Biogeosciences, 14, 389–401, 2017 www.biogeosciences.net/14/389/2017/ doi:10.5194/bg-14-389-2017 © Author(s) 2017. CC Attribution 3.0 License. Dynamics of canopy stomatal conductance, transpiration, and evaporation in a temperate deciduous forest, validated by carbonyl sulfide uptake Richard Wehr1, Róisín Commane2, J. William Munger2, J. Barry McManus3, David D. Nelson3, Mark S. Zahniser3, Scott R. Saleska1, and Steven C. Wofsy2 1Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721, USA 2John A. Paulson School of Engineering and Applied Sciences and Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA 3Aerodyne Research Inc., Billerica, MA 01821, USA Correspondence to: Richard Wehr ([email protected]) Received: 30 August 2016 – Published in Biogeosciences Discuss.: 2 September 2016 Revised: 3 January 2017 – Accepted: 3 January 2017 – Published: 26 January 2017 Abstract. Stomatal conductance influences both photosyn- evaporation peaks at the time of year when the soil is dry thesis and transpiration, thereby coupling the carbon and and the air is moist. Our method of ET partitioning avoids water cycles and affecting surface–atmosphere energy ex- concerns about mismatched scales or measurement types be- change. The environmental response of stomatal conduc- cause both ET and transpiration are derived from eddy co- tance has been measured mainly on the leaf scale, and theo- variance data. Neither of the two ecosystem models tested retical canopy models are relied on to upscale stomatal con- predicts the observed dynamics of evaporation or transpira- ductance for application in terrestrial ecosystem models and tion, indicating that ET partitioning such as that provided climate prediction. Here we estimate stomatal conductance here is needed to further model development and improve and associated transpiration in a temperate deciduous forest our understanding of carbon and water cycling. directly on the canopy scale via two independent approaches: (i) from heat and water vapor exchange and (ii) from car- bonyl sulfide (OCS) uptake. We use the eddy covariance method to measure the net ecosystem–atmosphere exchange 1 Introduction of OCS, and we use a flux-gradient approach to separate canopy OCS uptake from soil OCS uptake. We find that the Stomata are the adjustable pores through which carbon diox- seasonal and diurnal patterns of canopy stomatal conduc- ide (CO2/ enters and water vapor exits leaves. Stomata tance obtained by the two approaches agree (to within ±6 % strongly influence both carbon and water cycling and are a diurnally), validating both methods. Canopy stomatal con- key point of coupling between them (Lin et al., 2015). Stom- ductance increases linearly with above-canopy light inten- ata also influence surface–atmosphere energy exchange via sity (in contrast to the leaf scale, where stomatal conductance the latent heat of water vaporization. How readily a leaf’s or shows declining marginal increases) and otherwise depends a canopy’s stomata conduct gas is determined by their num- only on the diffuse light fraction, the canopy-average leaf- ber and degree of opening and is quantified by stomatal con- to-air water vapor gradient, and the total leaf area. Based on ductance. At the leaf level, stomatal conductance has been stomatal conductance, we partition evapotranspiration (ET) extensively modeled both empirically and theoretically, with and find that evaporation increases from 0 to 40 % of ET as both approaches describing it as proportional to net photo- the growing season progresses, driven primarily by rising soil synthesis, given fixed CO2 and water vapor concentrations in temperature and secondarily by rainfall. Counterintuitively, the air (Collatz et al., 1991; Medlyn et al., 2011). However, scaling leaf-level stomatal conductance up to the canopy is Published by Copernicus Publications on behalf of the European Geosciences Union. 390 R. Wehr et al.: Dynamics of canopy stomatal conductance, transpiration, and evaporation problematic because of the complex heterogeneity of the sion through the mesophyll. This approximation will be jus- canopy and its light environment (Bonan et al., 2011). Some tified a posteriori. empirical evidence suggests that on the canopy scale, stom- In addition to being taken up by the canopy, OCS is also atal conductance is not proportional to photosynthesis: as taken up or emitted by soils, with the direction and mag- photosynthetically active radiation (PAR) increases, canopy- nitude of the flux depending on soil type, temperature, and integrated stomatal conductance (estimated from eddy co- moisture (Ogée et al., 2016; Sun et al., 2016; Whelan et variance measurements of ecosystem–atmosphere heat and al., 2016). The interpretation of large-scale OCS concen- water vapor exchange) responds linearly while photosynthe- trations or fluxes therefore requires understanding of how sis responds nonlinearly (Wehr and Saleska, 2015). A linear both canopy and soil processes contribute to the overall response of canopy stomatal conductance to PAR has also