Atmospheric deposition and impact of forest thinning on the throughfall of mountain forest ecosystems in the Bavarian Alps

The input into two forested watersheds in the Bavarian Alps is characterised by 2000 mm mean annual precipitation. Evaporation from the tree crown surfaces varies between 25 and 30%. The input is influenced by multiple natural and man-made processes. In addition to the general climate, these are local weather conditions, the growing season, processes of combustion (traffic and domestic fuel), tourism, farming, and not least by processes in the canopy. The input of SO4-S is comparatively low, while high amounts of acidity and nitrogen are deposited. The input occurs mainly as dissolved salts. A considerable amount of the acidity deposited is buffered in the canopy by cation exchange and by calcareous dusts, which are blown out from the limestone and dolomite mountains nearby and redeposited on leaf surfaces. Forest thinning (removal of 40% of the stem volume) caused marked changes in the deposition pattern. Interception and deposition rates of individual ions were reduced by up to 45%. The amount and chemical composition of the throughfall occupied an intermediate position between bulk precipitation and the throughfall of the control watershed. Ions which are preferentially exchanged and leached from the leaves, e.g. potassium and organic compounds, were affected the most. (C) 1997 Elsevier Science B.V.