杨树人工林水分利用效率对开放式臭氧浓度升高的多尺度响应机制

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.

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。

随着我国工业化和城市化进程的加快，近地层臭氧浓度不断升高，严重威胁了树木碳固定和水分利用。本项目利用开顶箱（OTC）和开放式臭氧浓度升高模拟系统(O3-FACE)，从叶片、个体和林分尺度，研究了臭氧对树木水分利用效率的影响。气体交换研究结果表明，臭氧浓度升高抑制了叶肉导度和光合生化能力，导致树木幼苗叶片光合速率显著降低，但对气孔导度和蒸腾速率无显著影响。因此臭氧浓度升高导致叶片水分利用降低主要归咎于叶肉导度和光合生化能力下降。耦合气体交换和碳氧同位素结果发现，低臭氧浓度引起了气孔关闭，导致胞间CO2浓度下降，进而降低了叶片光合作用，但是增加了叶片的水分利用效率。而高浓度臭氧会引起气孔控制失灵，导致进入气孔臭氧通量增加，会显著降低光合生化能力，进而导致了光合速率降低，最终显著减少叶片水分利用效率。在生长季节内，随着臭氧暴露剂量的增加，叶片光合速率和水分利用效率持续降低，并加快叶片衰老和加速凋落。此外随着树木不断生长，杨树对臭氧抗性不断增强。气体交换和同位素的数据均表明臭氧浓度升高对4年生杨树叶片光合速率、气孔导度和水分利用效率无影响。株高、树干液流和胸径生长的多年监测数据表明，臭氧浓度升高未显著影响杨树的生长。但在2021年6-7月持续的高浓度臭氧导致了杨树水分消耗的增加，可能导致整株尺度上水分利用效率降低。综上所述，本项目通过开展多年实验揭示了不同臭氧剂量影响叶片尺度水分利用效率差异的机制，并建立了气孔臭氧吸收通量与水分利用效率的剂量关系。最后指出了幼苗和成年杨树对臭氧的敏感性不同。本项目成果将为评估地表臭氧影响树木固碳耗水规律提供了重要的科学依据。