Southwestern U.S. drought maps from pinyon tree‐ring carbon isotopes

Abstract

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 (δ 13 C = [ 13 C/ 12 C sample ÷ 13 C/ 12 C standard − 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> 13 C [ Warren et al. , 2001], so measurements of tree‐ring δ 13 C 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 (CO 2 ) for photosynthesis, which tends to discriminate against fixation of 13 CO 2 in favor of 12 CO 2 . In principle, under conditions of water stress the stomata close down and the reservoir of CO 2 available for continued photosynthesis is reduced, proportionally more 13 CO 2 is fixed, and the 13 C/ 12 C ratio of sugars eventually incorporated into tree rings increases (i.e., δ 13 C increases), and vice versa during moist conditions.