Ecological Interpretation of Hydrogen and Oxygen Isotope Ratios in Tree-Ring Cellulose

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9508671 The relationship between seasonal and regional patterns of precipitation and the use of those water inputs by plants has ramifications at several levels and time scales that are of both basic and applied interest. Identification of measurements that can provide quantitative assessment of specific water use patterns (that is, use of precipitation versus groundwater) by plants would make a major contribution to understanding that relationship. This research will test the hypothesis that stable isotope ratio analysis of hydrogen and oxygen isotopes at natural levels in wood will provide that information, at both short-term and long-term scales. Its goal is to develop, test, and apply quantitative models of the factors that influence isotope ratios of cellulose in tree rings. The tree ring information will allow reconstruction of seasonal and annual patterns of water use and humidity conditions during the growing season over the life of the tree. The model will be developed through laboratory experiments using four species of trees common in the western United States. It will be tested through field observations at sites chosen because they differ in water sources and humidity conditions. The model will then be used to reconstruct the history of water use (precipitation versus ground water), atmospheric humidity (leaf to air gradients) and relative water use efficiency (through carbon isotope discrimination) by the four species in Utah from seedling establishment through maturity. Once quantitative, mechanistic models of tree- ring isotope ratios are developed, they can be applied to unraveling historic patterns of seasonal precipitation across the landscape, sensitivity of physiological processes to changes in moisture sources and humidity, and aspects of water relations that contribute to the long-term changes in tree community structure. This r esearch will allow for scaling of physiological parameters from individual to landscape levels and will provide a direct link between physiology and global-level studies of biosphere-atmosphere interactions.