ecosystem–atmosphere OCS exchange. been inferred from sap flux measurements (Schäfer, 2011). Here we compare canopy stomatal conductance derived However, estimates of stomatal conductance based on sap from ecosystem–atmosphere heat and water vapor exchange or water vapor flux measurements involve assumptions (e.g. (Wehr and Saleska, 2015) to that derived from canopy OCS about evaporation, leaf temperature, and plant internal stor- uptake via Eq. (1), validating both methods by their agree- age) that have contributed substantial uncertainty (Schäfer, ment. The canopy OCS uptake is measured by using a sub- 2011; Wehr and Saleska, 2015). Our objective here is to test canopy flux-gradient approach to partition eddy covariance Wehr and Saleska’s (2015) method for estimating canopy measurements of net ecosystem–atmosphere OCS exchange stomatal conductance from the water vapor flux against a into canopy and soil components. Mesophyll conductance is new, independent method based on carbonyl sulfide (OCS) estimated using an empirical temperature-dependent func- – and then to use stomatal conductance to partition evapo- tion, and boundary layer conductance is estimated using a transpiration. theoretical model; we are not able to test these estimates OCS is currently a focus of ground- and satellite-based because the gas exchange is not sensitive to them. For the measurements (Kuai et al., 2014; Montzka et al., 2007) and biochemical conductance, we test two simple assumptions: model development (Berry et al., 2013), owing mainly to its (i) that it is constant and (ii) that it depends on temperature. potential as a large-scale proxy for gross primary production Based on the validated canopy stomatal conductance, we (GPP) (Blonquist et al., 2011). That potential stems from the partition eddy covariance measurements of total ecosystem– fact that OCS is taken up by leaves (Sandoval-Soto et al., atmosphere water vapor exchange (that is, evapotranspira- 2005), somewhat analogously to CO2. However, while hy- tion, or ET) into transpiration and evaporation, and exam- drolysis of both OCS and CO2 is catalyzed by the enzyme ine their diurnal and seasonal patterns. Various empirical and carbonic anhydrase (CA), the net rate of CO2 hydrolysis de- theoretical methods have been used to estimate these water pends on downstream reactions involving light while the rate fluxes in other ecosystems (Kool et al., 2014), but the present of OCS hydrolysis does not (Berry et al., 2013). Thus, the method is advantageous because all fluxes are derived from leaf OCS uptake is directly related not to GPP but to CA ecosystem-scale eddy covariance data, minimizing concerns activity and to the conductance of the diffusive pathway be- about mismatched scales or measurement types. Finally, we tween the air and the chloroplast (Commane et al., 2015). compare the estimated water fluxes to predictions by two That pathway consists of the leaf boundary layer (i.e. the ecosystem models. thin layer of stagnant air on the surface of the leaf), stomata, and mesophyll, with stomata typically being the most influ- ential component (Wehr and Saleska, 2015). On the ecosys- 2 Methods tem scale, the uptake of OCS by the canopy leaves should be 2.1 Site description given by −1 The data presented here were collected from May through D I D −1 C −1 C −1 C −1 F gCn g gb gs gm gCA ; (1) October of 2012 and 2013 at the Harvard Forest Environ- mental Measurements Site (HF-EMS) in Petersham, Mas- −2 −1 where F is the flux of OCS into the leaves (pmol m s /; sachusetts, USA. The site is located in a temperate decidu- Cn is the molar mixing ratio of OCS (to air) in the ous forest dominated by red oak and red maple (with some −1 canopy airspace (pmol mol /; gb, gs, and gm are the hemlock, white pine, and red pine) and has been described canopy-integrated conductances to OCS diffusion through in detail previously (Urbanski et al., 2007). Leaf area index the leaf boundary layer, stomata, and mesophyll, respectively (LAI) at the site is steady at about 5 after leaf expansion and −2 −1 (mol m s /; and gCA is the reaction rate coefficient for before autumnal abscission, although there are interannual OCS destruction by CA, expressed as a “biochemical con- differences of up to 10 % (Wehr and Saleska, 2015). ductance” (mol m−2 s−1/. Equation (1) approximates the dif- fusive pathway as being linear, thereby neglecting possible differences between the parallel pathways associated with in- dividual leaves, as well as the three-dimensionality of diffu- Biogeosciences, 14, 389–401, 2017 www.biogeosciences.net/14/389/2017/ R. Wehr et al.: Dynamics of canopy stomatal conductance, transpiration, and evaporation 391 2.2 OCS measurements 2.3 Soil chamber CO2 measurements The ecosystem–atmosphere exchange of OCS was mea- Of the eight automated soil chambers sampled, only four sured by eddy covariance using a closed-path quantum cas- were used here; these four were situated in a 5 × 5 m plot cade laser spectrometer (QCLS) manufactured by Aero- about 50 m to the south of the flux tower.