Controls on the D/H ratios of plant leaf waxes in an arid ecosystem

摘要

The extent to which leaf water D-enrichment (transpiration) and soil water D-enrichment (evaporation) affect the D/H ratio of plant leaf waxes remains a contentious issue, with important implications for paleohydrologic reconstructions. In this study we measure dD values of precipitation (dDp), groundwater (dDgw), plant xylem water (dDxw) and leaf water (dDlw) to understand their impact on the dD values of plant leaf wax n-alkanes (dDwax) in an arid ecosystem. Our survey includes multiple species at four sites across an aridity gradient (80–30% relative humidity) in southern California. We find that many species take up groundwater or precipitation without significant fractionation. D-enriched soil water is a minor source even in species known to perform and utilize waters from hydraulic lift, such as Larrea tridentata (+10&). Measurements of leaf water isotopic composition demonstrate that transpiration is an important mechanism for D-enrichment of leaf waters (+74 ± 20&, 1r), resulting in the smallest net fractionation yet reported between source water and leaf waxes (L. tridentata 41&; multi-species mean value is 94 ± 21&, 1r). We find little change in leaf water D-enrichment or net fractionation across the climatic gradient sampled by our study, suggesting that a net fractionation of ca. 90& may be appropriate for paleohydrologic reconstructions in semi-arid to arid environments. Large interspecies offsets in net fractionations (1r = 21&) are potentially troublesome, given the observed floristic diversity and the likelihood of species assemblage changes with climate shifts.