Effects of forest thinning on the streamwater chemistry of two forest watersheds in the Bavarian Alps

Two well-buffered mountain forest watersheds were studied with regard to st reamwater chemistry and the impact of forest thinning. The areas are locate d in the Bavarian Alps near lake Tegernsee, 50 km southwest of Munich. In o rder to get information about the influence of disturbances on the streamwa ter chemistry 40% of the trees were removed from one watershed after two ye ars of monitoring. The trees were removed by crane to avoid soil damage and erosion. The second watershed was used as a reference. The output at the weirs is characterized, in general, by a strong export of silicate, alkali and alkaline earth cations, and bicarbonate. Acidity and nitrogen accumulate in the watersheds. Both areas are subject to a continuo usly increasing load of acidity. The acidity is mainly buffered by cation e xchange and silicate hydrolysis. This is indicated by continuously decreasi ng pH values and the dominance of Ca2+ and silicic acid in the streamwater. Strong changes in the water chemistry along the flowpath indicate that rai nwater will be stored in the system for sometime, and water derived from th e headwater areas influences the discharge. During precipitation events the new rainwater replaces pre-event water, stored in soils, which had a longe r contact time with the biotic and abiotic soil matrix. Acute effects of the selective harvest are leveled in the streamwater by mu ltiple buffer reactions. The discharge of the disturbed watershed was incre ased by the reduction of interception and transpiration. Increased rates of mineralization and acidity loads caused an increase in ion concentration a nd solution load in the discharge from the disturbed area. NH4+ showed the strongest effects compared to the reference area and the pre-event period. The changes in the runoff returned to pre-event conditions about one year a fter the disturbance. (C) 1999 Elsevier Science B.V. All rights reserved.