玉米农田生态系统水分利用效率对干旱的响应机制

The IPCC Fifth Assessment Report predicted that both of the frequency and intensity of drought would increase significantly in most of the regions over the globe. Drought can have a direct effect on the plant transpiration and photosynthesis by changing the meteorological factors instantaneously, and it can also have an indirect and lagged effect on the carbon and water cycles through regulating the canopy structure of the vegetation. However, there is a general lack of consensus to the response of the ecosystem WUE to drought and its controlling mechanisms up to present, which clearly restricted the deep understanding of the coupling between carbon and water cycles. Based on the long-term measurement data of eddy covariance system, sap-flow, and ancillary environmental factors (e.g., soil water content, meteorological and biological factors), the direct and indirect effects of drought on ecosystem WUE were both quantified using the structural equation modeling (SEM) in a rainfed maize cropland. The effectiveness of WUE indicators will be explored by comparing their performances when analyzing their responses to drought. This study would establish a new way to make quantitative analysis of the response of ecosystem WUE to drought. Simultaneously, the results would provide important scientific basis for predicting the effect of the future climate change on ecosystem carbon and water cycles.

IPCC第五次报告预测，未来全球多数区域的干旱强度及频率均呈增加趋势。干旱不仅会改变瞬时环境条件，对植物光合及蒸腾过程造成直接影响，还通过调控植被冠层结构对碳水循环产生滞后的间接影响，进而改变生态系统的水分利用效率（WUE）。然而，目前生态系统尺度WUE对干旱的响应研究尚处于初始阶段，WUE对干旱的响应特征仍存在极大不确定性，其控制机制更鲜见研究，严重限制了对碳水循环耦合关系的深入理解。本项目拟以雨养玉米农田为研究对象，基于长期涡度相关通量及茎流仪蒸腾等观测资料，利用结构方程模型（SEM）分解干旱对WUE的直接影响和通过调节植被结构等对WUE的间接影响，阐明生态系统尺度不同WUE指标对干旱响应差异的形成原因，并探讨不同WUE指标在研究对干旱响应时的适用性。本研究将为定量分析生态系统WUE对干旱的响应探索一条新路，同时为预测未来气候变化对生态系统碳水循环的影响提供重要科学依据。

IPCC最新报告预测，未来全球多数区域的干旱强度及频率均呈增加趋势。干旱不仅会改变瞬时环境条件，对植物光合及蒸腾过程造成直接影响，还通过调控植被冠层结构对碳水循环产生滞后的间接影响，进而改变生态系统的水分利用效率（WUE）。然而，目前生态系统尺度水分利用效率对干旱的响应研究尚处于初始阶段，水分利用效率对干旱的响应特征仍存在极大不确定性，其控制机制更鲜见研究，严重限制了对碳水循环耦合关系的深入理解。本研究利用结构方程模型，通过分析玉米农田的长期观测结果发现：日尺度的净生态系统碳交换和蒸发散的主要影响因子基本一致，均为净辐射、气温、饱和水气压差以及叶面积指数，净辐射主要是直接影响植物的生理活动，而气温和饱和水汽压差主要通过调控叶面积进而对植被功能产生间接影响。由于土壤湿度对植被功能的直接和间接影响可能相反，因此总体的净影响往往不显著（简单的线性回归往往不能发现植被功能对土壤水分的显著响应）。另外，在干旱和湿润年份，玉米生态系统对环境因子的响应有所不同。例如，在干旱和湿润年份，玉米农田净生态系统生产力对土壤湿度的响应方向刚好相反，水分利用效率也表现出类似的响应特征；对于蒸发散来说，干旱可能会增加玉米蒸发散对温度变化的敏感性，并降低蒸发散对辐射变化的敏感性。上述结论为准确估算生产力及预测未来气候变化对生态系统碳水循环的影响提供了重要的科学依据。