Air pollution impact on Swedish forests-present evidence and future development

Research during the last five years has provided evidence that there is a long-term influence of air pollutants on forest ecosystems also in the southern parts of North Europe. High loads of acidity, sulphur and nitrogen affect soil conditions, trees as well as other organisms.In South and West Sweden changes in soil acidity (pH) have been registered during the last 60 years. The changes not only occur in the humus layer, but also in the lower part of the mineral soil. These latter changes cannot be explained without the action of strong acids originating from anthropogenic air pollution.Losses of elements like magnesium, calcium and potassium occur and phosphorus become less available to plants. An increased demand for plant mutrients is a consequence of the increased fall-out of nitrogen compounds. Nutrient imbalances of trees seem to be the result. Increased sensitivity to frost and drought as well as insects and pathogens is expected.The increased soil acidity and the eutrophication of soils caused by the continued input of nitrogen contributes to changes in plant communities.If we assume that there are no changes in deposition, land-use and management of the forests in SW Sweden, the better forest soils (brown forest soils) will have a continued acidification of humus and mineral soil layers resulting in high levels of aluminium and low levels of calcium. This will create a critical situation for roots and mycorrhiza. Soils that are already acid may not become more acidified, but will still be subject to losses of essential elements.Critical deposition levels or loads of acids (hydrogen ions) and nitrogen rendering no further deterioration of soils and leaching have been set to 0.1–0.2 keq·ha^-1yr^-1 for S Scandinavia (present level 1 keq·ha^-1yr^-1). For nitrogen the critical load is 10–20 kg N ha^-1yr^-1 (present range 10–25 kg ha^-1yr^-1).Contribution from Fourth World Wilderness Congress — Acid Rain Symposium, Denver (Estes Park), Colorado, September 11–18, 1987.