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

Abstract

Abstract. Stomatal 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 leafto-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.

Canopy uptake dominates nighttime carbonyl sulfide fluxes in a boreal forest

“…Besides uptake of COS by the soil and leaf stomatal diffusion there is no other process to our knowledge that would lead to uptake of COS in the ecosystem. This leads to the conclusion that the nighttime COS uptake is predominantly driven by vegetative uptake and supports the use of COS to estimate g sCOS (Wehr et al, 2017). Assuming that the soil is the only sink besides the vegetation, we can say that the nighttime vegetative uptake contributes to 17 % of the total daily COS uptake.…”

“…This means that vegetative uptake of COS can continue during the night if stomata are not completely closed . Caird et al (2007) showed that nighttime stomatal conductance exists in a wide variety of plant species and several studies report nighttime depletion of COS mole fractions (White et al, 2010;Belviso et al, 2013;Commane et al, 2013Commane et al, , 2015Berkelhammer et al, 2014;Maseyk et al, 2014;Wehr et al, 2017). The measurements presented in White et al (2010), Maseyk et al (2014), Berkelhammer et al (2014) and Wehr et al (2017) indicated that nighttime ecosystem COS fluxes were indeed dominated by the vegetation, and not by the soil.…”

“…However, the relation between COS fluxes and stomatal conductance measurements has not been studied under field conditions. Instead, Wehr et al (2017) used COS ecosystem fluxes to estimate stomatal conductance. This relation can especially be useful for estimating nighttime stomatal conductance, which cannot be accurately determined under humid conditions as the concentration gradient of water vapor in leaf chambers gets too small .…”

Stomatal control of leaf fluxes of carbonyl sulfide and CO<sub>2</sub> in a <i>Typha</i> freshwater marsh

“…To understand the relationship between daily integrated COS and CO 2 fluxes for regional flux inversion (e.g., Hilton et al, 2015), nighttime COS uptake needs to be constrained . Nighttime COS uptake has been found in a wheat field , a boreal pine forest (Kooijmans et al, 2017), and temperate forests (Berkelhammer et al, 2014;Commane et al, 2015;Wehr et al, 2017). However, most field studies based their findings upon indirect evidence of nighttime ecosystem COS uptake, with only one study reporting some direct leaf observations of nighttime uptake 30 (Berkelhammer et al, 2014).…”

Soil fluxes of carbonyl sulfide (COS), carbon monoxide, and carbon dioxide in a boreal forest in southern Finland

“…Since ocean COS emissions are geographically separated from the terrestrial COS sinks (leaf and soil), and anthropogenic emissions are usually concentrated as point sources, the spatial separation of dominant COS sources and sinks enables us to constrain land COS fluxes, and hence photosynthetic carbon uptake, from atmospheric COS observations (Campbell et al, 2008;Berry et al, 2013;Hilton et al, 2015). However, for the use of COS as a photosynthetic tracer, soil COS flux, which is unrelated to photosynthesis, needs to be understood and separated from the ecosystem COS flux that is the sum of leaf and soil fluxes Commane et al, 2015;Wehr et al, 2017).…”