Establishment of alluvial forest species in floodplains: the role of dispersal timing, germination characteristics and water level fluctuations

The establishment of seedlings of four floodplain forest species (Salix alba, S. triandra, S. viminalis and Populus nigra) on river banks along the River Waal, the Netherlands, was studied. Seed germination patterns and timing of seed dispersal were studied under field conditions. Effects of temperature and substrate moisture content on germination percentages and the viability of the seeds were investigated in a laboratory experiment. Along the River Waal, seed dispersal occurred in spring in the following sequence; S. viminalis, S. triandra, S. alba and P. nigra. Seeds germinated in narrow belts parallel to the river; seedlings of S. viminalis were found at a higher elevation than S. alba seedlings. The elevation of the seedlings on the river bank was related to the water level during the dissemination period for Salix spp., but not for P. nigra. Temperature had no effect on germination percentage within the range of 5-25°C. P. nigra showed a significantly longer seed viability than the Salix species and germinated at a lower soil moisture content. The zonation of seedlings observed in the field could be explained by the germination responses of the species; timing of seed dispersal and water level fluctuation appeared to be the major determinants of initial floodplain forest zonation.