Dubbert, M; Cuntz, M; Piayda, A; Werner, C: Oxygen isotope signatures of transpired water vapor – the role of isotopic non-steady-state transpiration under natural conditions, New Phytologist (2014)
• The oxygen isotope signature of water is a powerful tracer of water movement from plants to the global scale. However, little is known on the short-term variability of oxygen isotopes leaving the ecosystem via transpiration as high-frequency measurements are lacking. • A laser spectrometer was coupled to a gas-exchange chamber directly estimating branch-level fluxes in order to evaluate the short-term variability of the isotopic composition of transpiration (δE) and to investigate the role of isotopic non-steady-state under natural conditions in cork-oak trees (Quercus suber L.) during distinct Mediterranean seasons. • Measured 18O of transpiration (δE) deviated from isotopic steady-state throughout most of the day even when leaf water at the evaporating sites was near isotopic steady-state. High agreement was found between estimated and modeled δE assuming non-steady-state enrichment of leaf water. • Isoforcing, i.e. the influence of the transpirational 18O flux on atmospheric values, deviated from steady-state calculations but daily means were similar between steady-state and non-steady-state. However, strong daytime isoforcing on the atmosphere imply that short-term variations in δE have likely consequences for large-scale applications, e.g. partitioning of ecosystem fluxes or satellite-based applications.
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