Degeneration and Restoration of Species Diversity of a Desert Riparian Ecosystem

The Tarim River,which is the longest continental river in China,flows through an extremely arid area,the Taklimakan Desert.Natural vegetation in the Tarim River Basin plays a very important role in conservation of biodiversity and reduction of desertification.In the last 50 years,natural ecological processes in the Tarim River Basin have changed significantly in response to human activities related to the exploitation and utilization of water resources,especially in the lower reaches from the Daxihaizi Reservoir to Taitema Lake.To determine the degree of degradation and restoration of species diversity that has occurred,8 representative sections,42 plant plots and 16 plant species were selected along the river channel between Daxihaizi Reservoir and Taitema Lake.A data matrix of environmental factors was then derived based on these sections.Monitoring of plant communities and environmental factors since implementation of the ecological water conveyance project in 2 000 was then conducted to evaluate the degree of ecological degeneration and spatiotemporal improvement that has occurred in the biodiversity of the lower reaches of the Tarim River.The results show that the degenerated ecosystem in the study area can be divided into three types using cluster analysis,slight,moderate and seriously degenerated areas.The slight degeneration of ecosystem occurs along the upper section of the lower reaches of the river,where there is a relatively shallow groundwater depth and high biodiversity index and vegetation coverage.Serious degeneration of the ecosystem can be found in the area near Taitema Lake,where the groundwater level is deeper than 10 m and there are only a few plant species.Moderate degeneration of the ecosystem occurs along the river section between these two areas.Groundwater depth is the most important factor affecting changes in biodiversity during the process of ecological restoration.When groundwater depth is shallower than 5 m,the possibility of species restoration is as high as 100%,while it is only 46% when the groundwater level is deeper than 5 m.Taken together,these results suggest that degraded ecosystems in the region should be degenerated in a stepwise fashion.