Riparian plant species composition alternates between species from standing and flowing water bodies – Results of field studies upstream and downstream of weirs along the German rivers Lahn and Fulda

Abstract

Regulation measures like the installation of weirs led to distinct shifts in riparian plant communities and to a decline of typical riparian plant species occurring along the river shoreline, which is naturally poor in woody plants. These regulation measures include also run-of-river impoundments or weirs that cause distinct differences in hydrodynamic conditions in their direct proximity upstream and downstream. In the course of the year, the downstream reaches are exposed to significantly higher water level fluctuations than upstream. Thus, these reaches are assumed to provide also suitable habitat for riparian plant species of habitats being exposed to frequent disturbance by alternating water levels.We investigated the riparian vegetation in direct proximity upstream and downstream of nine weirs along the two regulated rivers Lahn and Fulda. Sampling was conducted in the transition zone from water to land directly above the actual water level during the summers 2016 and 2017. Differences in species composition were analyzed by means of NMS and indicator species analysis. Results of the indicator species analysis were used for further analyses regarding species’ functional traits and species’ habitat origin.A grouping of the relevés in accordance to the weir reach, dissimilarities in site conditions, significant indicator species for each weir reach and common indicator species upstream along both rivers point towards differences in species composition upstream and downstream. Due to rather constant water levels, upstream species composition consisted mainly of perennial species from reeds of still waters, swamp and alluvial forests and terrestrial habitats that were mainly competitors. By contrast, species from reeds of flowing waters, flooded meadows and grasslands and typical riparian plant communities (Bidention tripartitae, Chenopodion rubri) occurred more frequently downstream. Species downstream exhibited also less competitive power and short life cycles as a result of higher water level fluctuations.Upstream and downstream species composition clearly reflect the observed hydrodynamic conditions, restricting continuity to the area within two weirs. This distinctly differs from natural conditions, which leads to the establishment of novel riparian plant communities. As downstream reaches in direct proximity to weirs are related to higher water level fluctuations in the course of the year, these areas are of essential meaning as refuge for typical riparian species. Against this background, we recommend to decrease bank steepness downstream to increase the effect of water level fluctuations, which would lead to the establishment of suitable habitats for typical riparian plant species.