Anti-seasonal flooding drive substantial alterations in riparian plant diversity and niche characteristics in a unique hydro-fluctuation zone

Human-induced disturbances such as dam construction and regulation often alter the duration，frequency and seasonality of flooding and thus substantially influence plant characteristics in the hydro-fluctuation zones. However, the effect mechanism of anti-seasonal hydrological alterations on vegetation distribution patterns and niche characteristics in the water level fluctuation zones (WLFZs).is still unclear. In this study, 368 quadrats were selected to investigate the effects of the anti-seasonal hydrological regimes on the foristic composition, species diversity and niche characteristic in the hydro-fluctuation zone of the Three Gorges Reservoir (TGR), a unique riparian ecosystem, China. The results showed that the number of species per square meter (S), the Shannon-Wiener diversity index (H) and Simpson dominance index (D) of the plant guilds in the TGR increased significantly with elevation, which was inconsistent with humped diversity–disturbance relationship of the intermediate disturbance hypothesis, while the opposite trend was observed for the Pielou evenness index (E). The H, D, S and E from upstream to downstream firstly showed a significant increasing trend ( p <0.05), reached the highest in the middle reaches, and then decreased in the lower reaches. The vegetation was classified into 12 guild types but the vegetation composition showed a significant variation with a transition from xerophytes to mesophytes and hygrophytes with the increasing flooding time. Cynodon dactylon was the most dominant species based on its highest important value and niche breadth. And high niche breadth had a high niche overlap between species. Therefore, anti-seasonal hydrological alterations precipitated substantial reduction of plant diversity, species competition and exclusion among species by expanding the niche in the guilds. The vegetation in the unique riparian ecosystems was still in the primary stage of plant community succession with low species diversity, high niche overlap, intense competition and obvious single-species dominant communities. Compared to total nitrogen, total phosphorus and soil organic matter factors, the hydrological alteration filtering was more important in explaining the plant guild patterns and niche characteristics. Therefore, there may be some differences in the governance strategies adopted in different areas of the novel riparian ecosystems for vegetation restoration efforts of the riparian forests.