近地层臭氧（O3）浓度升高影响叶片气孔行为，直接影响林木碳同化和水分利用。从多个尺度上研究O3对林木水分利用效率的影响对系统理解和准确评估全球变化对生态系统碳循环和水循环及其耦合关系的影响具有重要科学意义。本项目拟选择我国北方广泛种植的杨树为研究对象，利用最接近自然生态环境的O3浓度增加模拟系统（O3-FACE），采取红外气体分析法、稳定同位素法、树干液流技术等野外原位观测与尺度扩展模型相结合的方法，分别从叶片、个体和林分水平研究O3浓度升高对林木碳同化-水分利用耦合过程的多时间尺度（瞬时、日、季节、年际变化）的影响机制，探究不同尺度水分利用效率的差异及其对环境因子的响应机理，为陆地生态系统碳-水耦合模型在时空尺度上的扩展提供理论依据。建立基于冠层导度的O3吸收量与水分利用效率的响应模型，评估环境O3浓度对杨树林固碳和耗水的区域影响，为人工林水分管理与防护提供科学依据。

Ground-level ozone (O3) concentration is rising at a high rate so that it often induces foliar stomatal closure or sluggish or out of control, finally leading to large changes in carbon assimilation through photosynthesis and water use through transpiration of leaves. Therefore, it is of great scientific significance to investigate the effects of O3 on forest water use efficiency at multiple scales in order to understand and evaluate the impacts of global change on ecosystem carbon cycle and water cycle systematically and accurately. In this project, two poplar clones widely planted in Northern China with significant difference in stomatal conductance will be selected. The multi-year impacts of elevated O3 on the coupling process (instantaneous,daily,seasonal and interannual variation) between carbon assimilation and water use will be investigated. The scale differences in water use efficiency and its response mechanism to environmental factors will be explored from the leaf, plant and stand levels using both field experiment and model analysis approaches, e.g.infrared gas analysis, stable isotope method and stem sap flower technique under free-air O3 concentration enrichment (O3-FACE), which is the most close to natural ecological environment among all gas enrichment simulation systems so far. The output of this project will provide a theoretical basis for the expansion of the carbon and water coupling model of terrestrial ecosystem at both spatial and temporal scales.The model on the relationship between O3 uptake and water use efficiency will be developed based on canopy conductance and thus the regional effects of O3 on carbon sequestration and water consumption in poplar forest will be evaluated. The output of this project will also provide a scientific basis for water mangement and protection of Poplar Plantation.