DROUGHT INDICATED IN CARBON‐13/CARBON‐12 RATIOS OF SOUTHWESTERN TREE RINGS

ABSTRACT: Stomatal closure during periods of moisture deficiency should theoretically lead to elevated ¹³C/¹²C ratios as reduction of available CO₂ leads to diminished photosynthetic discrimination against ¹³C in favor of ¹²C. Stable‐carbon isotope ratio chronologies developed from 5‐yr tree‐ring groups at 17 sites in six southwestern states were tested for a drought relationship by first fitting a spline curve to each chronology to remove the long‐term trend and calculating indices as the ratio of actual to spline curve value. The time series of “Del Indices” so developed are significantly correlated with 5‐yr mean Palmer Hydrological Drought Indices (post‐1930 period) and reconstructed July Palmer Drought Severity Indices from respective areas. Overall, in the period since 1790, the driest pentads were 1900–04 and 1960–64, whereas the wettest were 1980–84 and 1915–19. Maps of drought represented for two pentads seem to be reasonable representations, although spatial correlations of Del Indices with PHDI were generally not significant. These Del Index drought reconstructions may provide a useful measure of past physiological response to drought (stomatal closure), although the present cost of analysis would prevent this from being a routine method.