塔里木河下游胡杨径向生长与地下水的关系

本文通过分析塔里木河下游地区地下水位与胡杨径向生长量的相互关系，建立胡杨径向生长量随地下水位变化的函数模型，并从数理统计学的角度验证了胡杨生长的胁迫水位和临界水位。结果表明：1）随着地下水位的下降，胡杨径向生长量的均值逐渐减小。2）胡杨径向生长量与地下水位之间存在极显著的回归关系，其最佳回归模拟函数为四次多项式Y=0.003X4-0.080X3 0.731X2-3.060X 6.934，判定系数R2为0.857，P值为0.0001。3）通过分析可知：胡杨生长的胁迫水位是4.71 m，临界水位是8.62 m。胡杨正常生长的水位区间为0.5—4.71 m，受到胁迫的水位区间为4.71 m—8.62 m，当地下水位大于8.62 m时，胡杨将会呈现出衰败景象，此时，胡杨径向生长量的变化率逐渐向0接近，表明胡杨径向生长趋向停止。

The relationship between Populus euphratica's radial increment and groundwater level at the lower reach of Tarim River

The Tarim River, the longest inland river in China, is one of the famous continental rivers in the word. The climate in the Tarim River basin is a typical continental warm-temperate desert arid one. The annual precipitation varies from 17 mm to 34 mm, however, the annual evaporation reaches 2408-2671 mm. The ecological processes of the Tarim River basin have undergone great changes due to human disturbance over the past 50 years, especially along the river section at the lower reaches from the Daxihaizi Reservoir to the Taitema Lake, where the ecological environment is very fragile by nature. The problems, such as disappearance of surface runoff, decline of groundwater level, degradation of watershed ecosystems etc, has become more and more aggravated. In order to rescue the strategic "green corridor" in the lower reaches of Tarim River, ecological water input has been brought into effect since May 14^th, 2000. The groundwater level is the most important environmental factors affecting natural vegetation in the lower reaches of Tarim River. Accordingly,during the water transfer,12 monitoring sections with 58 monitoring wells and 58 vegetation sample plots were selected and established. Since the growth rings of trees can reflect the inter-annual variations in environmental conditions,in the current study we used a model to simulate the relationship between radial increment of P. euphratica and groundwater level at the lower reaches of Tarim River. The monitoring wells data of 4 typical sections: Yehepumahan(B), Yengisu(C), Kardayi(E) and Argan(G) were used in the analysis. Meanwhile, 3 Populus euphratica trees were randomly selected around the monitoring well and then 3 or 4 branches from each tree. Subsequently, the sample branches' radial increments were measured and averaged. Based on the monitoring data of groundwater level and the corresponding radial increments from 2000 to 2005, we used statistics to determine the groundwater levels at which the P. euphratica growth was inhibited or stopped by water stress. Our study revealed that: 1) The mean radial increment gradually decreased with the groundwater level declining. 2) The radial increment correlated to the groundwater level at a significance level of 0.01. The best regression model is a quadric equation, that is, Y=0.003X⁴-0.080X³+0.731X²-3.060X+6.934, with discriminate coefficient R² as 0.857 and P=0.00013) Based on the analysis, we found the stress groundwater level for P. euphratica's growth is 4.71 m and the critical groundwater level is 8.62 m. When the groundwater level ranged from 0.5 m to 4.71 m, P. euphratica normally grew; when it ranged from 4.71 m to 8.62 m, P. euphratica grew under stress; when it dropped to more than 8.62 m, the radial increment closed to 0, and the growth of P. euphratica nearly stopped. After then, further decline in groundwater had little effect on the P. euphratica's growth.