Variations in herbaceous vegetation structures and vegetation-environment relationships from floodplain to terrace along a large semi-humid river

Riparian zones along semi-humid rivers have significant lateral gradients of water, leading to the lateral pattern of riparian vegetation. However, less is known about the determinants of herbaceous vegetation structures between floodplain and terrace in semi-humid region. The study was designed to assess the variations in herbaceous vegetation composition, species diversity, and vegetation-environment relationships from floodplain to terrace. Fifty sampling lines that equally distributed in floodplain and terrace were established to collect herbaceous vegetation and corresponding environmental data. Results showed that soil moisture in floodplain was significantly higher than that in terrace, and herbaceous vegetation compositions shifted from hydric to mesic to xeric as soil moisture decreased. Phragmites australis and Artemisia selengensis were dominant species in floodplain, whereas Ambrosia trifid, Metaplexis japonica, Humulus japonicus, Glycine soja, Artemisia lavandulaefolia and Setaria glauca were dominant in terrace. Species diversity increased from floodplain to terrace and from upstream to downstream. Substrate and bank stability, rather than water availability, were the significant environmental drivers of herbaceous vegetation structures in water-rich floodplain. However, herbaceous vegetation structures were mainly determined by water gradient and human disturbance in water-poor terrace in semi-humid region.