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

森林用水效率（WUE）定义为单位蒸腾损失的水蒸气吸收的碳的比率。微气象测量表明，森林水分利用效率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。与观察到的森林 WUE 趋势相一致，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利用率或直接影响叶的生理来影响森林的水分利用效率。 WUE 的变化也可能导致森林流域河流流量的变化。 全球气候模型（GCM）的地表组成部分目前尚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，因此准确模拟它们的陆地-大气交换至关重要。本研究采用基于流域的方法来研究酸沉降变化对森林 WUE 的影响。树木年轮碳同位素将用于重建六个流域内的历史 WUE 时间序列，由于其底层基岩矿物学和地质历史之间的差异，这些流域受到酸沉积的不同影响。该研究还将利用在配对流域设计中改变土壤生物地球化学的实验处理（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）。将应用河流流量分析和基于模型的方法来检测酸沉降对森林 WUE 的影响。该项目将支持一名博士学位。学生将接受广泛的领域、实验室和建模技术方面的培训，以及至少八名本科生。高中生也将参与既定计划，包括现场和实验室活动。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演讲向更广泛的科学界传达，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.