A conservation paradox for riparian habitats and river corridor species.

Summary: The floodplain meadows of the River Meuse, one of the larger Northwest European streams, harbour a specific group of river corridor plant species. These species are mostly restricted to small habitat patches of dikes and natural levees as these locations provide the necessary microclimate conditions for this species group with a more southern Central European range. This spatial restriction of habitat together with the intensified agricultural use of the floodplains means that these habitat fragments have become rare in the river valley. The central question for the study was whether preservation of these relics is a sufficient means to preserve this group of species and the overall riparian diversity. We investigated the composition and diversity of the floodplain meadows of the alluvial plain of a 40km river reach. Emphasis was on the specific conditions of the dry river grasslands with river corridor plants. A mapping and sampling of vegetation and soil conditions over the alluvial plain was executed. This analysis was completed with a colonisation survey for the rare species of dry river grasslands in newly generated habitat after two consecutive flood peak periods of the mid-nineties and the beginning of this century. Comparative analysis of the recruitment over the different meadow types was done by defining a colonisation rate per type. In the DCA ordination the dry river grasslands appeared separated from the other floodplain meadow communities, and the rare river corridor plants showed a strong preference to the pioneer dry river grasslands. These communities are restricted to the gravel or sandy deposits on the more elevated parts of the floodplain. A significant isolation of the river corridor plant relics was revealed. As for the cause of this isolation, the colonisation analysis indicated recruitment limitation to be the major threat for survival of most of the river corridor plants. Colonisation rates were high for species from all vegetation types, the exceptional low values are caused by disconnection of flood contact. The recovery of populations depends strongly on flood contact and possibilities for recruitment in the creation of new habitat. Synthesis and applications: The hypothesis of conservation of relics of river corridor plant populations outside the river dynamic influence was withdrawn. This shows that the construction of conservation and rehabilitation strategies for species at risk needs a good knowledge of key processes that determine the species survival at the regional scale. Although the dry river grasslands and its river corridor species are sensitive to flooding, we determined the need for dynamic flooding processes in the creation of habitat and in the dispersal and recruitment, and thus the sustainable conservation of these species. For the investigated River Meuse reach, the flood dynamics prove essential for the habitat creation that strongly determines the restoration potentials at the reach scale. [Copyright &y& Elsevier]