Dynamics of canopy stomatal conductance, transpiration, and evaporation in a temperate deciduous forest, validated by carbonyl sulfide uptake

by Richard Wehr, Róisín Commane, J. William Munger, J. Barry McManus, David D. Nelson, Mark S. Zahniser, Scott R. Saleska, and Steven C. Wofsy


We used measurements of carbonyl sulfide, a structural analog to CO2 that is also taken up by leaves, as a tracer to validate measurements of the transpiration and evaporation of a whole forest canopy over time.  We discovered, surprisingly, that evaporation peaks at the time of year when the soil is dry and the air is moist. This pattern turns out to make physical sense but is not captured by current ecosystem models. Transpiration and evaporation were extracted from the total ecosystem-atmosphere water vapor exchange using estimates of stomatal conductance obtained by two independent, accordant methods: one based on measurements of the water and sensible heat fluxes coming from the forest, and the other based on the aforementioned measurements of the carbonyl sulfide flux into the forest.