Vortrag, Gesellschaft für Ökologie (GfÖ), Halle: 08.09.2003 - 12.09.2003
The spatial distribution of willow seedlings (Salix spp., Salicaceae) and its dependence of seed dispersal time, germination characteristics and fluctuations of water level were investigated in the years 2000 and 2001 in the laboratory and on study sites at the rivers Rodach and Main (Germany, Bavaria, Upper Franconia). The germination capacity of seeds of Salix caprea, S. cinerea, S. fragilis, S. purpurea, S. viminalis and S. triandra was investigated under three different laboratory conditions: on moist filter paper in closed petri dishes in light and dark, and swimming in water under light. On moist filter paper in light germination rates of all species were at least 80 %. In darkness, S. cinerea and S. triandra showed a significant lower germination rate. In water, germination rates of S. cinerea and S. fragilis were lower while S. purpurea seeds germinated better than on moist filter paper. Seed viability decreased very rapidly with increasing seed age. Salix fragilis, S. purpurea, S. viminalis and S. triandra lost their germination ability completely after four weeks, S. cinerea after six and S. caprea after eight weeks. According to seed dispersal time these species can be divided into early dispersers (S. viminalis, S. purpurea) and late dispersers (S. alba, S. fragilis, S. x rubens, S. triandra). The water level of the river during the time of early seed dispersal was significant higher than during the time of late dispersal. To investigate seedling establishment, two sample areas were established on a newly formed gravel bar without vegetation. Seedlings occurred only in a narrow band parallel to and near the water shore. Only 20 cm of the relief was colonized by willow seedlings, whereby the distribution of seedlings among the relief differed significantly between both dispersal groups. Early dispersers occurred in higher altitudes and farther from the water shore than late dispersers. Establishment of seedlings only took place in the range of water level fluctuations during the time of seed dispersal or a few centimeters higher. These results suggest that optimal germination conditions must be available during seed dispersal, but germination and temporary survival in darkness or water is possible. Sufficient germination conditions are provided primarily in the zone of water level fluctuations. Thereby, time of seed dispersal and distribution by wind and water in combination with water level fluctuations result in different spatial distribution of the two dispersal groups. This seems to be an important factor to avoid interspecific competition, to ensure establishment of seedlings on a preferably large area and to support the high diversity of Salix species in alluvial habitats.