Distribution of dominant plant species and characteristic of its communities on the foreland of Cele Oasis in relation to groundwater level.

Indigenous vegetation in the foreland of river oases at the extremely arid southern margin of the Taklimakan desert in Xinjiang,northwest China,is dominated by a few perennial plant species.As the population of the oases exploits the vegetation by harvest of construction wood,fuel and livestock feed,and by grazing of sheep and goats;indigenous vegetation plays an important role in the ecology and agro-economic system of the oases.In the foreland of river oases at this region,mean annual precipitation is less than 50 mm,since water was shifted to the oasis agriculture flooding reached rarely in the foreland,groundwater must have played a key role for plant survival.The hypotheses that distribution of dominant plant species and characteristics of its communities related to groundwater level were tested.The research was conducted at northeast area of Cele(Qira) River oasis.Ten transects were established at different groundwater level from 15.7 m to 0.9 m.The dominant species of Tamarix ssp.,Populus euphratica Oliv.,Alhagi sparsifolia Shap.,Karelinia caspica,Scorzonera divaricata,Phragmites communis(Cav.) Steudel,and Sophora alope croides were present at these transects.In each transect,groundwater level was measured at each well once a month during the year(2004).Near the wells,3-4 plots were chosen to investigate characteristics of plant community.The values of density,coverage,frequency,height,important value,and niche breadth,etc were investigated and calculated.The results show that variations of groundwater level impacted distributions and characteristics of dominant plant communities.However,in addition to groundwater level,the community composition and species diversity were also influenced by physiognomy.Comparing the similarity coefficient,the differences between dominant plant communities were very significant under different groundwater level.Comparing other results on desert vegetation to groundwater level,variations of community distribution was similar in big spatial scale.However,there were significant differences in community type and community succession,especially under the condition of very low groundwater level.With groundwater level rising from down to up,these communities transformed from Alhagi community with the lowest groundwater level,Tamarix ssp.community with low groundwater level,Populus community with middle groundwater level,Phragmites community with high groundwater level,to Sophora community with the highest groundwater level.The groundwater level being up to 6.0 m,the community type,composition and species diversity were obviously increased.For the dominant species,niche breadth exhibited the biggest efficiency of resource utilization in Tamarix,which means that it had best adaptability to environment in foreland of the oasis.