The influence of channelisation on riparian plant assemblages

1. A comparative study of species diversity and assemblage patterns of herbaceous ground‐flora communities in riparian areas was performed along natural mid‐sized lowland streams and their channelised counterparts. The areas had open vegetation and were positioned along 18 similar‐sized third to fourth order stream reaches. 2. Alpha diversity was significantly higher along natural streams both at the sample plot and reach scale. Sample plot diversity peaked at intermediate distance from the natural stream reaches, whereas it increased with increasing distance from the channelised reaches. Both gamma diversity, measured and estimated from species‐area curves, and beta diversity, which is a measure of the change in diversity between areas, was similar along the two types of streams. 3. Alpha diversity correlated with several of the measured and calibrated environmental variables. The positive correlation between bank slope and alpha diversity indicates that flooding plays a key role in maintaining high levels of diversity along natural streams. 4. Species composition varied significantly between the two stream types. A cluster analysis identified four clusters of which two clusters (one and three) primarily included species associated with sample plots in areas along natural streams. Most cluster one and three species were also identified as indicator species for this stream type. 5. Canonical correspondence analysis revealed that cluster one species were less productive species associated with high total soil carbon and nitrogen contents, whereas cluster three species were highly productive species associated with high soil moisture levels, probably partly resulting from flooding. Our results suggest that distance from the stream channel imposes a probabilistic gradient that sustains co‐occurrence of these two communities in riparian areas along natural mid‐sized lowland streams.