Effects of the Three Gorges Dam on preupland and preriparian drawdown zones vegetation in the upper watershed of the Yangtze River, P.R. China

After the construction of the Three Gorges Dam (TGD) on Yangtze River, the preupland region and preriparian zone were submerged as a result of a substantial increase in the water level. To assess the overall effects of the Three Gorges Reservoir (TGR) impoundment on the drawdown zone vegetation, we compared the preupland drawdown zone (PU-DZ) vegetation with its adjacent upland and the preriparian drawdown zone (PR-DZ) vegetation with the natural riparian zone (NRZ) of the Yangtze River. Twelve sites were surveyed in each of these four habitats in September 2009. The results showed that the TGR impoundment had different effects on the PU-DZ and PR-DZ vegetation. The species composition of the PU-DZ vegetation changed substantially after the impoundment. The species number per 100 m2 increased significantly in the PU-DZ compared with the adjacent upland. Most of the species were forbs and ferns, especially annual and biennial species. However, the total cover in the PU-DZ was lower than that of the adjacent upland due to the destruction of tree and shrub communities. In contrast, the total cover and species composition of the PR-DZ did not change as seriously as the PU-DZ vegetation for the existing tolerant perennials. Significantly fewer species were found per 100 m2 in the PR-DZ than in the NRZ. Moreover, the species numbers per 100 m2 for trees and shrubs, forbs and ferns and graminoids decreased significantly, but the proportions of cover for these groups were unchanged, except for trees and shrubs decreased significantly. This result indicated that all species groups were extensively suppressed in the PR-DZ.