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Forest fire and lake-water acidity in a northern Swedish boreal area: Holocene changes in lake-water quality at Makkassjon

Korsman, T., Segerstrom, U.
Journal of ecology 1998 v.86 no.1 pp. 113-124
pollen, boreal forests, lakes, acidity, water quality, Bacillariophyta, alkalinity, color, charcoal, paleoecology, forest fires, watersheds, deciduous forests, coniferous forests, acidification, sediments, soil organic matter, fisheries management, humus, Sweden
1 A Holocene sediment profile from a northern Swedish boreal forest lake was analysed for diatoms, pollen and charcoal. The diatom data were used for inferences of lake-water pH, alkalinity and colour, while the pollen and charcoal records enabled assessment of catchment vegetation changes and fire. This palaeoecological study demonstrated long-term changes in lake-water acidity as a result of catchment vegetation changes and fire. 2 The pollen record showed the transition from a deciduous-dominated forest (Betula and Alnus) 9000-8000 years BP (calibrated years) to the present coniferous-dominated forest (mainly Pinus) in the catchment of Makkassjon. Local peaks in the steadily decreasing Betula pollen curve corresponded with increases in the charcoal concentration. Picea pollen did not appear until the uppermost half of the sediment section (c. 2500 BP), and never exceeded 10% relative abundance. 3 The Holocene development in lake-water chemistry, inferred from diatom 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 alkalinity decreased by c. 1.3 pH units and c. 80 micro eq l-1, respectively, while diatom-inferred colour increased from 10 to 40 mg Pt l-1 in the same period. These changes occurred simultaneously with a change from deciduous to coniferous forest. Using redundancy analysis (RDA) with the tree pollen as predictor variables, the Alnus, Betula or Pinus pollen record captured 41-85% of the variance in lake-water pH and alkalinity. 4 During the period of natural alkalization fire became a dominant disturbance factor in the catchment. The charcoal record was a significant predictor of the variance in diatom-inferred pH and alkalinity during the period with frequent fires, as determined by RDA analysis with a time-restricted Monte Carlo permutation test. The inferred changes in pH and colour suggested that the reduction of soil humus played a major role in the alkalization of the acid-sensitive lake. 5 The results highlight questions about the important mechanisms for predicting the long-term effects of fire on surface-water-chemistry, which will be of crucial importance if changes in fire regime take place as a consequence of global warming.