An isotopic approach to partition evapotranspiration in a mixed deciduous forest

Abstract

Transpiration (T) is perhaps the largest fluxes of water from the land surface to the atmosphere and is susceptible to changes in climate, land use and vegetation structure. However, predictions of future transpiration fluxes vary widely and are poorly constrained. Stable water isotopes can help expand our understanding of land–atmosphere water fluxes but are limited by a lack of observations and a poor understanding of how the isotopic composition of transpired vapour (δT) varies. Here, we present isotopic data of water vapour, terrestrial water and plant water from a deciduous forest to understand how vegetation affects water budgets and land–atmosphere water fluxes. We measured subdiurnal variations of δ18OT from three tree species and used water isotopes to partition T from evapotranspiration (ET) to quantify the role of vegetation in the local water cycle. We find that δ18OT deviated from isotopic steady‐state during the day but find no species‐specific patterns. The ratio of T to ET varied from 53% to 61% and was generally invariant during the day, indicating that diurnal evaporation and transpiration fluxes respond to similar atmospheric and micrometeorological conditions at this site. Finally, we compared the isotope‐inferred ratio of T to ET with results from another ET partitioning approach that uses eddy covariance and sap flux data. We find broad midday agreement between these two partitioning techniques, in particular, the absence of a diurnal cycle, which should encourage future ecohydrological isotope studies. Isotope‐inferred estimates of transpiration can inform land surface models and improve our understanding of land–atmosphere water fluxes.

Water taken up through the bark is detected in the transpiration stream in intact upper‐canopy branches

“…Besides, neglecting the contributions of bark and/or foliar water uptake could also lead to an overestimation of the risk of hydraulic failure (Earles et al, 2016; Mayr et al, 2014), and eventually of the risk of mortality, as these alternative water uptake pathways (through the leaves and/or the bark) could contribute to partially alleviate drought stress (Breshears et al, 2008). Obtaining estimates of bark and/or foliar water uptake might not be straightforward for most study sites, but coupling complementary approaches to eddy covariance and/or sapflux measurements, such as online measurements of δ 2 H and δ 18 O or other atmospheric tracers (e.g., COS), could help identify missing sources contributing to water uptake (Aron et al, 2020; Wehr et al, 2017; Yakir & Sternberg, 2000).…”

Evapotranspiration partitioning based on field‐stable oxygen isotope observations for an urban locust forest land

“…Among the various approaches mentioned above, the stable water isotope method distinguishes ET based on the concept that E and T have different isotopic compositions. (e.g., Aron et al, 2020;Dubbert et al, 2013Dubbert et al, , 2014Iraheta et al, 2021;Rothfuss et al, 2010;Wang et al, 2010;Williams et al, 2004;Wu et al, 2017;Wu et al, 2021). Soil evaporation is more fractionated in comparison to vegetation transpiration at equal initial isotopic boundary conditions (Rothfuss et al, 2020;Wu et al, 2017).…”