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

摘要

Two well-buffered mixed mountain forest watersheds were studied with regard to stream water chemistry and the impact of forest thinning. The areas are located in the Bavarian Alps near Lake Tegernsee, 50 km SW of Munich (Germany). In order to obtaininformation on the influence of disturbances on the stream water chemistry, 40% of the trees were removed in October 1992 from one watershed after 2 yr of monitoring (1990-92), using a crane to avoid soil damage and erosion. The second watershed was used as a reference. The water output at 90degree C V-weirs installed at the foot of the watersheds for the monitoring was characterized, in general, by a strong export of silicate, alkali and alkaline earth cations, and bicarbonate. Acidity and nitrogen accumulated in the watersheds. Both areas were subject to a continuously increasing load of acidity, which was mainly buffered by cation exchange and silicate hydrolysis. This was indicated by continuously decreasing pH values and the dominance of Ca2+ and silicic acid in the stream water. Strong changes in the water chemistry along the flowpath indicate that rainwater will be stored in the system for sometime, and water derived from the headwater areas influences the discharge. During precipitation events the new rainwater replaces pre-event water, stored in soils, which had a longer contact time with the biotic and abiotic soil matrix. Acute effects of the selective harvest are levelled in the stream water by multiple buffer reactions. The discharge of the disturbed watershed was increased by the reduction of interception and transpiration. Increased rates of mineralization and acidity loads caused an increase in ion concentration and solution load in the discharge from the disturbed area. NH4+ showed the strongest effects in comparisons with the reference area and the pre-event period. The changes in the runoff returned to pre-event conditions about one year after the disturbance.