Optimal stomatal behavior with competition for water and risk of hydraulic impairment
by Wolf A., Anderegg W. R. L., Pacala S. W. (2016)
October 31, 2016 –PNAS
Plants lose water and take up carbon through stomata, whose behavior has major influences on global carbon and water fluxes. Yet both competition for water and the potential fitness costs of hydraulic damage during water stress could alter how stomata behave. Here, we add variable xylem conductivity to water and carbon costs of low-xylem water potentials to the classic stomatal optimization and a pure carbon-maximization optimization. We show that both optimizations can reproduce known stomatal responses to environmental conditions but that the pure carbon-maximization optimization is also consistent with competition for water. We describe a new measure—the marginal xylem tension efficiency—that can be used to test stomatal optimizations.
For over 40 y the dominant theory of stomatal behavior has been that plants should open stomates until the carbon gained by an infinitesimal additional opening balances the additional water lost times a water price that is constant at least over short periods. This theory has persisted because of its remarkable success in explaining strongly supported simple empirical models of stomatal conductance, even though we have also known for over 40 y that the theory is not consistent with competition among plants for water.
We develop an alternative theory in which plants maximize carbon gain without pricing water loss and also add two features to both this and the classical theory, which are strongly supported by empirical evidence:
(i) water flow through xylem that is progressively impaired as xylem water potential drops and
(ii) fitness or carbon costs associated with low water potentials caused by a variety of mechanisms, including xylem damage repair.
We show that our alternative carbon-maximization optimization is consistent with plant competition because it yields an evolutionary stable strategy (ESS)—species with the ESS stomatal behavior that will outcompete all others.
We further show that, like the classical theory, the alternative theory also explains the functional forms of empirical stomatal models.
We derive ways to test between the alternative optimization criteria by introducing a metric—the marginal xylem tension efficiency, which quantifies the amount of photosynthesis a plant will forego from opening stomatal an infinitesimal amount more to avoid a drop in water potential.