DISTRIBUTION OF FIELD LAYER SPECIES IN SWEDISH DECIDUOUS FORESTS IN 1929-54 AND 1979-88 AS RELATED TO SOIL-PH

Literature data on vascular plant distribution in deciduous forests were compared in 1929-54 and 1979-88. pH in the topsoil was related to the occurrence of 13 common vascular plants. The median of pH (H2O) was 5.2 for the old (n=105) and 4.4 for the new plots (n=132). New plots of pH>5 were scarce and soils of pH<4 were limited to the new plots. Comparison of the pH related frequency curves gave four patterns: (A) a displacement of frequency curves towards lower pHs for the new plots (Stellaria holostea, Milium effusum, Melica uniflora); (B) a widening of the frequency curves into lower pH for the new plots (Viola riviniana/reichenbachiana, Galium odoratum, Lamium galeobdolon, Oxalis acetosella, Maianthemum bifolium); (C) a raised frequency curve for the new plots at all pHs (Stellaria nemorum, Poa nemoralis, Rubus idaeus, Mercurialis perennis); (D) a lowered frequency curve for the new plots at all pHs (Deschampsia flexuosa). It is deduced, that the pH related frequency curves are not stable over long periods as e.g. the studied 40-60 years. The increased amounts of nitrogen and acidifying substances in deposition probably caused an altered soil chemistry and a change in competition among nitrogen favoured and acid tolerant species. As the soil in the new plots was more acidic, the total frequency was higher for many species. However, the frequency of most species decreases with soil pH, and if soil acidification continues without further plant adaptation to low soil pH, there will be a decrease of many species. © 1990 Kluwer Academic Publishers.