THE EFFECT OF GROUNDWATER LEVEL ON VEGETATION IN THE MIDDLE AND LOWER REACHES OF THE TARIM RIVER,XINJIANG,CHINA
塔里木河中下游地区不同地下水位对植被的影响

Since shallow groundwater is the main source of vegetation growth in arid zones, groundwater level is one of the important eco-environmental factors affecting natural vegetation in the middle and lower reaches of the Tarim River in Xinjiang of China. Therefore, it is important to understand the relationship between the groundwater level and the vegetation. Accordingly, 12 monitoring sections with 58 monitoring wells and 58 vegetation sample plots were selected and established. Based on several years of monitoring data, we describe the influence of different levels of groundwater level on soil moisture content, vegetation (species and coverage) and the proline (Pro) and abscisic acid (ABA) content in the leaves of Populus euphratica. Regression models on groundwater level-vegetation coverage (Y=159.32e -0.314 8X, R 2=0.819 3, p<0.01) and groundwater level-species (Y=9.113e -0.162 3X, R 2=0.606 7, p<0.01) were both significant. The effect of groundwater level on vegetation function is expressed through soil moisture content, and ground water level also was significantly related to soil moisture content by the following regression models: Y=64.898e -0.515X, R 2=0.727, p<0.01 (when the groundwater levels are between 1 to 4 m of the soil surface); and, Y=21.147e -0.178X, R 2=0.658, p<0.01 (when groundwater levels are between 4 to 12 m below the surface). The soil moisture content changes significantly when the groundwater level drops to 3.5-4.0 m below the surface, henceforth, 3.5 m is regarded as the lowest groundwater level acceptable for restoration of the natural meadow vegetation. By analyzing the changes of the Pro and ABA content in the leaves of P. euphratica, we determined that water stress develops in these populations when groundwater levels drop below 5.0 m depth and that the ABA content is a more sensitive indicator of water stress than Pro content in the leaves. This relationship is described by the following equation: ABA content=0.703 5e 0.408X, R 2=0.830 4, p<0.01. Our results indicate that ground water level is a critical ecological factor controlling the vegetation in the lower reaches of the Tarim River. Some xerophytic trees, meadows and bushes were restored by changing groundwater levels; hence, any vegetation restoration efforts in this region will need to manage ground water levels to be successful.