REVERSING ACIDIFICATION IN A FOREST ECOSYSTEM - THE GARDSJON COVERED CATCHMENT

The rate at which soil and surface-water acidification can be reversed by reducing atmospheric deposition is of great importance for Fenno-S candia where so many lakes and streams are already acidified. Some mod els predict that recovery of aquatic ecosystems may take over a centur y in severely acidified areas even after major reductions in acid depo sition. Predictions of a protracted recovery from acidification are be ing tested on a severely acidified forest catchment near Lake Gardsjon in SW Sweden. Atmospheric deposition to the entire 6300 m2 catchment is intercepted beneath the tree canopy by a transparent plastic roof t hat was completed in April 1991. Since then, throughfall has been repl aced with water adjusted chemically to approximate ''clean'', pre-indu strial precipitation with a pH of 5.5, and no nitrogen or nonmarine su lfate. Two years after the switch to clean throughfall on the covered catchment, sulfate concentrations in runoff have decreased by some 25% (down 95 mueq L-1), together with a somewhat smaller decrease in alum inum, but the pH has not changed. Given the ten-year time perspective of the experiment, it is still too early to say whether recovery at th is site will be as protracted as some models suggest.