ANALYZING MODERN DROUGHT RECORDS IN SOUTHEASTERN GEORGIA USING CARBON AND OXYGEN ISOTOPES AND TREE RINGS

Stable isotope analyses have proven to be useful alternatives to ring-width analyses for elucidating the impacts of various climate parameters on tree growth. The combination of ring-width and stable isotope analyses, evaluated in the context of instrumental meteorological records, may allow us to develop better strategies for interpreting long-term drought fluctuations in the dendrochronological record. Previous research from Valdosta, Georgia, using seasonally resolved oxygen isotope and ring width data, revealed significant relationships between both proxies and the regional instrumental Palmer Drought Severity Index (PDSI) for 1895–present. We hypothesize that this relationship can be strengthened by the addition of carbon isotope ratios, which are useful to distinguish moisture stress, and therefore assist in the interpretation of ¹⁸O-enriched isotope ratios (i.e., moisture source vs. moisture stress). For this study, annual rings from longleaf pine (Pinus palustris Mill.) were subdivided into their seasonal earlywood (EW) and latewood (LW) components and measured to the nearest 0.001 mm. EW and LW α-cellulose isotopic compositions were analyzed and time-series correlations were conducted using all three proxies (oxygen isotopes, carbon isotopes, and EW/LW widths) against monthly PDSI data to determine significant months for tree growth for each proxy. Significant months were then seasonalized and re-analyzed using correlation analyses. Ring-width studies typically only allow for the analysis of a small portion of the growing season. The addition of stable isotope proxies will allow for the analysis of drought effects over a greater portion of the growing season. Initial results indicated significant relationships between d¹⁸O and ring-width values with respect to drought that spanned the entire growing season (March–October). Correlations between LW d¹⁸O and drought were impacted by the effects of frequent tropical cyclone systems (hurricanes) on LW isotopic compositions. The carbon isotope ratios increase the correlation between seasonal isotopes and drought.