Dynamics of canopy stomatal conductance, transpiration, and evaporation in a temperate deciduous forest, validated by carbonyl sulfide uptake
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 relie...
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-01-01
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| Series: | Biogeosciences |
| Online Access: | http://www.biogeosciences.net/14/389/2017/bg-14-389-2017.pdf |
| Summary: | 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 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. |
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| ISSN: | 1726-4170 1726-4189 |