CONCENTRATION VARIATIONS IN RAIN AND CANOPY THROUGHFALL COLLECTED SEQUENTIALLY DURING INDIVIDUAL RAIN EVENTS

Canopy throughfall was collected sequentially during individual rain events in a Norway spruce stand at Klosterhede in Denmark (1989) and a Douglas-fir stand at Speuld in the Netherlands (1993). Simulataneously, precipitation was sampled sequentially. Each fraction was analyzed for pH, Na+, K+, Ca2+, Mg2+, NH4+, Cl-, NO3-, and SO4(2-). Analysis of successive precipitation collected at both sites generally showed the highest concentrations of Na+, Cl-, Mg2+, Ca2+, NO3-, and SO4(2-) in the initial fraction of most rain events followed by decreasing ion concentrations in subsequent samples. For K+, and especially for NH4+, several concentration maxima were observed throughout the events. At Speuld, the sequential throughfall showed a slight increase in concentration in the beginning, followed by a continuous decrease. At the end of the events, concentrations of throughfall increased again. At Klosterhede, decreasing ion concentrations of all ions were observed throughout some events, while constant and even increasing ion concentrations were seen throughout others. The ion concentrations in throughfall were inversely connected to the intensity of the throughfall. At both sites, the concentrations in throughfall in the final part of the event never reached a constant value and were always higher than in the simultaneously collected rain water. This indicates that the sources for the ions in throughfall were never emptied. The gradual and steadily changing concentration curves suggest that the processes involved in dry deposition and canopy exchange are slow. It was further found that the concentrations of NH4+ and SO4(2-) in the initial throughfall at Speuld sometimes exceeded concentrations which could damage plant tissues.