Deposition and canopy exchange processes in central-German beech forests differing in tree species diversity

Atmospheric deposition is an important nutrient input to forests. The chemical composition of the rainfall is altered by the forest canopy due to interception and canopy exchange. Bulk deposition and stand deposition (throughfall plus stemflow) of Na⁺, Cl^?, K⁺, Ca²⁺, Mg²⁺, PO ₄ ^3? , SO ₄ ^2? , H⁺, Mn²⁺, Al³⁺, Fe²⁺, NH ₄ ⁺ , NO ₃ ^? and N_org were measured in nine deciduous forest plots with different tree species diversity in central Germany. Interception deposition and canopy exchange rates were calculated with a canopy budget model. The investigated forest plots were pure beech (Fagus sylvatica L.) plots, three-species plots (Fagus sylvatica, Tilia cordata Mill. or T. platyphyllos Scop. and Fraxinus excelsior L.) and five-species plots (Fagus sylvatica, T. cordata or T. platyphyllos, Fraxinus excelsior, Acer platanoides L., A. pseudoplatanus L. or A. campestre L. and Carpinus betulus L.). The interception deposition of all ions was highest in pure beech plots and was negatively related to the Shannon index. The stand deposition of K⁺, Ca²⁺, Mg²⁺ and PO ₄ ^3? was higher in mixed species plots than in pure beech plots due to higher canopy leaching rates in the mixed species plots. The acid input to the canopy and to the soil was higher in pure beech plots than in mixed species plots. The high canopy leaching rates of Mn²⁺ in pure beech plots indicated differences in soil properties between the plot types. Indeed, pH, effective cation exchange capacity and base saturation were lower in pure beech plots. This may have contributed to the lower leaching rates of K⁺, Ca²⁺ and Mg²⁺ compared to the mixed species plots. However, foliar analyses indicated differences in the ion status among the tree species, which may additionally have influenced canopy exchange. In conclusion, the nutrient input to the soil resulting from deposition and canopy leaching was higher in mixed species plots than in pure beech plots, whereas the acid input was highest in pure beech plots.