Effects of forest clear-cutting and soil disturbance on the biology of small forest brooks

Abstract

As a part of the larger Nurmes research project, we studied the effects of clear-cutting and soil disturbance (ditching, ploughing, mounding) on the biology of small forest brooks. After these forestry activities, incoming light, temperature, and nutrient content of the water increased significantly. Mean algal biomass peaked during the first summer after clear-cutting. During the first year following soil disturbance, algal biomass initially decreased because of turbidity, but began to rise again as suspended solids in the water decreased in the summer months. Species composition changed, too; after clear-cutting, Cryptomonas and Chlamydomonas species increased their densities (48% of the algal biomass) and following soil disturbance, the species numbers and densities of Conjugatophyceae (52% of the algal biomass) were higher than before. Canonical correspondence analysis suggested that increased nutrient levels and acidity were the main factors behind the changes in the flora following clear-cutting. The most important effect of soil disturbance was the increased water temperature. Effects of forest clear-cutting on algal productivity in the brooks remain evident at least for three years and those of soil disturbance for an even longer period. Restricted clear-cutting, with a protective zone left uncut around the brooks, appears to reduce the effects.