生态系统水分利用效率研究进展

Ecosystem level water use efficiency:A review

水分利用效率(WUE)是反映生态系统水碳循环相互关系的重要指标,开展生态系统水平WUE的时空变异性的研究有助于预测气候变化对生态系统水碳过程的影响。目前不同研究常常基于不同的算法估算生态系统WUE,一方面不同算法因包含了不同复杂程度的水过程而有着不同的内涵,另一方面各种算法又因包含了相同的核心过程而有着密切的联系。长期以来人们通过传统的生物量动态调查和生态系统水文过程的测定来估算生态系统的WUE,但该方法大大限制了在短时间尺度上对生态系统WUE进行分析,近年来发展起来的以涡度相关为代表的新技术的应用使得研究生态系统WUE在多个时空尺度上的变异特征取得了突破性的进展。生态系统WUE的主要影响因子与叶片尺度相似,主要有空气饱和水气压差(VPD)、土壤水分、大气CO2浓度、Ci/Ca等,另外,生态系统水分平衡特征也有着重要影响。比较分析表明,森林与草地生态系统WUE的日变化和季节变化存在显著的差异,同时森林和农田生态系统的WUE整体高于草地、荒漠和冻原。当前生态系统WUE的研究尚处于初始阶段,许多工作仍需深入开展,其中,多时间尺度以及生态系统间WUE的时空变异特征及机理的对比研究可能是未来工作的热点。

Water use efficiency （WUE） is a very important index for understanding the relationship between ecosystem carbon and water cycles. Studies on the spatiotemporal variations in WUE at the ecosystem level are essential for effectively predicting the impact of global climate change on ecosystem functions. At present, several algorithms are used for calculating ecosystem WUE. On the one hand, the different algorithms result in differences in the meanings for the different complexity of water processes involved, and on the other hand, they are closely associated with each other for the similar key processes involved. The traditional techniques for estimating ecosystem WUE are based on the measurements of vegetation biomass and soil hydrological parameters, but these approaches have limitation with estimating ecosystem WUE at finer timescales. Some new methods developed in the past years, especially the eddy covariance technique are the landmarks for studying ecosystem WUE at multiple timescales. Similar to the cases in the leaf level, the main factors affecting ecosystem WUE are vapour pressure deficit （VPD）, soil moisture, air CO2 concentration, etc. Besides, the water balance characteristics in ecosystem also play an important role in controlling ecosystem WUE. There are distinct differences in the daily and seasonal dynamics of ecosystem WUE between forest and grassland ecosystems. Meanwhile, the forest and crop ecosystems generally have higher WUE than grassland, desert and tundra ecosystems. Overall, current studies on ecosystem WUE are just at the early stage, and it is fairly needed to make further investigations into the variations in WUE at different spatiotemporal scales and their underlying mechanisms.