The dominant environmental driver of leaf water stable isotope enrichment differs for 2H compared to 18O
A number of important isotopic biomarkers derive at least part of their signal from leaf water stable isotope composition (e.g., leaf wax 2H, cellulose 2H and 18O, lignin 18O). In order to interpret these isotopic proxies, it is therefore helpful to know which environmental signal most strongly controls leaf water stable isotope enrichment. We collated observations of the stable isotope compositions of leaf water, xylem water, and atmospheric vapour, along with air temperature and relative humidity, to test whether the dominant driver of leaf water enrichment for 2H could differ from that for 18O. Our dataset comprises 620 observations from 39 sites with broad geographical coverage. We limited our analysis to daytime observations, when, for the most part, physiological processes forming isotopic proxies based on leaf water take place. The Craig-Gordon equation was generally a good predictor for daytime leaf water stable isotope composition for both 2H (R2=0.93, p<0.001) and 18O (R2=0.70, p<0.001). The Craig-Gordon equation requires knowledge of relative humidity, air temperature, and the stable isotope compositions of xylem water and atmospheric vapour. However, it is not possible to invert the Craig-Gordon equation to solve for one of these parameters unless the others are known. Here we show that the two isotopic signals of 2H and 18O are predominantly driven by different environmental variables: leaf water 2H correlated most strongly with the 2H of atmospheric vapour (R2=0.79, p<0.001), whereas leaf water 18O correlated most strongly with air relative humidity (R2=0.45, p<0.001). We conclude that these two isotopes are not simply mirror images in the environmental information that they carry in leaf water, with crucial implications for interpretation of isotopic biomarkers which record leaf water isotopic composition.