Establishing a grassland signature in veins: 18O in the leaf water of C3 and C4 grasses.

We show that 18O evaporative enrichment of bulk leaf water in grass species can be significantly more enriched than predicted by the Craig-Gordon model, with C4 grasses considerably more enriched than C3 grasses. Our results suggest that the unanticipated 18O leaf water enrichment of grasses is attributable to the progressive evaporative enrichment along parallel veins (a function of both leaf length and interveinal distance), a pattern that does not occur in Dicotyledonous species. We propose that the differential 18O enrichment of grasses will result in distinct C18O16O biospheric signals from grassland and forest ecosystems, allowing for further partitioning of terrestrial carbon fluxes.