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.
