PLANT COLONIZATION ON RIVER GRAVEL BARS - THE EFFECT OF LITTER ACCUMULATION

The riparian areas of the Garonne river were severely flooded in lune 1992. After those floods, germinations of seed pioneer species have be en observed. Simultaneous high rates of germination and of mortality o f seedlings occurred on the substrates which consisted mainly of grave l. Leaf litter accumulation led to different results in function of th e mass of litter applied : accumulations of 150 to 300 g/m(2) allowed a high rate af germination and decreased the rate of mortality. When 6 00 to 1200 g/m(2) of litter were applied, the germination rates decrea sed Litter accumulations of 150 g/m(2) corresponded to maximum densiti es and species richness, and seemed to generate, for the studied river section, optimal conditions of colonization of gravel bars by seedlin gs of riparian pioneer species.