Preliminary Effects of Impounding on Vegetation in Drawdown Zone of the Three Gorges Reservoir Region

Vegetation of the drawdown zone of the Three Gorges Reservoir is a hotspot for Chinese researchers and governments.Responses of the vegetation to the reservoir impounding are vital both for assessments on the Three Gorges Dam construction and for future drawdown zone vegetation restoration.To obtain information on local vegetation responses in already inundated zones(up to 156 m)and future water levels (up to 175 m)in 2009,we established sixtyeight 5 m×5 m permanent plots along the Yangtze River at 12 sites from Banan to Zigui upstream from the Three Gorges Dam in August 2008.We compared vegetation in zones already submerged (145-156 m A.S.L.) and vegetation in zones to be submerged (156-175 m A.S.L.) in terms of plant species composition,plant species diversity,and herbaceous layer aboveground biomass.Our results demonstrated that plant species composition of submerged vegetation,which were dominated by annuals and perennial forbs,were significantly different from vegetation to be submerged,which were mainly dominated by shrubs.The most dominant forbs/herbs in the submerged vegetation included perennials, Cynodon dactylon,Hemarthria altissima and Capillipedium assimile,and annuals Setaria viridis,Digitaria ciliaris, and Leptochloa chinensis.Most woody plants in the submerged zone were dead except for Ficus tikoua,Pterocarya stenoptera，and Vitex negundo.In addition,shrub species richness was significantly lower and annual species richness was significantly higher in the vegetation below 156 m A.S.L.than the vegetation distributed at elevations from 156 to 175 m.As for plant species diversity,the richness of vascular plants and ferns，and that of perennial forbs/herbs in the vegetation of the submerged zone were significantly lower than the vegetation to be submerged,which showed no significant difference for annual species richness.Based on aboveground biomass,no significant differences were detected in herbaceous layer biomass between the vegetation in the submerged zone and the zone to be submerged.We concluded that plant species composition of local vegetation was greatly altered by submergence and plant species diversity greatly decreased due to impounding of the reservoir.Annuals will dominate future vegetation below 175 m A.S.L.in the drawdown zone of the Three Gorges Reservoir Region.To select suitable plants to restore the vegetation in the drawdown zone,lifehistory,physiology,and morphology traits should be taken into account,and,in particular,lifehistory strategy should be thoroughly examined.Dominant annuals,perennial forbs,and shrubs found below 156 m have a potential value for future vegetation restoration in this region.