Dynamics of canopy stomatal conductance, transpiration, and evaporation in a temperate deciduous forest, validated by carbonyl sulfide uptake
Munger, J. William
McManus, J. Barry
Nelson, David D.
Zahniser, Mark S.
Saleska, Scott R.
Wofsy, Steven C.
AffiliationUniv Arizona, Dept Ecol & Evolutionary Biol
MetadataShow full item record
PublisherCOPERNICUS GESELLSCHAFT MBH
CitationDynamics of canopy stomatal conductance, transpiration, and evaporation in a temperate deciduous forest, validated by carbonyl sulfide uptake 2017, 14 (2):389 Biogeosciences
Rights© Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License.
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AbstractStomatal conductance influences both photosynthesis and transpiration, thereby coupling the carbon and water cycles and affecting surface–atmosphere energy exchange. The environmental response of stomatal conductance has been measured mainly on the leaf scale, and theoretical canopy models are relied on to upscale stomatal conductance for application in terrestrial ecosystem models and climate prediction. Here we estimate stomatal conductance and associated transpiration in a temperate deciduous forest directly on the canopy scale via two independent approaches: (i) from heat and water vapor exchange and (ii) from carbonyl sulfide (OCS) uptake. We use the eddy covariance method to measure the net ecosystem–atmosphere exchange of OCS, and we use a flux-gradient approach to separate canopy OCS uptake from soil OCS uptake. We find that the seasonal and diurnal patterns of canopy stomatal conductance obtained by the two approaches agree (to within ±6 % diurnally), validating both methods. Canopy stomatal conductance increases linearly with above-canopy light intensity (in contrast to the leaf scale, where stomatal conductance shows declining marginal increases) and otherwise depends only on the diffuse light fraction, the canopy-average leaf-to-air water vapor gradient, and the total leaf area. Based on stomatal conductance, we partition evapotranspiration (ET) and find that evaporation increases from 0 to 40 % of ET as the growing season progresses, driven primarily by rising soil temperature and secondarily by rainfall. Counterintuitively, evaporation peaks at the time of year when the soil is dry and the air is moist. Our method of ET partitioning avoids concerns about mismatched scales or measurement types because both ET and transpiration are derived from eddy covariance data. Neither of the two ecosystem models tested predicts the observed dynamics of evaporation or transpiration, indicating that ET partitioning such as that provided here is needed to further model development and improve our understanding of carbon and water cycling.
NoteOpen Access Journal.
VersionFinal published version
SponsorsUS Department Of Energy (DOE)'s Office of Science's Terrestrial Ecosystem Science program [DE-SC0006741]; Agnese Nelms Haury Program in Environment and Social Justice at the University of Arizona; DOE's Small Business Innovation Research program [DE-SC0001801]; DOE via Ameriflux network; US National Science Foundation (NSF); NSF LTER program; US National Oceanic and Atmospheric Administration's Atmospheric Chemistry, Carbon Cycle and Climate program; US National Aeronautics and Space Administration's Carbon Cycle Science program; NSF's Science and Technology Center for Multiscale Modeling of Atmospheric Processes