TEMPORAL VARIATIONS IN HYDROGEN AND CARBON ISOTOPES: UNDERSTANDING HOW PHYSIOLOGICAL DIFFERENCES BETWEEN TAXONOMIC CLASSES AFFECT ISOTOPIC FRACTIONATION IN PLANT LEAF WAXES

Hydrogen (δD) and carbon (δ¹³C) isotope composition of individual waxes from terrestrial plant are widely used as proxies for paleo-precipitation δD and paleoenvironmental change. Despite considerable data on the relationships between water and wax dD and environmental factors and leaf wax δ¹³C, there is considerable uncertainty in how the timescale, physiological variation, and water stress affects hydrogen and carbon isotopic fractionation in plants with different water use strategies and stomatal regulation. We sampled leaves from four riparian plant species with distinct functional types growing on the banks of the Fenton River (Storrs, CT) throughout the 2013 growing season, to assess controls of hydrogen and carbon isotope discrimination. Our data shows that plants from different functional classes produce distinct temporal patterns in δ¹³C and δD related to timescales of wax synthesis and stomatal regulation. Common Reed Canary grass (P. arundinacea) began the growing season with D-enriched leaf waxes, which became steadily more depleted throughout the growing season, while its carbon isotopes shows steady isotopic enrichment throughout the growing season. This opposing pattern of enrichment and depletion between carbon and hydrogen occurs in the deciduous angiosperm American Hazerlnut (C. americana) as well. During leaf flush (weeks 1-5) hydrogen is significantly enriched (approx. 40‰), while carbon is depleted (approx. 3‰). For the White Pine (P. strobus), we see depletion of hydrogen isotopes throughout the growing season, while carbon isotopes show minimal changes. These patterns indicate that differences in stomatal regulation and WUE for different plant functional types drives the large differences in carbon and hydrogen fractionation. Overall, soils record a site-averaged response that integrates multiple plants and may provide an ecosystem level indicator of environmental conditions. However, data from individual leaves can be coupled with plant-specific information such as stomatal density and size, to examine temporal changes in plant water use efficiency and carbon assimilation.