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Leaf-wax n-alkanes ²H/¹H ratios are widely used as a proxy in climate reconstruction. Although the broad nature of the relationship between n-alkanes δ²H values and climate is appreciated, the quantitative details of the proxy remain elusive. To examine these details under natural environmental conditions, we studied a riparian broadleaf angiosperm species, Populus angustifolia, growing on water with a constant δ²H value and monitored the δ²H values of leaf-wax n-alkanes and of stem, leaf, stream, and atmospheric waters throughout the entire growing season. Here we found the δ²H values of leaf-wax n-alkanes recorded only a 2-wk period during leaf flush and did not vary for the 19 weeks thereafter when leaves remained active. We found δ²H values of leaf-wax n-alkanes of P. angustifolia record conditions earlier in the season rather than fully integrating the entire growing season. Using these data, we …