Abstract: The construction of the Three Gorges Dam(TGD) has led to the disappearance of original plant species and communities, and new predominant plant communities had been formed to adapt the riparian zone environment changes. The study of predominant plant species distribution in those areas is important for the ecological restoration of Three Gorges Reservoir riparian zones. In this study, we investigated the plant species diversity before(2007) and after(2015) the dam impoundment at a special site in Zhong county, used populations Important Values(IVs) to estimate the relative importance of species of this area. The results showed that:(1) There were106 vascular plant species(belong to 96 genera in 47 families) before the dam impoundment(2007), and Perennial herbs, annual herbs, and shrubs were co-dominated in this area. After dam impounding(2015), the vascular plant species dropped dramatically down to 58(belonging to 51 genera in26 families), with shrub losing their dominated role. The perennial herbs percentages in all species increased from 40.57%(2007) to 43.10%(2015), while annual herbs percentages increased from 28.30% to 48.28%, respectively.(2) The IVs of Cynodon dactylon increased from 1.72%(2007) to 45.06%(2015); while the IVs of Arthraxon hispidus and Imperata cylindrical decreased respectively from 6.81% and 16.48% to both zeros. The predominant species in the bottom of studied area changed from Arthraxon hispidus, Imperata cylindrical and Alternanthera philoxeroides to Cynodon dactylon, Echinochloa crusgalli and Polygonum hydropiper; the predominant species in the middle of studied area changed from Arthraxon hispidus and Imperata cylindrical to Cynodon dactylon, Xanthium sibiricum, Bidens pilosa and Alternanthera philoxeroides; and the top area predominant species changed from Imperata cylindrica to Cynodon dactylon, Xanthium sibiricum and Setaria viridis.(3) Before the dam impoundment in 2007, the plant species between elevation 158 m to 173 m were accounted as 42-66, and there were no obvious changes with the elevation variations. After the impoundment in 2015, the plant species between elevation 158 m and 173 m decreased to 14-43, and the species densities increased along the elevation increases. Meanwhile, the species diversity index and species evenness index didn’t show obvious changing trends along the elevation variations. The results suggested the dam impoundment had caused the plant species in the studied area decreased about 50%. Many species that couldn’t suffer the submergence had been vanishing or marginalizing. However, some perennials that have strong clone abilities showed obviously competitive preponderance and the annuals whose seeds had strong dispersal abilities could evade the winter submergences through seed dormancy and riparian habitats adaptation. The plants were affected by the difference of submergence depth, submergence time and submergence frequency, leading to the significant differences in species composition and distribution at different elevations, particularly for those predominant species changes.
Key words：Cynodon dactylon; annuals; perennials; species richness; water fluctuation