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

We compiled hydrogen and oxygen stable isotope compositions (δ²H and δ¹⁸O) of leaf water from multiple biomes to examine variations with environmental drivers. Leaf water δ²H was more closely correlated with δ²H of xylem water or atmospheric vapour, whereas leaf water δ¹⁸O was more closely correlated with air relative humidity. This resulted from the larger proportional range for δ²H of meteoric waters relative to the extent of leaf water evaporative enrichment compared with δ¹⁸O. We next expressed leaf water as isotopic enrichment above xylem water (Δ²H and Δ¹⁸O) to remove the impact of xylem water isotopic variation. For Δ²H, leaf water still correlated with atmospheric vapour, whereas Δ¹⁸O showed no such correlation. This was explained by covariance between air relative humidity and the Δ¹⁸O 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 ²H and ¹⁸O 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.