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Leaf-wax n -alkanes 2 H/ 1 H ratios are widely used as a proxy in climate reconstruction. Although the broad nature of the relationship between n -alkanes δ 2 H 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 δ 2 H value and monitored the δ 2 H values of leaf-wax n- alkanes and of stem, leaf, stream, and atmospheric waters throughout the entire growing season. Here we found the δ 2 H 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 δ 2 H 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 modeled precipitation δ 2 H values during the time of wax synthesis. We observed that the isotope ratios of this precipitation generally were 2 H-enriched compared with mean annual precipitation. This model provides a mechanistic basis of the often-observed 2 H-enrichment from the expected fractionation values in studies of broadleaf angiosperm leaf-wax δ 2 H. In addition, these findings may have implications for the spatial and temporal uses of n -alkane δ 2 H values in paleoapplications; when both plant community and growth form are known, this study allows the isolation of the precipitation dynamics of individual periods of the growing season.