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

We compiled hydrogen and oxygen stable isotope compositions (δ(2)H and δ(18)O) of leaf water from multiple biomes to examine variations with environmental drivers. Leaf water δ(2)H was more closely correlated with δ(2)H of xylem water or atmospheric vapour, whereas leaf water δ(18)O was more closely correlated with air relative humidity. This resulted from the larger proportional range for δ(2)H of meteoric waters relative to the extent of leaf water evaporative enrichment compared with δ(18)O. We next expressed leaf water as isotopic enrichment above xylem water (Δ(2)H and Δ(18)O) to remove the impact of xylem water isotopic variation. For Δ(2)H, leaf water still correlated with atmospheric vapour, whereas Δ(18)O showed no such correlation. This was explained by covariance between air relative humidity and the Δ(18)O 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 (2)H and (18)O 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.