RESPONSES OF GROUND VEGETATION TO PROLONGED SIMULATED ACID-RAIN IN SUB-ARCTIC PINE-BIRCH FOREST

The effects of prolonged simulated acid rain on percentage cover of gr ound vegetation, and on growth and reproduction of two dominating dwar f shrubs (Empetrum nigrum and Vaccinium vitis-idaea) were examined in a field experiment in the Finnish Subarctic, in an area with low ambie nt levels of sulphur and nitrogen deposition. Acid rain treatments inc luded moderate (pH 3.8) and high (pH 2.9) concentrations of either H2S O4, or HNO3, or a mixture of them, and were compared with irrigated (p H 6) and dry control plots. Long-term application of acid rain caused significant alteration in the cover and composition of ground vegetati on. Effects of acid rain depended on the accompanying anion and on pH. Sub-plots under different canopy tree species differed in responses i ndicating that spatial heterogeneity is important in predicting the ef fect of acidifying pollution on this plant community. In the bottom la yer, acid rain caused significant reduction in cover of the cyanobacte rial lichens Nephroma arcticum and Peltigera spp. Decrease in cover of fruticose lichens, mainly composed of Cladina spp., more likely resul ted from additional watering. In the field layer, acid rain containing moderate concentrations of NO3- caused an increase in cover of gramin oid species. There were only slight alterations in growth and cover of the two dominant evergreen dwarf shrubs, E. nigrum and V. vitis-idaea , indicating that these species are tolerant to acid rain of as low as pH 3. Even some positive responses of dwarf shrubs were observed, dep ending on canopy tree. Application of acid rain of pH 3 to plots under pine trees caused an increase in cover of V. vitis-idaea and, when th e nitric acid only was applied, a short-term increase in the number of new shoots of E. nigrum. In contrast to vegetative growth, reproducti on of the dwarf shrubs was more strongly affected by acid rain, but th is also depended on local conditions and anion composition of acid rai n. On 'pine' plots, rain of pH 3 reduced the number of berries and flo wer buds on terminal current shoot of E. nigrum, however, this was par tially compensated by an increase in berry production at the ramet lev el. Simulated acid rain had mainly negative effects on berry productio n by V. vitis-idaea.