Tree-ring widths have long been a useful North American drought proxy. 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. In the U.S. Southwest, where evaporation exceeds precipitation, drought may be the dominant influence on plant δ13C, 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.