基于树木年轮技术的塔里木河下游河岸胡杨林生态需水量研究

生态需水量计算是干旱脆弱生态区恢复重建的一个关键问题。分析了塔里木河下游荒漠河岸林关键种-胡杨树木年轮近90年来的变化特征及对气候水文过程的响应,并基于树木年轮技术提出了维系荒漠河岸林不同恢复状态的生态需水量。研究结果表明,塔里木河下游胡杨树木年轮主要承载的是区域水文历史变化信息,可以作为定量评估生态输水工程的生态恢复效应和定量计算植被生态需水量的新指标;胡杨标准年轮宽度指数与生长年生态输水量呈显著正相关(P0.05),近15年生态输水对胡杨年轮宽度指数增加的平均贡献率为42.96%;若要维持塔里木河下游英苏段垂直河道300 m范围内的胡杨达到近90年来的平均生长水平,则需在生长年下泄生态需水量0.84×10~8m~3,若要达到断流前(1933—1974年)的平均径向生长,则每个生长年内生态需水量应达到0.91×10~8m~3。

Estimation of ecological water demand of a desert riparian forest using tree rings of Populus euphratica in the Lower Tarim River

Estimation of the ecological water demand of natural plants is a key issue for the restoration and reconstruction of a fragile ecological region in arid areas. Tree rings of Populus euphratica in the Yingsu section of the lower reaches of the Tarim River and their responses to climate change and hydrological conditions were analyzed using the nonparametric Mann-Kendall trend test, Mann-Kendall-Sneyers test, moving t-test, one-way ANOVA, and Pearson correlation. The ecological water demand of a desert riparian forest in the Yingsu section was then estimated based on the tree ring characteristics using regression and stepwise regression models. The results showed that the abrupt temporal changes of temperature and precipitation were different from that of the standard chronology of P. euphratica in the lower reaches of the Tarim River. Moreover, there was no significant relationship between temperature and standard chronology of P. euphratica or between precipitation and standard chronology of P. euphratica in the lower reaches of the Tarim River. The Pearson correlation analysis also revealed no significant relationship between temperature and groundwater depth or between precipitation and groundwater depth in the lower reaches of the Tarim River. However, the width indexes of tree rings of P. euphratica were significantly and negatively related to the groundwater depth, and they were significantly and positively related to the amount of ecologically conveyed water from August to July in the Yingsu section of the lower reaches of the Tarim River. These results proved that historical information regarding water conditions could be accurately reproduced by the tree rings of P. euphratica in the lower reaches of the Tarim River. Therefore, the tree rings of P. euphratica could be used for quantitatively assessing the restoration effects of ecological water conveyance to desert riparian forests and calculating the ecological water demand for natural vegetation in the lower reaches of the Tarim River. The average contribution of ecological water conveyance to the increased tree ring width index was 42.96% in the Yingsu section of the lower reaches of the Tarim River. Estimated from the equation of the tree rings of P. euphratica and the amount of ecologically conveyed water, an annual ecological water conveyance of 0.84×10⁸ m³ was expected to maintain the average growth (i.e., average tree ring index of 0.997 during 1933-2015) of P. euphratica in the Yingsu section within a distance of 300 m to the river channel in the lower reaches of the Tarim River. Furthermore, an annual water conveyance of 0.91×10⁸ m³ was estimated to keep P. euphratica at an average radial growth rate of 0.972 during the period of 1933-1974 (i.e., before the main channel of the Tarim river near Yingsu section dried up).