Leaf wax n ‐alkane δ D values are determined early in the ontogeny of Populus trichocarpa leaves when grown under controlled environmental conditions

The stable hydrogen isotope ratios ( δ D) of leaf wax n ‐alkanes record valuable information on plant and ecosystem water relations. It remains, however, unknown if leaf wax n ‐alkane δ D values record only environmental variation during the brief period of time of leaf growth or if leaf wax n ‐alkane δ D values are affected by environmental variability throughout the entire lifespan of a leaf. To resolve these uncertainties, we irrigated Populus trichocarpa trees with a pulse of deuterium‐enriched water and used compound‐specific stable hydrogen isotope analyses to test if the applied tracer could be recovered from leaf wax n ‐alkanes of leaves that were at different stages of their development during the tracer application. Our experiment revealed that only leaf wax n ‐alkanes from leaves that had developed during the time of the tracer application were affected, while leaves that were already fully matured at the time of the tracer application were not. We conclude from our study that under controlled environmental conditions, leaf wax n ‐alkanes are synthesized only early in the ontogeny of a leaf. Our experiment has implications for the interpretation of leaf wax n ‐alkane δ D values in an environmental context, as it suggests that these compounds record only a brief period of the environmental variability that a leaf experiences throughout its life.