DROUGHT INDICATED IN CARBON-13lCARBON-12 RATIOS OF SOUTHWESTERN TREE RINGS1

Stomata1 closure during yriods of moisture deficiency should theoretically lead to elevated C/12C ratios as reduction of available C02 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. (KEY TERMS: stableearbon isotopes; C-13lC-12; pinyon pine; Pinus edulis; drought; Palmer Drought Indices; Southwest.) INTRODUCTION relates to changes in the 13C/12C ratio of carbon fixed by plants (Francey and Farquhar, 1982). In principle, when abundant COP is available, plants tend to discriminate against 13C02 relative to 12C02 during photosynthesis. When stomatal closure cuts off COP supply, continued carbon fixation is then necessarily less discriminating in order to sustain operation under reduced C02 availability. We have found evidence that those tree rings (actually 5-yr ring groups) with high 13C/12C ratios are also narrow rings (drought conditions) (Leavitt and Long, 1985a; 1986; 1987). Elevated 13C/12C ratios would also be favored by high COP fixation rates depleting COP (Francey and Farquhar, 1982). Thus, the narrow rings associated with high ratios (Leavitt and Long, 1986) could also result from relatively high fixation rates (high gross photosynthesis) with accompanying increased respiration, producing reduced net photosynthesis in (hot) drought periods (Fritts, 1976). This link between stomatal closure and 13C/12C ratios in tree rings forms the basis for a new potential method of climate reconstruction that merits exploration. In this paper we derive isotopic drought indices from long 613C ("del C-13") chronologies determined from One of the primary methods of reconstructing sites in six southwestern states, compare them to other precipitation and drought in the western U.S. has been climate measures (direct and proxy), and construct through tree-ring width measurements and analysis maps based on these indices. The value of 613C is (e.g., Stockton and Meko, 1975; Meko, et al., 1980; obtained by: Michaelsen, et al., 1987). Narrow rings are consistent 13C/12C(sample) with one or more of a number of physiological responses 613C (%o units) = [mC,uC(standard) 1 1 x 1000 (1) to stress associated with low precipitation andlor high temperatures (Fritts, 1976). one of these physiologic-al responses during the growing season is increased where del C-13 in permil (%0) units is expressed as the daytime stomatal closure, and it is this response of 13C/12C ratio of the sample with respect to the same decreased stomatal conductance in particular that ratio of the PDB carbonate standard (Craig, 1957). lpaper No. 88028 of the Water Resources Bulletin. Discussions are open until December 1,1989. ' ~ e s ~ e c t i v e l ~ , Assistant Professor, Department of Geology, University of Wisconsin-Parkside, Kenosha, Wisconsin 53141; and Professor, Departments of Geosciences and Hydrology & Water Resources, University of Arizona, Tucson, Arizona 85721. WATER RESOURCES BULLE1-IN Leavitt and h n g . . .