Dominance of competitors in riparian plant species composition along constructed banks of the German rivers Main and Danube

Abstract

Hardening of shorelines has been extensively implemented in many parts of the developed world. This also applies to banks of German Federal Waterways, which are mostly fixed by ripraps to prevent bank erosion as a consequence of wave disturbance by shipping traffic. Since ripraps notably alter functions of riparian ecosystems and nature conservation demands recently gained in importance along waterways, alternatives for ripraps play an increasing role.Front-fixed banks are ripraps parallel to the shoreline and embedded in the waterway’s channel with an unsecured bank of low steepness behind them. Thus, they are suitable to prevent banks from erosion. However, it is unclear how they can contribute to the ecological enhancement of riparian vegetation along waterways.Therefore, we compared riparian vegetation of ripraps and front-fixed banks with unsecured banks along the German rivers Main and Danube to assess the ecological efficiency of front-fixed banks. Disturbance by alternating water levels was the lowest at ripraps, whereas disturbance levels were higher at front-fixed and unfortified banks. We used an ordination and indicator species analysis to reveal differences in species composition. The results of the indicator species analysis were analyzed regarding species’ biotope origin, light and moisture demand and life strategy. We analyzed species diversity and calculated functional diversity indices to display the prevalent ecosystem processes.Higher variation in species composition, common indicator species, a strong vertical layering of woody riparian vegetation and similar site conditions at front-fixed and unfortified banks revealed higher similarities in species composition between them than to other bank type combinations. Limnic species occurred with a higher frequency at unfortified and front-fixed banks, whereas terrestrial species were more frequent at ripraps. Light-tolerant species were more common at ripraps, whereas species’ demand for moisture was higher at front-fixed and unfortified banks than at ripraps. Cr-strategists occurred more frequently with increasing disturbance level, whereas c-strategists were more common at ripraps.Ripraps had the highest species diversity levels whereas functional diversity tended to be higher at front-fixed and unfortified banks. This indicates a higher trait complementarity and thus a higher specialization towards alternating water levels, wherefore a higher degree of naturalness can be assumed in these habitats.Nature conservation efforts along German Federal Waterways should focus on the restoration of flooding dynamics, as species typical for riverine habitats are more common at front-fixed and unfortified banks than at ripraps. Since front-fixed banks bear traits suited for the reconnection of rivers and their floodplains and for bank protection, they are a suitable alternative to promote biodiversity along German Federal Waterways.