The effects of river embankment and forest fragmentation on the plant species richness and composition of floodplain forests in the Meuse Valley, Belgium

We studied the effect of disruption of alluvial forests from natural river flooding on their vascular plant diversity in the river Meuse floodplain in Belgium. The river Meuse is a rain-fed river, originating at an altitude of 409 in above sea level and discharging into the North Sea some 900 km further downstream. The Meuse river was channelized for most of its course in Belgium and The Netherlands during the last two centuries. In the study area a continuous embankment was gradually realised with a system of winter dykes. This enabled us to sample forests along a gradient of isolation from the river and flooding frequency. Flooding frequency was the most important correlate of community composition of the forests. Forests still under influence of the river were significantly richer in river species and significantly poorer in woody species than forests disconnected from the river. They also had a higher beta species diversity and tended to have a higher alpha diversity. Disconnected forests seem to loose species but they do not gain species at the same rate. We suggest that the two most important ecological processes behind this are 1) the poor colonisation capacity of typical forest plant species, which is mainly due to dispersal limitation and 2) the absence of natural disturbance events, which stimulates the dominance of certain competitive species in these forests. Only the re-establishment of lateral river connectivity and natural dynamics can stop this process.