River regulation intensity matters: Riverbank vegetation is characterized by more typical riverbank plant species with increasing distance from weirs

Abstract

The installation of weirs leads to extensive alterations in species composition of riverbank vegetation, as they distinctly change the natural disturbance regime of rivers. Due to their infrastructural significance, the removal of most weirs and impoundments is unlikely, which is why restoration measures along impounded rivers are strongly limited.To evaluate the restoration potential of weir-near (maximum 400 m distance upstream and downstream from weir) and weir-distant (minimum 1000 m distance from weir) river stretches, we sampled 72 total relevés (24 weir-distant, 24 weir-near upstream, and 24 weir-near downstream) along the German river Lahn in Hesse and Rhineland Palatinate. Weir-distant stretches do not have lockages directly surrounding any weirs like they do in weir-near areas; consequently, the impoundment effect is further minimized, which leads to reduced regulation intensity. Differences in species composition were evaluated by non-metric, multidimensional scaling and indicator species analysis. Species diversity, functional diversity, and csr-signatures were compared to test for differences between upstream, downstream, and weir-distant sites using a Kruskal-Wallis test by ranks and a Posthoc-Kruskal-Nemenyi-Test. Indicator species were used to analyze the distribution of typical species from the transition zone of riverbanks.Weir-distant vegetation was distinctly different from weir-near vegetation, revealing more similarities to typical floodplain species and species adapted to flooding and changing water levels, higher species diversity, and a somewhat higher functional diversity. R-strategists were more prevalent along the weir-distant reaches; this also applies to species from flooded meadows and grasslands. As these results indicate a more typical species composition for riverbank habitats, weir-distant river stretches are likely to bear a higher potential for successful restoration measures than weir-near river stretches. Thus, regulation intensity is an important factor when planning riverbank restoration measures. As summer annual species from the Bidentetea alliance were rare, we recommend the ecological value of regulated riverbank stretches be assessed by means of species that naturally occur directly above the summer annual species zone. Within our study, these species originate from flooded meadows and grasslands that occurred irrespective of the weir distance and corresponded to water level fluctuation intensity.