Diurnal variations in the \delta18O of water vapor observed in the Pacific Northwest coniferous forests

Changes in the ²H and ¹⁸O of atmospheric water vapor provide information for integrating aspects of gas exchange within forest canopies. In this study we showed that overstory transpiration dominated \delta¹⁸O values of canopy water vapor (\delta¹⁸O_{v}) in an old-growth coniferous forest in the Pacific Northwest of United States. Diurnal fluctuations nearly 5 \permil in δ ^{18}O_v were observed at 3 heights both above and within the canopy. Values of \delta¹⁸O_{v} were \sim -18 \permil (VSMOW scale) before dawn, and gradually decreased to \sim -23 \permil at noon before slowly becoming more enriched again in the late afternoon. This diurnal pattern reflected a balance of δ ^{18}O signatures associated with early-transpired water and the water transpired at later hours when isotopic steady state was established. Small transpiration flux relative to leaf water contents in the early morning prevents leaf transpiration quickly approaching steady state. Using a non-steady state model, we compared the measured and predicted δ ^{18}O values of leaf water over the course of a day and showed a lagged response of leaf water enrichment. Using an isotopic approach, we determined that canopy transpiration accounted for \> 80 % of total evapotranspiration for two summer days in this old-growth forest, which agreed closely with above- and within-canopy eddy covariance measurements. Partitioning evapotranspiration into transpiration and evaporation fluxes using δ ^{18}$O as a tracer needs to carefully assess whether steady state assumption is satisfied.