Tree-ring widths have long been a useful North American drought proxy [e.g., Cook et al., 1999, 2004]. A potentially rich, new tree-ring proxy associated with the tree's leaf-level moisture status is stable-carbon isotope composition (δ13C = [13C/12Csample ÷ 13C/12Cstandard − 1[ × 1000), which is determined by both the rate of carbon assimilation and the rate of gas conductance through leaf stomata [Farquhar et al., 1982]. In the U.S. Southwest, where evaporation exceeds precipitation, drought may be the dominant influence on plant δ>13C [Warren et al., 2001], so measurements of tree-ring δ13C in a network of southwestern sites has allowed for spatially mapping this ecophysiological indicator back to A.D. 1600.
Stomatal portals are the primary avenues of water loss and carbon gain in plants, providing carbon dioxide (CO2) for photosynthesis, which tends to discriminate against fixation of 13CO2 in favor of 12CO2. In principle, under conditions of water stress the stomata close down and the reservoir of CO2 available for continued photosynthesis is reduced, proportionally more 13CO2 is fixed, and the 13C/12C ratio of sugars eventually incorporated into tree rings increases (i.e., δ13C increases), and vice versa during moist conditions.