ESTABLISHMENT OF ALLUVIAL FOREST SPECIES IN FLOODPLAINS - THE ROLE OFDISPERSAL TIMING, GERMINATION CHARACTERISTICS AND WATER-LEVEL FLUCTUATIONS

The establishment of seedlings of four floodplain forest species (Sali x alba, S. triandia, S. viminalis and Populus nigra) on river banks al ong the River Waal, the Netherlands, was studied. Seed germination pat terns 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 lab oratory experiment. Along the River Waal, seed dispersal occurred in s pring in the following sequence: S. viminalis, S. triandra, S. alba an d P. nigra. Seeds germinated in narrow belts parallel to the river; se edlings of S. viminalis were found at a higher elevation than S. alba seedlings, The elevation of the seedlings on the river bank was relate d to the water level during the dissemination period for Salix spp,, b ut not for P. nigra. Temperature had no effect on germination percenta ge within the range of 5-25 degrees C. P. nigra showed a significantly longer seed viability than the Salix species and germinated at a lowe r soil moisture content. The zonation of seedlings observed in the fie ld could be explained by the germination responses of the species; tim ing of seed dispersal and water level fluctuation appeared to be the m ajor determinants of initial floodplain forest zonation.