Submitted to: Functional Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/29/2010
Publication Date: 1/25/2011
Citation: Manzoni, S., Vico, G., Katul, G., Fay, P.A., Polley, H.W., Palmroth, S., Porporato, A. 2011. Optimizing stomatal conductance for maximum carbon gain under water stress: A meta-analysis across plant functional types and climates. Functional Ecology. 25:456-467. Interpretive Summary: Plant carbon uptake by the process of photosynthesis is the point where atmosphere and biosphere meet, and where the biosphere exerts a significant effect on the gaseous composition of earth's atmosphere. Plant carbon uptake therefore has major ramifications for the concentration of CO2 in the atmosphere, and the rate of global warming. The rate of CO2 uptake by plant photosynthesis critically depends on water availability, and plants have evolved sensitive mechanisms for slowing water loss from leaves during photosynthesis. Accurately accounting for the carbon gain to water loss ratio is necessary for building models that accurately predict grassland plant responses to future climate scenarios. Scientists at the Grassland Soil and Water Research Laboratory along with collaborators from Duke University have conducted a study showing that an optimization approach borrowed from the physiological literature can be used to improve the characterization of this ratio, resulting in better estimates of carbon gain and water loss in grasslands. This result will lead to better estimates of productivity and carbon sequestration in grasslands under future CO2 and climate scenarios.
Technical Abstract: Stomatal responses to environmental variables, in particular atmospheric CO2 concentration and soil water status, are needed for quantifying the controls on carbon and water exchanges between plants and the atmosphere. Building on previous leaf-scale gas exchange models and stomatal optimality theory, here we develop a new stomatal conductance parameterization accounting for the effects of water stress. Previous works derived optimal stomatal conductance as a function of marginal water use efficiency, lambda (which indicates the cost of water losses for the plant), and hypothesized that this parameter should increase with water stress. We show that in general lambda changes nonlinearly with leaf water potential, with an increase during mild water stress, followed by a decrease under severe stress when leaf internal limitations to photosynthesis become more important than stomatal limitations. The optimal stomatal model accounting for the nonlinear effects of water potential on lambda captures well the typical patterns of decreasing photosynthesis and transpiration observed when the leaf is subject to water stress. Atmospheric CO2 is shown to increase the marginal water use efficiency, consistent with the frequently observed decrease in transpiration under elevated CO2 conditions. The proposed parameterization is useful to assess vegetation responses to the compounded changes in water and CO2 availability.