Pragmatic hydraulic theory predicts stomatal responses to climatic water deficits

Abstract

Ecosystem models have difficulty predicting plant drought responses, partially from uncertainty in the stomatal response to water deficits in soil and atmosphere. We evaluate a 'supply-demand' theory for water-limited stomatal behavior that avoids the typical scaffold of empirical response functions. The premise is that canopy water demand is regulated in proportion to threat to supply posed by xylem cavitation and soil drying. The theory was implemented in a trait-based soil-plant-atmosphere model. The model predicted canopy transpiration (E), canopy diffusive conductance (G), and canopy xylem pressure (P ) from soil water potential (P ) and vapor pressure deficit (D). Modeled responses to D and P were consistent with empirical response functions, but controlling parameters were hydraulic traits rather than coefficients. Maximum hydraulic and diffusive conductances and vulnerability to loss in hydraulic conductance dictated stomatal sensitivity and hence the iso- to anisohydric spectrum of regulation. The model matched wide fluctuations in G and P across nine data sets from seasonally dry tropical forest and piñon-juniper woodland with < 26% mean error. Promising initial performance suggests the theory could be useful in improving ecosystem models. Better understanding of the variation in hydraulic properties along the root-stem-leaf continuum will simplify parameterization.

Why are leaves hydraulically vulnerable?

“…A widespread decline of K leaf under moderate dehydration would have major implications. Strong K leaf vulnerability relative to stems would drive stomatal responses under soil and atmospheric drought and thereby control carbon and water fluxes from leaves and canopies, with feedback on the climate system (Sperry et al , 2016). …”

Plant water potential improves prediction of empirical stomatal models

“…Despite much progress in the past several decades [13–20], full mechanistic understanding of stomatal function remains incomplete, and thus many ecosystem models use empirical stomatal algorithms derived from leaf-level gas exchange [21–23], often during wet soil conditions. A growing number of ecosystem models include supply-side water limitation through simulating plant hydraulic transport and water potential [24–29], and thus it is likely to be instructive to test models that simulate leaf water potential against the standard empirical models [28]. Considering common empirical models, the Ball-Berry-Leuning is perhaps the most widely used empirical stomatal conductance model and gives stomatal conductance as a simple function of photosynthetic rate, atmospheric carbon dioxide concentration, and either relative humidity or vapor pressure deficit (VPD) [21,23].…”

“…Temporal variation in leaf water potential is influenced both by VPD and soil water potential, and the functional form of stomatal response to leaf water potential differs distinctly between these two drivers and across species [28]. Indeed, the stomatal response to leaf water potential is likely one of the central elements of a plant’s drought response strategy [44–49].…”

“…Indeed, the stomatal response to leaf water potential is likely one of the central elements of a plant’s drought response strategy [44–49]. The water potential sensitivity of stomata is predicted to be tightly coupled to species’ xylem anatomy and the xylem’s vulnerability to cavitation [28,37,48,50]. Xylem traits will thus influence a plant’s stomatal strategy and the coupled hydraulic-stomatal continuum is likely to be critical in predicting which plants might be vulnerable to drought-induced tree mortality [11,51–53].…”

Climate pattern-scaling set for an ensemble of 22 GCMs – adding uncertainty to the IMOGEN version 2.0 impact system

“…Mercado et al, 2011), and inclusion of a full representation of a coupled carbon-nitrogen cycle in JULES (Zaehle et al, 2010). Furthermore, it is desirable to test the effects of adding height competition into the vegetation dynamics module of JULES, in order to add ecological succession modelling (Smith et al, 2001;Moorecroft et al, 2001), as well as assess the impacts of improved representation of stomatal conductance (Medlyn et al, 2011;Kala et al, 2015) and plant hydraulics (Sperry et al, 2016) on simulated land carbon and water cycles couplings to climate. The latter could extend as far as testing any hormonal signalling in the hydraulic linkages between soil moisture and stomata response, an effect well known by the physiological community but heretofore never tested in a full large-scale gridded land surface model .…”