A direct measurement of the stable isotopes of transpired water vapor in a northern Michigan forest

The stable isotopes of oxygen and hydrogen in water vapor track hydrologic processes as phase changes of water preferentially partition heavy isotopes (18O and 2H) into the condensate and light isotopes (16O and 1H) into the vapor phase. As a result, the isotopic composition of water vapor can be used to identify water fluxes and cycling through natural environments. Forest water vapor is comprised of terrestrial (evaporation and transpiration) and atmospheric (tropospheric mixing, precipitation, and condensation) components. Within the isotopic record of forest water vapor, stable isotopes of transpired water (δT) comprise an important component but is typically either assumed to be non-fractionating or estimated indirectly. However, on small time scales (minutes to hours), non-steady state forest systems experience isotopic enrichment during early morning and late afternoon when transpiration rates are low. We deployed two Picarro Cavity Ring-Down spectrometers (L2120-i and L2130-i, respectively) in the University of Michigan Biological Station (UMBS) forest near Pellston, MI to measure the isotopic composition of near-surface ambient water vapor and the transpired vapor component directly. Both ambient and transpired water vapor were measured at three heights above the forest floor (2, 10, and 20 m) during August 2016. To measure species-specific water use, δT was measured on red maple (Acer rubrum) and northern red oak (Quercus rubra), two of the dominant tree types in the UMBS forest. This work represents the first direct measurement of δT in the UMBS forest and will help decouple local and species-specific hydrologic cycling. Beyond UMBS, this measurement will allow for a better understanding of species-specific plant hydraulics and help identify when the steady state approximation of transpiration is valid, which can be used to study water use and forest health.