Community assembly of riparian vegetation in the middle and lower reaches of the Yangtze River

In this study, hygrophyte communities at 37 sites in 13 cities along the middle and lower reaches of the Yangtze River (Yichang-Tongling section) were investigated during the wet and dry seasons. Both species composition and phylogenetic structure were examined at each site in the wet and dry seasons. We also revealed the drivers of community assembly combining environmental and spatial factors. Results showed that: (1) α-diversity in the wet season was higher than that in the dry season. There was a positive correlation between hydrological conditions and α-diversity in the wet season, while α-diversity in the dry season was mainly correlated with temperature and soil total nitrogen (TN). (2) The phylogenetic structure in the wet season showed an aggregation, implying that environmental filtering played a dominant role in community assembly; in the dry season, a different pattern was observed between the net relatedness index (NRI) and nearest taxon index (NTI), suggesting the evolution at recent (terminal) phylogenetic nodes. (3) There was a significant relationship between the α-diversity of species and phylogeny, indicating that these two dimensions can represent each other to a certain extent. (4) The composition mechanism of riparian vegetation in the middle and lower reaches of the Yangtze River differed between the wet and dry seasons. Species and phylogenetic structure were both influenced by environmental filtering and dispersal limitation in the wet season, with environmental filtering playing a dominant role in this process. In the dry season, community composition was only somewhat influenced by environmental filtering at the species level. (5) The key drivers of community assembly in our study were temperature at the large scale and aquatic and nutrient conditions of the soil at the small scale. This study should contribute to the management and protection of riparian vegetation in the middle and lower reaches of the Yangtze River.