title: 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 (Pcanopy) from soil water potential (Psoil) and vapor pressure deficit (D). Modeled responses to D and Psoilwere 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 Pcanopyacross 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.
authors.affiliation: Department of Biology, University of Utah, Salt Lake City, UT, 84112, USA. j.sperry@utah.edu.
authors.forename: John S
authors.initials: JS
authors.lastname: Sperry
authors.affiliation: Department of Biology, University of Utah, Salt Lake City, UT, 84112, USA.
authors.forename: Yujie
authors.identifier: http://orcid.org/0000-0002-3729-2743
authors.initials: Y
authors.lastname: Wang
authors.affiliation: Smithsonian Tropical Research Institute, PO Box 0843-03092, Balboa, Panama.
authors.forename: Brett T
authors.initials: BT
authors.lastname: Wolfe
authors.affiliation: Department of Geography, State University of New York, Buffalo, NY, 14260, USA.
authors.forename: D Scott
authors.initials: DS
authors.lastname: Mackay
authors.affiliation: Department of Biology, University of Utah, Salt Lake City, UT, 84112, USA.
authors.forename: William R L
authors.initials: WR
authors.lastname: Anderegg
authors.affiliation: Earth and Environmental Sciences Division, Los Alamos National Lab, Los Alamos, NM, 87545, USA.
authors.forename: Nate G
authors.initials: NG
authors.lastname: McDowell
authors.affiliation: Biology Department, University of New Mexico, Albuquerque, NM, 87131, USA.
authors.forename: William T
authors.initials: WT
authors.lastname: Pockman
mesh_terms: D002980:Climate; D004058:Diffusion; D055864:Droughts; D006813:Humidity; D008954:Models, Biological; D054046:Plant Stomata / Q000502:physiology*; D018526:Plant Transpiration / Q000502:physiology; D012987:Soil / Q000737:chemistry; D014867:Water / Q000502:physiology*; D052584:Xylem / Q000502:physiology
chemical_list: D012987:Soil; D014867:Water
keywords: climate change drought; hydraulic limitation; modeling climate change impacts; plant drought responses; plant water transport; stomatal regulation; xylem cavitation; xylem transport