be utilized as space for time substitutions to predict potential responses of plant traits to future climate change (Pfenningwerth et al. 2017). Performance-related foliar plant traits are thought to be highly sensitive to climatic environments and may co-vary with climatic variation associated with elevational gradient (Pratt and Mooney 2013; Read et al. 2014).
Until recently, considerable amounts of research has been conducted on community structure and diversity of plant species in mountain ecosystems in South Korea (for instance, Park et al. 2003; Choo and Kim 2005; Lee et al. 2013). However, most of them are taxonomy-based and trait-based approach has not been applied. Accordingly, information on variation in plant functional traits across various environmental gradients is not available. Therefore, in the present study, foliar functional traits of plant species Acer pseudosieboldianum were investigated along elevation ranging from 600 to 1200 m to understand how functional traits (leaf traits) vary across elevation-associated environmental gradient.
Materials and method
Study area and focal species
This study was conducted in Baegunsan mountain (35° 6′ N, 127° 37′ E) in Gwangyang-si, Jeollanam-do province, South Korea. Mean annual temperature and yearly precipitation of this area are 13 °C and 1271.5 mm respectively (based on 2017 averaged data from Baegunsan observation station at 515 m in altitude). The study area is characterized by deciduous trees such as Quercus mongolica, Carpinus laxiflora, Quercus variabilis, Quercus serrata, Exochorda serratifolia, and Stewartia pseudocamellia (Kim and Jeong 2015).
Acer pseudosieboldianum is a deciduous small tree or shrub, native to Korea. Although this species is not a dominant tree in the study area, it occupies across the entire elevation gradient in the study area, and most individuals have similar height (1.5~2 m) (their ages were not estimated but may be not mature yet considering their heights); therefore, the variation in leaf traits between individuals resulting from difference in height and light availability may be considered minimal.
Environmental gradient quantification
Mean annual temperature and yearly precipitation across elevation in the study area were estimated using data from the Korea Meteorological Administration. As data specific for each elevation gradient in the study area were not available, data (for between January and December 2017) from 22 weather observation stations located in Jeolla- and Gyeongsang-do province, at similar latitudes but various altitudes, were obtained and temperature and precipitation values were regressed against altitude to determine the relationship.
Plant sampling and leaf trait measurements
Sampling sites were selected for every 100 m in altitude across the elevation gradient (from 700 to 1200 m in altitude) on one west-facing slope of the Sangbong peak (1222.2 m, 35° 6′ N, 127° 37′ E) of the Baegunsan mountain. At each sampling site, 4–10 individuals of the A. pseudosieboldianum tree of similar height (approximately 1.5~2 m) were randomly chosen. The numbers of trees sampled at each sampling site were determined depending on the availability of individuals on site. From each individual, five fully expanded, non-senescent leaves from multiple branches were collected. Leaves showing any noticeable symptoms of herbivore attack were avoided. Sampling was conducted from late May to early June in 2018.
Collected leaves (on a branch) were sealed in a closed plastic bag and taken to the laboratory, then immediately scanned using an electronic scanner, or stored in a fridge (4 °C) for no more than 12 h until scanning. From scanned images, leaf area, LA (mm2), was calculated using ImageJ (Rueden et al. 2017). Scanned leaves were dried in a dry oven at 80 °C for 48 h, and then weighted to obtain leaf dry mass. To calculate specific leaf area, SLA (mm2 mg−1), the ratio between fresh leaf area and leaf dry mass, areas, and dry mass of five leaves of an individual tree were pooled.
For leaf nutrient analysis, dried leaves were powdered using a mortar and pestle and homogenized. For leaf P, ground leaves were digested using HNO3 in a microwave oven and analyzed with an inductively coupled plasma-optical emission spectrometer (ICP-OES) (OPTIMA 730DV, PerkinElmer, USA). For leaf N and C, leaf powder was analyzed using a micro elemental analyzer (Flash 2000, Thermo Fisher Scientific, USA). Total C and N contents were reported as percentage of dry mass (%).
For characterizing soil features of each sampling site, three soil samples were taken at random from the 0–10 cm soil layer of each sampling site (except for the site at the lowest elevation) using a hand shovel and transferred to the laboratory. After homogenization, soil samples were air dried and sieved with 2-mm mesh for further analysis. Total soil N (%) were determined using Kjeldahl analysis. NH3-N and NO3-N were estimated via colorimetric titration after the extraction with 2 M KCl solution and following Kjeldahl distillation. Available P was determined with Lancaster extraction method. Soil organic matter (%) was determined with Walkley-Black titration. Soil pH was measured with a pH meter.
Linear regression was performed between elevation (m a.s.l) and all measured soil and leaf variables (soil: total N, NH3-N, NO3-N, organic matter, pH; leaf: LA, SLA, total N, total C, total P, C/N ratio). For soil available P, quadratic regression was applied as this model explained variation of the data slightly better than linear regression model (p = 0.047 at F-test). All analyses were performed with the R software 3.5.0 (R Development Core Team 2011).
Results and discussion
Leaf nutrient concentrations (nitrogen, phosphorus, carbon) increased significantly along an elevational gradient (Fig. 3c, d, e). Leaf C/N ratio decreased with elevation (Fig. 3f). The results of the present study are consistent with other studies. It has been reported that plants at higher elevations have lower growth rates and higher leaf nutrient content per unit area (Pfennigwerth et al. 2017). In some area, nitrogen deposition increased with elevation, which caused high plant nitrogen concentrations (Fowler et al. 1988). In Morecroft and Woodward (1996), increases in leaf nitrogen and phosphorus concentrations along elevation were observed, and the results were associated with decreases in leaf biomass (mainly carbon). In the present study, carbon concentration increased with elevation but leaf C/N ratio decreased, which may suggest the partial support for the hypothesis in Morecroft and Woodward (1996). It is to note that there are also studies showing the decrease in leaf nitrogen and phosphorus (Soethe et al. 2008; Zhao et al. 2016). The acquisition of nitrogen and phosphorus in plants is known to be influenced by climate, soil conditions, phylogeny, and different physiological growth strategies among species, and the pattern of nitrogen and phosphorus in leaf may reflect variation in climate, soil nutrient, and plant growth form along elevation (Zhao et al. 2016).
In the present study, key leaf functional traits (leaf area, SLA, nutrients) of understory woody plant species Acer pseudosieboldianum varied along elevation, with some traits increased and others decreased. Plant communities usually consist of a variety of plant species with ranges of functional traits; therefore, means of variation in those traits are often utilized to understand functional features of plant communities. The results of the present study suggest that variation in functional traits at within-species level may have significant impacts on overall feature of functional traits, so intraspecific variation of individual plant species needs to be considered.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2015R1C1A1A01053841), (2017-0994).
Availability of data and materials
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KJN designed the study, carried out sampling, laboratory analysis and drafted the manuscript. EJL participated in the design of the study, participated in the laboratory analysis, and performed the statistical analysis. All authors read and approved the final manuscript.
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