Species Traits and Recolonization Processes after Flood Disturbances in Riverine Macrophytes

Macrophyte recolonization after two major flood disturbances (winter 1990 and autumn 1991) was studied for 3 years in 11 contiguous permanent quadrats located along a transect from one bank to the other across a former channel of the Rhône River. Floods induced changes in substrate grain-size from fine to coarser sediment and swept away all the plants 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 the center of the channel in coarse sediment. The transect was fully colonized by macrophytes the second year after a major flood. All these species, as well as those observed before the 1990 flood, can be considered as pioneer species adapted to frequent disturbances. Information identified from personal previous observations and collected in literature concerning selected species traits was analysed statistically using a fuzzy-coding technique and confronted with date of re-establishment of each species after disturbance. Species composition before disturbance and selected species traits allow us to predict species composition in disturbed areas. The first species to re-establish were able to produce turions or other non-subterranean vegetative organs. Species that disseminate both by lateral spread and regeneration by stem fragments re-establish later, before helophytes that also disseminate by lateral spread but flower each year.