宁夏河东沙地典型防护林树种水分来源及水分利用策略研究

Water is one of the essential substances to maintain the plant system. The growth and distribution of plants are closely associated with the degree of water use, especially in a arid region. How do plants make use of water and what are the changes in the mode of plants in the seasonal variations? The answers of these questions all depend on the study of the plant water sources. Water sources for plants are different by species, water environmental condition and botany life cycle, and even the plant water use strategy does. It is necessary to dig further into the underlying mechanism that controlling the water consumption of transpiration by taking the water sources and water use strategies into account. In this study, water movement tracing in Soil-Plant-Atmosphere system by stable hydrogen and oxygen isotopes technology combined with field investigation provide a new option to study the water sources and water use strategies of typical shelter-forest tree species (Populus bolleana, Ulmus pumila, Caragana korshinskii and Tamarix ramosissima) response to seasonal variations on a multi-time scale in Hedong sandy land of Ningxia. To clarify the influence of environmental factors on hydrogen and oxygen isotope distribution characteristics, we integrate the dynamic variation regularity of stable isotopes characteristics (δ D and δ18O) of water cycle component in different time scales, combined with field sampling and stable isotope analysis of rainfall, soil water, groundwater, condensation water and plant xylem water, as well as simultaneous records of micro-meteorological factors, hydrology, soil properties, plant root distribution and other environmental factors. Then a local meteoric water line (LMWL) equation is established. Such integration will provide the basis for investigating the soil-vegetation system response to seasonal dynamics of precipitation to disclose plant water use mechanism, as well as the changes in soil water of different layer. We also study the water isotope compositions and the contribution rate of different potential water sources based on direct inference method and multi-source mass balance method expecting to detect long-term patterns of plant water use, which will provide quantitative analysis on plant water sources, competition for water among different functional types, hydraulic lift by deep root plants, and rainwater use patterns by plants under natural conditions. It is expected that the elucidation of the water use strategies of different tree species and the mechanism of plant water adaption to drought environment will help to accurately estimate water consumption of forest and will provide the scientific basis for optimizing forest structure allocation of regional vegetation restoration and reconstruction in arid region.

植物的生长状况和分布格局都与水分的可利用程度紧密相关，尤其是在干旱半干旱地区，植物如何利用水分、植物在季节变化中的水分利用模式有何变化，这些问题的回答都离不开植物水分来源的研究。干旱沙区不同季节植物水分来源及水分利用策略研究，对评估蒸腾耗水对生态系统水量平衡的潜在效应具有重要意义。本项目基于氢氧稳定同位素示踪技术，以宁夏河东沙地典型防护林树种为研究对象，从多时间尺度研究大气-植被-土壤系统水分氢氧同位素的变化节律，分析环境因子对氢氧同位素分布特征的影响；研究不同季节植被-土壤系统对降水的动态响应，阐明植物水对各层土壤水变化的响应机制；采用直观法和多元混合模型法，定量区分不同季节降水、土壤水、地下水、凝结水等潜在水源对植物水分利用的贡献及利用途径，进而揭示沙区不同树种水分利用策略，为干旱半干旱区植被恢复与重建及林分结构的优化配置提供科学依据。

本项目以宁夏河东沙地典型防护林树种柠条、沙枣、刺槐和丝绵木为研究对象，观测研究大气-植被-土壤系统水分氢氧同位素的变化节律，定量区分植物水分来源，揭示沙区不同树种的水分利用策略。结果表明：（1）研究区降水氢氧稳定同位素组成存在明显的季节性变化，冬半年降水氢氧同位素组成明显偏正于夏半年；夏半年局地大气降水线（LMWL）的斜率、截距均低于冬半年和全年LMWL（δD=6.24 δ18O-5.63），夏半年降水受云下二次蒸发作用的影响程度更大。降水中δ18O的气温效应显著，气温效应为（0.211±0.031）‰•℃-1，降水量效应不显著。研究区降水的水汽来源主要为西风水汽，同时受局地蒸发水汽的影响。（2）随土壤深度增加，各林地土壤水δ18O值整体呈现出先减小后趋于稳定的趋势，其中沙枣林地土壤水δ18O变化范围为-7.3‰~-3.4‰，刺槐和丝绵木林地土壤水δ18O变化范围为-7.437‰~-1.496‰。凝结水δD和δ18O的变化范围分别为-71.1‰~-25.9‰和-7.5‰~-4.7‰，表现为夏季最大，秋季最小，年际差异较大；地下水的δD和δ18O春季和秋季δD和δ18O较小，夏季δD和δ18O相对较大，δD和δ18O范围分别为-61.53‰~-58.25‰、-7.47‰~-6.96‰。（3）不同季节植物水分利用策略不同，柠条随季节变化可在不同土层间转换主要的水分来源，生长季初期主要利用浅层土壤水（贡献率43.7%），6-9月转为利用中层土壤水，其利用率增加了13%；生长季后期，对中层土壤水的利用降低，而对浅层土壤水利用率增加至35.4%。沙枣、刺槐和丝绵木生长季主要利用中层土壤水，利用率分别为34.6%、37.2%、37.3%，但各月有差异，5-6月，刺槐主要利用浅层土壤水，沙枣和丝棉木主要利用中层土壤水，生长季中期至后期，刺槐对浅层土壤水的利用率降低，增加了对中层和深层土壤水的利用，而丝绵木增加了浅层土壤水的利用。（4）不同坡度柠条水分利用策略存在明显的季节差异，生长季初期，随着坡度的增大，柠条对中层土壤水利用率呈先增大再逐渐减小的趋势；生长季中期至后期，随坡度的增大，其对土壤水的利用先变浅再加深。刺槐蒸腾耗水量约为丝绵木蒸腾耗水量的1.5倍，在土壤较干旱的季节，刺槐和丝棉木主要通过水分利用空间上的差异来应对干旱。研究成果可为区域植被建设及林分结构优化配置提供依据。