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 measurements from the same uncultivated forest sites in the middle of 1930 were compared 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. Therefore, the acidification may principally have been caused by long-distance pollution, especially since the local number of inhabitants has been relatively low in the region since 1944 for historical reasons. Old pH values correlate highly significantly with the decline. Significantly 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 acidification-induced leaching from the till slopes surrounding these sites. (C) 1997 Elsevier Science B.V.