It has been proposed that vegetation and soil changes resulting from changes in land use cause surface-water acidification. The expansion of spruce forest, from natural colonization and from afforestation, has been one of the major changes that has taken place in the vegetation of South Sweden during this century. Spruce has been favoured at the expense of broad-leafed trees by forest management and has been planted on open land, abandoned farm land and in forests. Since the 1920s, the area covered by spruce forest has increased by 2.3 million haf in Gotaland and Svealand, and the frequency of spruce trees in the forests has increased from 11.5 to 33.5% (data from Department of Forest Survey, Swedish University of Agricultural Science, Umea). Gotaland and Svealand comprise the southernmost third of Sweden, the area that suffers most from lake acidification. Spruce colonization alters soil conditions. In several investigations, in which conditions in spruce and birch stands have been compared, significantly lower pH values have been recorded in spruce forest soils. It has been suggested that spruce expansion also leads to lake-water acidification, but this has not been confirmed. Unfortunately, it is difficult to design an investigation aimed at studying the acidification effects of spruce forest under prevailing levels of atmospheric pollution because there are problems in distinguishing between true vegetation-soil effects, effects of air pollution, and combined effects. To assess whether spruce forest
per se
causes lake-water acidification, we have studied the effects of the natural immigration of spruce that reached northern Sweden from the northeast about 3000 years ago, before there was any acid precipitation from fossil-fuel combustion. Palaeoecological studies indicate that spruce colonized land that was occupied by birch, alder and pine.
