The relation between geomorphological features and species richness in the low flow channel of the Warche, downstream from the Bütgenbach dam (Ardennes, Belgium)

Abstract

The Butgenbach dam, built on the Warche River (Ardennes, Belgium) in 1931 brought about two major changes: a significant reduction in the frequency and magnitude of the minimum discharges during the growing season (April–September) and the formation of numerous new geomorphological features (islets, pebble bars and rock outcrops) in the low flow channel. These changes have lead to an increase in the number of vascular plants downstream from the dam. Between 1994 and 1997, 74 species (bryophytes and phanerogams) were found downstream from the dam and 12 species were identified upstream. Downstream, most of the species are typical of damp semi-natural grasslands and of the banks of the Haute Ardennes—environments both characterised by oligotrophy. Euryoecious species (with a wide ecological range) and nitrophile species (that indicate pollution of the Warche from urban effluents and agricultural fertilisers) are also present. However, the distribution of plant species in the low flow channel is very heterogeneous. The number of species varies from one geomorphological unit to another. Species are more numerous on islets (54 species) than on rock outcrops (35 species) and gravel bars (28 species). On the islets, the number of species present varies in accordance with the degree of erosion. The islets that are most eroded and those that are least eroded display a poor range of flora. The degree of erosion influences the depth of silt, pebbles and litter on the islets. With regard to rock outcrops, the wealth of flora present depends on the form they take. Stratified outcrops are richer in flora than protruding outcrops. The number of plant species present on gravel bars depends on the frequency and the scale of remobilisation of pebbly material by floods. Following large floods, the vegetation cover and the number of different species decreases. But, if the sites remain stable, the number of species first of all increases, only then to decrease due to the proliferation of competitive species such as Phalaris arundinacea L. The ranges of species found on the different geomorphological features do not display a high degree of similarity. Canonical correlation analysis reveals that the density and the total number of species on geomorphological features are most strongly influenced by the proportion of fine particulates (<2 mm).