ACIDIFICATION OF FOREST TOP SOILS IN 60 YEARS TO THE SOUTHWEST OF HELSINKI

Topsoils of 30 sites on the southern coast of Finland in an area 5 km x 8 km were studied for their pH in 1995. Published old pH measurement s from the same uncultivated forest sites in the middle of 1930 were c ompared to the new values. The highly significant mean +/- SEM decline had been 0.57 +/- 0.11 pH units, with an annual mean decline of 0.01 units. The region is on a southwest protruding isthmus at the Gulf of Finland, where southwest winds prevail about 25% in frequency. Therefo re, the acidification may principally have been caused by long-distanc e pollution, especially since the local number of inhabitants has been relatively low in the region since 1944 for historical reasons. Old p H values correlate highly significantly with the decline. Significantl y different acidifications were not found in different soil types and under different predominant tree species-birch, pine and spruce. Four sites had an increase in pH, which might be attributed to acidificatio n-induced leaching from the till slopes surrounding these sites. (C) 1 997 Elsevier Science B.V.