Do H-2 and O-18 in leaf water reflect environmental drivers differently?

We compiled hydrogen and oxygen stable isotope compositions (delta H-2 and delta O-18) of leaf water from multiple biomes to examine variations with environmental drivers. Leaf water delta H-2 was more closely correlated with delta H-2 of xylem water or atmospheric vapour, whereas leaf water delta O-18 was more closely correlated with air relative humidity. This resulted from the larger proportional range for delta H-2 of meteoric waters relative to the extent of leaf water evaporative enrichment compared with delta O-18. We next expressed leaf water as isotopic enrichment above xylem water (Delta H-2 and Delta O-18) to remove the impact of xylem water isotopic variation. For Delta H-2, leaf water still correlated with atmospheric vapour, whereas Delta O-18 showed no such correlation. This was explained by covariance between air relative humidity and the Delta O-18 of atmospheric vapour. This is consistent with a previously observed diurnal correlation between air relative humidity and the deuterium excess of atmospheric vapour across a range of ecosystems. We conclude that H-2 and O-18 in leaf water do indeed reflect the balance of environmental drivers differently; our results have implications for understanding isotopic effects associated with water cycling in terrestrial ecosystems and for inferring environmental change from isotopic biomarkers that act as proxies for leaf water.