This project asks the general question of how ecosystems composed of different types of plants use water and process carbon dioxide. Specifically, this project will evaluate the differences in water-use and carbon processing in grassland versus shrubland ecosystems of the semi-arid, southwestern U.S. In this region, water-use by vegetation can influence stream behavior and the subsequent availability of water for human endeavor. Additionally, how vegetation in this region processes carbon from the atmosphere may influence the carbon sink-versus-source characteristics of North America. This project will evaluate water-use and carbon processing at five sites in both riparian and upland settings each with a gradient of vegetation types (between grass-dominated and shrub-dominated) will be evaluated. The project will measure the photosynthetic behavior of whole ecosystems, along with patterns of vegetation water-use as assessed via large-scale meteorological techniques. Rainfall simulations will be employed in grasslands and shrublands to understand how a change in rainfall events affects the way these ecosystems function. This work is motivated by the expected shift in vegetation composition within North America's biomes in the next century. This vegetation change is arguably the most ubiquitous and significant manifestation of global change. The current increase in the density of woody plants in North America's grassland and savanna ecosystems may be an important contributor to the North American carbon sink. How such a change in vegetation composition affects landscape water budgets is unknown, yet may directly influence society in water-limited regions of the globe. On top of these issues are the expected shifts in the timing and magnitude of rainfall in North America. Together these issues suggest the need for a greater understanding of how vegetation type, water-use, carbon processing and variation in climate (primarily rainfall) are interlinked. This project will contribute substantially to the goals of the National Science Foundation in the production of interdisciplinary knowledge and education of students. This program represents a collaboration among several researchers with distinct backgrounds and areas of expertise and hence will foster novel interdisciplinary approaches cutting across plant physiological ecology, ecosystem ecology and hydrology. Graduate students, including females and underrepresented groups, will be involved in all aspects of the science proposed. Graduate students working on this project will disseminate the information and enthusiasm derived from this project to others through K-12 Teaching / Research Fellowships. The research goals and individual members of this project are critical components of the long term focus of the NSF Science and Technology Center "Sustainability of semi-Arid Hydrology and Riparian Areas" (SAHRA-www.sahra.arizona.edu). This project will further the goals of the Upper San Pedro Partnership, a consortium of 20 agencies and organizations working to meet the water needs of area residents while protecting the San Pedro River (www.usppartnership.com). Undergraduate students employed in this project will be recruited from a program at the University of Arizona (Undergraduate Biology Research Program) that actively encourages research opportunities for under-represented groups.
