SPECIES TRAITS AND RECOLONIZATION PROCESSES AFTER FLOOD DISTURBANCES IN RIVERINE MACROPHYTES

Macrophyte recolonization after two major flood disturbances (winter 1 990 and autumn 1991) was studied for 3 years in 11 contiguous permanen t quadrats located along a transect from one bank to the other across a former channel of the Rhone River. Floods induced changes in substra te grain-size from fine to coarser sediment and swept away all the pla nts present before disturbance. Re-establishment of species started on or near the banks in fine sediment areas that can act as refuges for propagules during disturbance or as regeneration niches for propagules brought in by floods. From the banks, the species expanded towards th e center of the channel in coarse sediment. The transect was fully col onized by macrophytes the second year after a major flood. All these s pecies, as well as those observed before the 1990 flood, can be consid ered as pioneer species adapted to frequent disturbances. Information identified from personal previous observations and collected in litera ture concerning selected species traits was analysed statistically usi ng a fuzzy-coding technique and confronted with date of re-establishme nt of each species after disturbance. Species composition before distu rbance and selected species traits allow us to predict species composi tion in disturbed areas. The first species to re-establish were able t o produce turions or other non-subterranean vegetative organs. Species that disseminate both by lateral spread and regeneration by stem frag ments re-establish later, before helophytes that also disseminate by l ateral spread but flower each year.