| Hydrogen and Oxygen Isotope Ratios of Water Vapor in Coniferous Forests of the Pacific Northwest, USA | |||||||||
| By:Chun-Ta Lai | |||||||||
| Introduction Changes in the 2H and 18O of atmospheric water vapor provide information for integrating aspects of gas exchange within forest canopies. We show diel variations in d2H and d18O values of water vapor measured above/within an old-growth coniferous forest, and investigate factors that contribute to the variation. | |||||||||
| Study area: Wind River Canoy Crane Research Facility, Southern Washington | |||||||||
| Field sample collection 1 We conducted a field experiment by collecting samples from ecosystem water pools in WRCCRF. We collected leaf, stem, soil and rain waters at different times during this field experiment. | |||||||||
| Field sample collection 2 We collected atmospheric water vapor by using a cryogenic trap to condense air from 3 different heights at 0.5, 10 and 60 meters. The vapor was collected every 3 hours, day and night, for 3 consecutive days. All the samples were prepared for d2H and d18O analysis. | |||||||||
Theory | |||||||||
| The isotopic composition of water vapor reflects the balance between 3 isofluxes: contributions from the atmosphere above, transpiration and soil evaporation. | |||||||||
| Using a simple box model, we successfully modeled diurnal patterns of oxygen isotope ratios in water vapor measured in this coniferous forests. From Lai et al. (2006) PCE 29:77 | |||||||||
| Relationship between d18O and d2H in water vapor, separated by collection time, and source (soil and stem) waters. The solid line represents the local meteoric water line based on river water samples collected in western Oregon and Washington. The dashed-line represents a linear regression that combines all the nighttime vapor (21:00 – 5:00) and source water for this forest. The regression results in a shallower slope that implies evaporation fractionation may have occurred to the source water. In general, daytime vapor show impacts from many processes (e.g. atmospheric mixing and non-steady state transpiration), while nighttime vapor, to a first order approximation, equilibrates with source waters. However, a single equilibrium fractionation factor cannot explain the wide range of nighttime vapor, likely due to contributions from waters of multi-sources (e.g. soil and mosses). | |||||||||
| Mosses covered much of the soil surface in the Wind River research forest. | |||||||||
References: Lai, C.-T., J. Ehleringer, B. Bond, and K.T. Paw U (2006), Contributions of evaporation, isotopic non-steady state transpiration, and atmospheric mixing on the d18O of water vapor in Pacific Northwest coniferous forests, Plant, Cell and Environment, 29(1), 77-94, doi:10.1111/j.1365-3040.2005.01402.x | |||||||||