Acidic Input to a Beech and Spruce Forest

Summary

In a wooded area in north-west Germany the chemical composition of atmospheric aerosols and rainwater sampled above and below a beech and a spruce canopy were analysed to investigate dry and wet deposition fluxes from the atmosphere to the vegetation-soil system. The concentrations of the aerosol components were by 17% – 41% smaller below than above the canopy indicating a considerable aerosol uptake by the tree-canopies. This aerosol uptake showed a minimum for components with a mass mean diameter (MMD) of about 1 μm for both spruce and beech trees. The concentrations of rainwater constituents were considerably higher in the throughfall below the canopy than in the open field. These increases were most pronounced for the spruce forest. The soil of the spruce forest received a free acidity input via throughfall about 5 times as high as it was brought down in rainwater above the canopy. It could be shown that most of these increases are due to dry deposition of gaseous and particulate matter onto the vegetation during the periods without rain. For aerosol components a dry deposition velocity for a spruce and beech forest between 0.4 cm s^-1 and 2 cm s^-1 could be determined. A minimum was found for Mn (MMD of about 1 μm) in agreement with the dependence of the filter effect on particle size. It seems that dry deposition of gaseous sulfur dioxide and not wet deposition of acid rain contributes most to the input of free acidity to a spruce forest ecosystems in this rural area.