Collaborative Research: Forest Water Use and the Influence of Acid Deposition

森林水分利用效率（WUE）定义为单位蒸腾蒸发损失的碳吸收量之比。微气象测量表明，森林WUE在过去二十年中急剧增加，超过了大气二氧化碳浓度增加的预期值。与观测到的森林用水效率趋势相一致的是，北美和欧洲大部分地区的酸沉降明显减少。有证据表明，酸沉降可能通过改变森林土壤中养分的有效性或直接影响叶片生理来影响森林水分利用效率。用水效率的变化也可能导致森林集水区流量的变化。目前，全球气候模式（GCMs）的陆地表面成分并未考虑森林对酸沉降水平变化的假设响应。由于二氧化碳和水蒸气是两种最重要的温室气体，因此精确模拟它们的陆地-大气交换是至关重要的。本研究采用一种基于流域的方法来研究酸沉降变化对森林水分利用效率的影响。树木年轮碳同位素将用于重建六个集水区的历史用水效率时间序列，这些集水区由于其底层基岩矿物学和地质历史的差异而受到酸沉积的不同影响。该研究还将利用改变配对集水区土壤生物地球化学的试验性处理方法（缅因州贝尔布鲁克、新罕布什尔州哈伯德布鲁克和西弗吉尼亚州费诺实验森林）。溪流流量分析和基于模型的方法将被用于检测酸沉降对森林WUE的影响。该项目将支持一名博士生，他将接受广泛的领域、实验室和建模技术方面的培训，以及至少八名本科生。高中学生也将通过已建立的项目参与，并将包括他们参与实地和实验室活动。项目团队的所有高级成员都有从代表性不足的群体中招募学生的记录。结果将通过期刊出版物和会议报告传达给更广泛的科学界，并将组织一次特别会议，目的是与建模界分享研究成果。

Forest Water Use Efficiency (WUE) is defined as the ratio of carbon uptake per unit water vapor loss via transpiration. Micrometeorological measurements suggest that forest WUE has dramatically increased over the last two decades, in excess of what would be expected from increases in atmospheric carbon dioxide concentrations. Coinciding with observed trends in forest WUE have been marked decreases in acid deposition throughout much of North America and Europe. There is evidence that acid deposition may impact forest WUE, either by altering the availability of nutrients in forest soils or by directly affecting foliar physiology. Changes in WUE could also lead to changes in stream discharge from forested catchments. The hypothesized response of forests to changing levels of acid deposition is not currently considered in the land surface components of global climate models (GCMs). Since carbon dioxide and water vapor are the two most important greenhouse gases, it is vital to accurately model their land-atmosphere exchange. The proposed research uses a catchment-based approach to investigate the effects of changing acid deposition on forest WUE. Tree ring carbon isotopes will be used to reconstruct historical WUE time series within six catchments that have been differentially impacted by acid deposition due to distinctions between their underlying bedrock mineralogy and geological histories. The research will also capitalize on experimental treatments that have altered soil biogeochemistry in paired catchment designs (Bear Brook, ME; Hubbard Brook, NH; and Fernow Experimental Forest, WV). Stream discharge analysis and model-based approaches will be applied to detect the impact of acid deposition on forest WUE. The project will support a Ph.D. student, who will receive training in a wide range of field, laboratory, and modeling techniques, and at least eight undergraduate students. High school students will also be involved through established programs, and will include their participation in both field and lab activities. All senior members of the project team have track records of recruiting students from underrepresented groups. Results will be communicated to the broader scientific community through journal publications and conference presentations, and a special conference session will be organized with an objective of sharing research findings with the modeling community.