Contributions of evaporation, isotopic non‐steady state transpiration and atmospheric mixing on the δ 18 O of water vapour in Pacific Northwest coniferous forests

Changes in the 2 H and 18 O of atmospheric water vapour provide information for integrating aspects of gas exchange within forest canopies. In this study, we show that diurnal fluctuations in the oxygen isotope ratio ( δ 18 O) as high as 4‰ were observed for water vapour ( δ 18 O vp ) above and within an old‐growth coniferous forest in the Pacific Northwest region of the United States. Values of δ 18 O vp decreased in the morning, reached a minimum at midday, and recovered to early‐morning values in the late afternoon, creating a nearly symmetrical diurnal pattern for two consecutive summer days. A mass balance budget was derived and assessed for the 18 O of canopy water vapour over a 2‐d period by considering the 18 O‐isoflux of canopy transpiration, soil evaporation and the air entering the canopy column. The budget was used to address two questions: (1) do δ 18 O values of canopy water vapour reflect the biospheric influence, or are such signals swamped by atmospheric mixing? and (2) what mechanisms drive temporal variations of δ 18 O vp ? Model calculations show that the entry of air into the canopy column resulted in an isotopically depleted 18 O‐isoflux in the morning of day 1, causing values of δ 18 O vp to decrease. An isotopically enriched 18 O‐isoflux resulting from transpiration then offset this decreased δ 18 O vp later during the day. Contributions of 18 O‐isoflux from soil evaporation were relatively small on day 1 but were more significant on day 2, despite the small H 2 16 O fluxes. From measurements of leaf water volume and sapflux, we determined the turnover time of leaf water in the needles of Douglas‐fir trees as ≈ 11 h at midday. Such an extended turnover time suggests that transpiration may not have occurred at the commonly assumed isotopic steady state. We tested a non‐steady state model for predicting δ 18 O of leaf water. Our model calculations show that assuming isotopic steady state increased isoflux of transpiration. The impact of this increase on the modelled δ  18 O vp was clearly detectable, suggesting the importance of considering isotopic non‐steady state of transpiration in studies of forest 18 O water balance.