C4 herbs dominate the reservoir flood area of the Three Gorges Reservoir.
Dam operations can dramatically degenerate riparian vegetation. To improve the restoration practices of reservoir riparian vegetation, it is important to understand which and how a dominant species physiologically and ecologically maintain high fitness in this type of ecosystems. We explored the compositional change of riparian plants during the long-term flood-dry-flood cycle in the reservoir flood area (RFA) of the Three Gorges Reservoir Area (TGRA), China. In total 769 vascular plant species (belonging to 415 genera in 122 families) existed in the study area before damming (prior to 2006, i.e. the natural riparian zone). Following damming (2008-2018), plant species diversity rapidly declined, with only 51 species identified in 2018 (45 genera in 22 families). Before damming, perennial herbs, annual herbs and shrubs co-dominated the study area. After damming, the proportion of shrubs decreased significantly, and the proportion of annuals to total plants increased by 20%. Alien invasive species proportion increased from 5% to 18%. Notably, the proportion of C4 species increased significantly from 7% to 31%. Ten of the 16 dominant species in RFA since 2015 were C4 Poaceae species. Our study indicates that dam construction could cause severe biodiversity loss of riparian plants and draw alien species invasion. Besides, C4 herbs would dominate the RFA. A higher photosynthetic rate could help C4 plants grow faster to cope with the nitrogen deficiency and short growth cycles in RFA. Hence, screening C4 herbs for vegetation restoration might aid in maintaining biodiversity and ecosystem functions in flood-dry-flood reservoir flood areas.