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

Changes in the H-2 and O-18 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 (delta O-18) as high as 4 parts per thousand were observed for water vapour (delta O-18(vp)) above and within an old-growth coniferous forest in the Pacific Northwest region of the United States. Values of delta O-18(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 O-18 of canopy water vapour over a 2-d period by considering the O-18-isoflux of canopy transpiration, soil evaporation and the air entering the canopy column. The budget was used to address two questions: (1) do delta O-18 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 delta O-18(vp)? Model calculations show that the entry of air into the canopy column resulted in an isotopically depleted O-18-isoflux in the morning of day 1, causing values of delta O-18(vp) to decrease. An isotopically enriched O-18-isoflux resulting from transpiration then offset this decreased delta O-18(vp) later during the day. Contributions of O-18-isoflux from soil evaporation were relatively small on day 1 but were more significant on day 2, despite the small (H2O)-O-16 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 approximate to 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 delta O-18 of leaf water. Our model calculations show that assuming isotopic steady state increased isoflux of transpiration. The impact of this increase on the modelled delta O-18(vp) was clearly detectable, suggesting the importance of considering isotopic non-steady state of transpiration in studies of forest O-18 water balance.