DISTURBANCE REGIMES AND VEGETATION DYNAMICS - ROLE OF FLOODS IN RIVERINE WETLANDS

This study tested whether the frequency of flood disturbances was able to slow down or stabilize vegetation succession in former braided cha nnels over a decade. According to the Patch Dynamics Concept and to su ccession theory, species richness and diversity should be high but sta ble in the frequently (40 days/year) flooded channel, and should chang e over time in the infrequently (1 day/year) flooded one. Within the f requently disturbed channel, composition of vegetation as well as spec ies richness and diversity appeared stable through dynamic equilibrium over the decade. Only one zone, because of particular geomorphologica l features that decreased disturbance intensity, developed highest div ersity and richness as expected from the Intermediate Disturbance Hypo thesis. The highest disturbance effect decreased species richness and was related to a higher spatial heterogeneity of the substrate (number of grain-size classes). In the other zones, richness and diversity ap peared to be lowest where disturbance frequency was lowest or disturba nce intensity was highest. From 1981 to 1987, the infrequently flooded channel underwent succession, and species richness increased in the m ajor part of the channel, whereas diversity increased only in its extr eme parts.