Dam inundation simplifies the plant community composition.

The construction of dams has caused riparian habitat degradation and ecosystem service loss globally. It is critical to assess the response of riparian plant communities to inundation gradients for their conservation. Recent evidence suggests that plant community assemblages are governed by flooding stress, soil nutrient availability, climate (environmental filtering) and dispersal, speciation, local extinction (dispersal filtering), but it remains unclear which dominates the riparian ecosystem regulated by a dam. Thus, this article aims to elucidate the relative importance of environmental and dispersal filtering to variations in plant communities to understand community assembly mechanisms in riparian ecosystems. Here we used plant community data related to four elevations in the riparian zone of the Three Gorges Dam Reservoir in China to show that species richness and diversity, community height, and the cover of total, annual, and exotic plant categories decreased, while the cover of perennial and native plant groups increased under higher flooding stress. Community composition varied substantially with elevation, and species composition tended to converge with increased inundation, characterized by flood-tolerant species. The community composition underwent stronger environmental filtering at low elevations and stronger dispersal filtering at high elevations, with stronger environmental filtering across riparian ecosystems. Therefore, we conclude that dam inundation drives community assemblages of riparian plants by the combined effects of environmental and dispersal filtering. Still, their relative contribution varies between elevations, and environmental filtering is more important in shaping community assembly. This study is the first to confirm that plant community assembly in the dam-regulated riparian area is determined by both niche-based and stochastic processes. Thus, we highlighted the importance of considering inundation intensity, propagule sources, and river connectivity when implementing restoration projects.