Abstract
Plant responses to drought occur across many time-scales, with stomatal closure typically considered to be a critical short-term response. Recent theories of optimal stomatal conductance linked to plant hydraulic transport have shown promise, but it is not known if stomata update their hydraulic “shadow price” of water use (marginal increase in carbon cost with a marginal drop in water potential) over days, seasons, or in response to recent drought. Here, I estimate the hydraulic shadow price in five species – two semi-arid gymnosperms, one temperate and two tropical angiosperms – at daily timescales and in wet and dry periods. I tested whether the shadow prices varies predictably as a function of current and/or lagged drought conditions. Diurnal estimates of the hydraulic shadow price estimated from observed stomatal conductance, while variable, did not vary predictably with environmental variables. Seasonal variation in shadow price was observed in the gymnosperm species, but not the angiosperm species, and did not meaningfully influence prediction accuracy of stomatal conductance. The lack of systematic variation in shadow price and high predictive ability of stomatal conductance when using a single set of parameters further emphasizes the potential of hydraulic-based stomatal optimization theories.
Highlights
Stomatal response to environmental variation has a major influence on ecosystem carbon, water, and energy fluxes, and is important for global carbon and water cycles (Berry et al, 2010)
Because stomatal conductance is influenced by a suite of whole-plant traits and signals, for example leaf water potential that is mediated by hydraulic transport from roots to leaves (e.g. Sperry et al, 2016), longerterm changes in plant drought responses have the potential to alter stomatal sensitivity to the environment
For P. edulis, the best fitting and most parsimonious model selected by AIC with non-lagged variables explained 17% of the variation (p=0.04) and contained the highest leaf water potential recorded during that day, the number of stomatal conductance measurements, and the date
Summary
Stomatal response to environmental variation has a major influence on ecosystem carbon, water, and energy fluxes, and is important for global carbon and water cycles (Berry et al, 2010). Stomatal behavior is likely to influence ecosystem response to climate change and carbon cycle feedbacks of the terrestrial biosphere (Berry et al, 2010; Franks et al, 2013; Swann et al, 2016). Stomata have the opportunity to behave in a “Bayesian” manner by updating their behavior (e.g. water potentials at which stomatal closure begins) based on previous environmental conditions the plant has experienced, likely mediated by these longer-term plant responses such as biosynthesis of the hormone ABA (Brodribb & McAdam, 2017)
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