Throughfall fluxes in a secondary spruce (Picea abies), a beech (Fagus sylvatica) and a mixed spruce–beech stand

Bulk precipitation, throughfall (TF) and stemflow (SF, for beech only) were monitored from May 02 to October 03 in adjacent stands of pure spruce, pure beech and mixed spruce–beech in Lower Austria. The main objective of this study was to specify regulating factors of TF and to address the impact of tree species composition on atmospheric input. We were especially interested, whether components of TF in the mixed stand indicate non-linear effects that would not be obvious in the single species stands. The mean annual nitrogen (NH4 + NO3) and sulfur (SO4) TF (+SF) fluxes were in the low to moderate range, increasing from the beech (N: 9.1 versus S: 5.5 kg ha−1) over the mixed (N: 11.6 versus S: 6.1 kg ha−1) to the spruce stand (N: 16.6 versus S: 9.2 kg ha−1). Two different methods were used to separate net throughfall (NTF) into the dry deposition (DD) and canopy exchange (CE) components. The regression analysis method on the event-to-event variation in NTF was useful for explaining the regulating factors for H+ and K but failed for other substances, since we used multiple rain event collections, which mixed up both components. According to these results it is concluded that cation leaching was mainly driven by organic anions and bicarbonate but insignificantly by a cation exchange reaction with H+, what may be considered a natural process in the absence of heavy acidic deposition. The filtering approach method, applying sulfate instead of widely used sodium as tracer for calculation of DD, yielded reasonable estimates. This model revealed that during the dry summer 03 DD rates were much higher than during the wet summer 02 for all studied elements. However, canopy leaching rates of K, Ca and Mg showed the opposite trend, since this process is mainly controlled by the amount of precipitation. Calculated total deposition (TD) fluxes as estimates of true inputs to the system were different from measured TF fluxes and justify the conclusion that the mixed spruce–beech stand showed an approximately linear response for H+, K, NH4, NO3, SO4, Cl and PO4, if compared to the TD fluxes of the single species stands, but an antagonist response for Ca and Mg, since TD fluxes of these cations were almost identical for the beech and the mixed stand.