基于稳定同位素技术的黑河下游不同林龄胡杨的吸水深度研究

Study on the depth of water uptake by Populus euphratica trees of different ages in the lower reaches of the Heihe River,based on the stable isotope techniques

水是影响陆地生态系统植被分布最重要的因素之一,特别是在干旱地区,水资源将是限制植物生长的关键因素。不同植物具有不同的吸水深度,植物的吸水深度可影响它的分布区域,因此,精确测量植物的吸水深度对预测全球气候变化下植被的时空分布具有重要意义。稳定同位素技术逐渐被公认为是一种精确测量植物吸水深度的有效方法。利用稳定同位素技术分析了不同林龄胡杨的吸水深度和不同林龄胡杨的水分利用效率,对确定胡杨生存的合理地下水位、制定胡杨保育恢复策略具有重要意义。通过对黑河下游不同林龄胡杨木质部水及其不同潜在水源δD、δ^(18)O的测定分析,并运用深度模型计算了不同林龄胡杨的吸水深度,探讨了不同林龄胡杨的水分利用效率。结果发现:1)不同林龄胡杨木质部δ^(18)O差异显著,胡杨幼苗、成熟木、过熟木的δ^(18)O分别为-5.37‰、-6.03‰、-6.92‰;2)不同林龄胡杨的平均吸水深度不同,且具有随林龄变老而选择利用更深的土壤水分的特点:胡杨幼苗的平均吸水深度为37 cm,胡杨成熟木的平均吸水深度为145 cm,胡杨过熟木的平均吸水深度为149.5 cm。3)不同林龄胡杨水分利用效率不同,随林龄的变老水分利用效率具有降低的趋势。

Water availability is one of the most important factors affecting vegetation distribution in terrestrial ecosystems, especially in arid regions. Plant species vary in their abilities to absorb water from different soil depths. The depth from which plant species obtain water directly determines their distributions. Therefore, quantifying the depth of water uptake of dominant species iscritically important to defining and predicting vegetation spatiotemporal distributions under global change. Stable isotopes of oxygen and hydrogen have become an important tool for identifying the plant water uptake sources; this is because various water sources possess distinct oxygen or hydrogen isotope signatures. However, there has been little research on shifts in water sources with phenological development. Research on the mean depth of water uptake of trees of different ages is essential for formulating conservation strategies for the riparian tree, Populus euphratica. This study assessed the contributions of different potential water sources to P. euphratica. We used the Romero-Saltos model and levelsof stable oxygen and hydrogen isotopes （δD, δ18O） in the xylem of P. euphratica individuals of different ages and in soil water and groundwater along the lower Heihe River. For all the three age classifications （ young forest, mature forest and over-mature forest）, the shallowest soll water sample （5 cm in depth） had the highest or near highest measured values of δ18O： 4.4 ‰, 4.1‰, and 1.5 ‰, respectively. δD and δ18O decreased with increasing depth. The greatest rates of decrease occurred near the surface, reflecting evaporation of soil water near the surface. The similarity between deep soil and groundwater 6D and Bis O values of for all forest ages strongly suggests that the deep soil water is derived from groundwater. We found significant differences in δ18O values in the xylem among different ages of P. euphratica. The δ18O values of young, mature and over-mature forest reached -5.37‰, -6.03‰, and -6.92‰ respectively, reflecting the reliance of older trees on deeper sources of water with lower δ18O values, closer to those of groundwater. The mean depth of water uptake P. euphratica varied with age. The mean depths of water uptake of seedlings, mature and over-mature forest were 37, 145, and 149.5 cm, respectively. This indicates that P. euphratica accesses deeper soil moisture with age. The δ13 C values of young, mature and over-mature forest were - 26.30‰, - 27.53‰, and - 29.03‰, respectively. Similarly, we found significant differences in δ13O values of leaves among different ages of P. euphratica. The water use efficiency of young, mature, and over-mature forest were different, and tended to decrease with age. For all ages of forest, soil moisture peaked between 100 and 200 cm depth, which it is close to the mean water uptake depth of P. euphratica of all ages.