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

森林从大气中吸收二氧化碳，从土壤中吸收水分，从而调节溪流中的水流。一些证据表明，后工业革命时期美国东部的高酸沉积（“酸雨”）可能从根本上改变了这些过程之间的平衡。在此期间，假设酸沉积减少了土壤中的钙、镁和钾等营养物质。这可能需要树木通过它们的系统从土壤中抽出更多的水，以获得足够的营养供应，从而导致溪流中水流的随后损失。随着过去20-30年酸沉降的减少，森林土壤能够保留更多的养分。这可能会减少树木需要泵入其系统的水量。因此，在保持同样的生长速度的同时，树木向大气中释放的水可能会减少，从而提高它们对水的利用效率，并留下更多的水流入溪流。目前，全球气候模式的陆地表面成分并未考虑森林对酸沉降水平变化的这种假设反应。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。该研究采用了一种基于流域的方法来调查酸沉降变化对森林水利用效率的影响。树木年轮碳同位素将用于重建六个集水区的历史森林水利用效率时间序列，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）。河流流量分析和基于模型的方法将用于检测酸沉降对森林水利用效率和河流流量的影响。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，并将参加实地和实验室活动。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。

Forests sequester carbon dioxide from the atmosphere and take up water from soils, thus regulating the flow of water in streams. Some evidence suggests that the balance among these processes may have been fundamentally altered by high acid deposition ("acid rain") in the eastern United States during the post-Industrial Revolution period. During this period it is hypothesized that acid deposition decreased nutrients in the soil such as calcium, magnesium and potassium. This may have required trees to pump more water from the soil through their systems to obtain an adequate supply of nutrients, causing a subsequent loss of water flowing in streams. With the decline in acid deposition over the last 20-30 years, forests soils are able to retain more nutrients. This may reduce the amount of water that trees need to pump through their systems. Thus while maintaining the same rate of growth, trees may release less water to the atmosphere, improving their water use efficiency and leaving more water to feed into streams. This hypothesized response of forests to changing levels of acid deposition is not currently considered in the land surface components of global climate models. 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 water use efficiency. Tree ring carbon isotopes will be used to reconstruct historical forest water use efficiency 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 water use efficiency and streamflow. 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 participate 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.