FOREST-FIRE AND LAKE-WATER ACIDITY IN A NORTHERN SWEDISH BOREAL AREA - HOLOCENE CHANGES IN LAKE-WATER QUALITY AT MAKKASSJON

1 A Holocene sediment profile from a northern Swedish boreal forest la ke was analysed for diatoms, pollen and charcoal. The diatom data were used for inferences of lake-water pH, alkalinity and colour, while th e pollen and charcoal records enabled assessment of catchment vegetati on changes and fire. This palaeoecological study demonstrated long-ter m changes in lake-water acidity as a result of catchment vegetation ch anges and fire. 2 The pollen record showed the transition from a decid uous-dominated forest (Betula and Alnus) 9000-8000 years sp (calibrate d years) to the present coniferous-dominated forest (mainly Pinus) in the catchment of Makkassjon. Local peaks in the steadily decreasing Be tula pollen curve corresponded with increases in the charcoal concentr ation. Picea pollen did not appear until the uppermost half of the sed iment section (c. 2500 sp), and never exceeded 10% relative abundance. 3 The Holocene development in lake-water chemistry, inferred from dia tom analysis, was divided into three periods: a natural acidification, a natural alkalization and a fertilization/liming period. During the long-term natural acidification period, the diatom-inferred pH and alk alinity decreased by c. 1.3 pH units and c.80 mu eq I-L, respectively, while diatom-inferred colour increased from 10 to 40 mg Pt 1(-1) in t he same period. These changes occurred simultaneously with a change fr om deciduous to coniferous forest. Using redundancy analysis (RDA) wit h the tree pollen as predictor variables, the Alnus, Betula or Pinus p ollen record captured 41-85% of the variance in lake-water pH and alka linity. 4 During the period of natural alkalization fire became a domi nant disturbance factor in the catchment. The charcoal record was a si gnificant predictor of the variance in diatom-inferred pH and alkalini ty during the period with frequent fires, as determined by RDA analysi s with a time-restricted Monte Carlo permutation test. The inferred ch anges in pH and colour suggested that the reduction of soil humus play ed a major role in the alkalization of the acid-sensitive lake. 5 The results highlight questions about the important mechanisms for predict ing the long-term effects of fire on surface-water chemistry, which wi ll be of crucial importance if changes in fire regime take place as a consequence of global warming.