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Tree-ring carbon isotope data and drought maps
for the U.S. Southwest

The Physiological Basis
All the sites exhibited both
(a) a long-term downward trend (e.g., Kane Springs in Fig. 2), interpreted as the effect of decling δ13C ratios of atmospheric CO2 caused by fossil-fuel burning and land-use change (Leavitt and Long, 1988, 1989b), and
(b) high-frequency fluctuations that in many cases were coherent among sites hundreds of kilometers apart.

Analysis of those short-term fluctuations indicates they are related to climate, notably the moisture status of the plant (Leavitt and Long, 1989a). Other studies have likewise found a strong relationship of tree-ring δ13C with various moisture indicators such as drought indices (e.g., Leavitt, 1993; Leavitt et al., 2002), precipitation (e.g., Saurer et al., 1995; Hemming et al., 1998), soil moisture (e.g., Dupouey et al., 1993) and relative humidity (e.g., Saurer and Siegenthaler, 1989). The mechanism for this relationship is related to low moisture promoting stomatal closure (reduced stomatal conductance), reduced discrimination against δ13C in leaf photosynthesis, and therefore higher δ13C values of the resulting photosynthate (and tree rings) (e.g., Francey and Farquhar, 1982).
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