BULK PLANT AND SINGLE ORGAN CARBON ISOTOPIC VARIABILITY ALONG A PRECIPITATION GRADIENT

Plant fractionation of carbon isotopes during the uptake and fixation of CO₂ varies as a function of climate, but the extent and dependence of this variability on environmental conditions is not well understood. In a large-scale study of the variability of plant δ¹³C with precipitation, Diefendorf et al., (2010) found an isotopic shift to heavier values in drier environments. In this study we investigate the isotopic variability that occurs in C₃ plants and focus on a precipitation gradient limited to less than 1000 mm/yr. We also investigate the carbon isotopic variability in individual plant organs and how this contributes to the overall isotopic variability in C₃ photosynthesis of a particular environment. We find a weak trend with the average isotopic composition of C₃ plants at a given site, with heavier isotopic values being related to decreasing precipitation. This trend is less pronounced than the isotopic shift observed by Diefendorf et al. (2010) and Kohn (2010). While the larger dataset was only weakly correlated with MAP, our analyses reveal a negative correlation between MAP and δ¹³C for gymnosperms (R² = 0.60), which is potentially useful for paleoprecipitation reconstructions. We also find a significant amount of isotopic variability among individual plant organs, up to 5‰ in some species. This inter-organ variability has significant implications for paleoclimatology and paleovegetation reconstructions.