FACTORS CONTROLLING C ISOTOPE VALUES IN GINKGO BILOBA AND ITS USE AS A PALEOCLIMATE INDICATOR

Significant relationships between carbon isotope ratios and a variety of variables including mean annual precipitation (MAP) have been established for angiosperms. However, little attention has been paid to gymnosperms, which were the dominant vegetation type during the Permian and Mesozoic, and deciduous gymnosperms in particular have not been studied. The species Ginkgo biloba has potential significance as a paleoclimate indicator because it is the only extant species in the order Ginkgoales, which has a fossil record extending back to the Permian. Modern Ginkgo trees were analyzed in order to test for a correlation between δ¹³C value and a number of climate variables including MAP and mean annual temperature (MAT). Over 235 samples were analyzed, including >200 leaves as well as wood and reproductive structures. Samples represent sites with MAP ranging from 475 mm yr^-1 to 1767 mm yr^-1 and with MAT ranging from 6.7°C to 20.4°C. Variation in δ¹³C was quantified by comparing (1) values from different organs, canopy position, and short shoot vs. long shoot position, within a single tree, and (2) within-population variation for a large sample of trees from a warm-temperate region (Michigan) and Mediterranean region (California). Preliminary results show a relationship between MAP and δ¹³C value similar to that for angiosperms. Within populations, δ¹³C values of all organs ranged from -28.97 to -25.74 ‰ (Michigan) and from -31.18 to -23.87 ‰ (California), although leaves alone have a smaller range and varied by 2‰. Ongoing work will disentangle organ vs. developmental stage differences within a single tree and within populations of trees.