Acidic Precipitation Research in Finland

Abstract

Because forests and lakes are such essential elements of the Finnish landscape, it is important to study the ecological effects of acidic precipitation and to control air pollution accordingly so that all major damages can be prevented. A special interagency research program was initiated on acidic precipitation in Finland in 1985.

Finland escapes the highest air pollution levels because the country is rather remotely located on the outskirts of Europe. However, in the winter as high daily averages of sulphur dioxide concentration as 20 to 40 μgX m^-3are observed in forested “background” areas in southern Finland. Total sulphur deposition ranges from 1.5 to 2.0 gX m^-2X yr^-1in southeastern Finland to 0.3 to 0.5 gX m^-2X yr^-1in northwestern Finland. In this way there is a declining gradient of sulphur levels from southeast to northwest. Interestingly, the rain pH is rather high (on the average 5.0) in southeastern Finland due to the high level of alkaline dust deposition in that area. The highest concentrations and depositions of nitrogen compounds are measured in southwestern Finland where, for example, the deposition of oxidized nitrogen is estimated at 0.2 to 0.4 gX m^-2X yr^-1. Nitrogen deposition estimates are, however, more uncertain than those of sulphur deposition.

A declining trend of alkalinity has been observed over the past decades in small lakes as well as in rivers. This has been documented also in areas where land use has not changed and cannot contribute to the trend. Fish and crayfish populations are at risk in small forested lakes. Unlike southern Norway, for example, major declines have thus far not occurred. The Norwegian watersheds have extremely thin soil layers. Finnish soils are also fairly thin but the ecosystems are, nevertheless, somewhat less sensitive to acidification than those in Norway. Yet, many precursor effects to a forthcoming fish population decline have been documented in Finnish aquatic biota.

Forest growth on the average has substantially increased in Finland throughout this century. A particularly strong increase of forest growth has been observed over the past 20 years in southeastern Finland. A major cause is forest management. Cuttings have been less than the forest growth, implying an increase of forest inventory. Additional growth has then been formed upon this extra capital. However, air pollutants may contribute to the increase of forest growth. Nitrogen deposition acts as a forest fertilizer, sulphur deposition mobilizes calcium and potassium reserves, and the increase of atmospheric carbon concentration stimulates photosynthesis.

An increase of forest growth due to air pollutants is viewed as a warning signal. It indicates a rapid change in forest ecosystems, which is not based on sustainable growing conditions, and possibly precedes forest decline. Recent forest damage surveys indicated marked canopy defoliation in southeastern Finland, that is, in the area of high sulphur and nitrogen levels, and a strong increase of forest growth. According to statistical analyses, unfavorable weather conditions or bad forest management are not the only causes of this canopy defoliation. An additional warning signal has been a strong decline of epiphytic lichens observed in southern Finland since the 1950s and obviously associated with sulphur pollution.

Sulphur deposition levels are still regarded as too high in Finland. Gradually nitrogen deposition is also causing nutrient imbalances in ecosystems. Recommendations of actions to control and prevent air pollution damage include development of surface water liming and forest vitality fertilization, improvement of silvicultural practices, and, in particular, implementation of further emission reduction.