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Summary: Stomatal optimization models can improve estimates of water and carbon fluxes with relatively low complexity, yet there is no consensus on which formulations are most appropriate for ecosystem‐scale applications. We implemented three existing analytical equations for stomatal conductance, based on different water penalty functions, in a big‐leaf comparison framework, and determined which optimization principles were most consistent with flux tower observations from different biomes.We used information theory to dissect controls of soil water supply and atmospheric demand on evapotranspiration in wet to dry conditions and to quantify missing or inadequate information in model variants. We ranked stomatal optimization principles based on parameter uncertainty, parsimony, predictive accuracy, and functional accuracy of the interactions between soil moisture, vapor pressure deficit, and evapotranspiration.Performance was high for all model variants. Water penalty functions with explicit representation of plant hydraulics did not substantially improve predictive or functional accuracy of ecosystem‐scale evapotranspiration estimates, and parameterizations were more uncertain, despite having physiological underpinnings at the plant level.Stomatal optimization based on water use efficiency thus provided more information about ecosystem‐scale evapotranspiration compared to those based on xylem vulnerability and proved more useful in improving ecosystem‐scale models with less complexity. [ABSTRACT FROM AUTHOR]

Highlights: [•] Stomatal optimization theories have neglected the effect of dynamic soil moisture. [•] Here soil moisture is considered to provide a closure for the optimization problem. [•] The new solutions link marginal water use efficiency to air [CO2] and soil water. [•] Previously inconsistent theoretical results and data are generalized and explained. [Copyright &y& Elsevier]

Soil and plant hydraulics constrain ecosystem productivity by setting physical limits to water transport and hence carbon uptake by leaves. While more negative xylem water potentials provide a larger driving force for water transport, they also cause cavitation that limits hydraulic conductivity. An optimum balance between driving force and cavitation occurs at intermediate water potentials, thus defining the maximum transpiration rate the xylem can sustain (denoted as Emax). The presence of this maximum raises the question as to whether plants regulate transpiration through stomata to function near Emax., To address this question, we calculated Emax across plant functional types and climates using a hydraulic model and a global database of plant hydraulic traits., The predicted Emax compared well with measured peak transpiration across plant sizes and growth conditions ( R = 0.86, P < 0.001) and was relatively conserved among plant types (for a given plant size), while increasing across climates following the atmospheric evaporative demand. The fact that Emax was roughly conserved across plant types and scales with the product of xylem saturated conductivity and water potential at 50% cavitation was used here to explain the safety-efficiency trade-off in plant xylem., Stomatal conductance allows maximum transpiration rates despite partial cavitation in the xylem thereby suggesting coordination between stomatal regulation and xylem hydraulic characteristics. [ABSTRACT FROM AUTHOR]

Abstract: An effective description of water transport in the soil–plant–atmosphere continuum (SPAC) is needed for wide-ranging applications in hydrology and climate-vegetation interactions. In this contribution, the theory of water movement within the SPAC is reviewed with emphasis on the eco-physiological and evolutionary constraints to water transport. The description of the SPAC can be framed at two widely separated time scales: (i) sub-hourly to growing season scales, relevant for hydro-climatic effects on ecosystem fluxes (given a set of plant hydraulic traits), and (ii) inter-annual to centennial scales during which either hydraulic traits may change, as individuals grow and acclimate, or species composition may change. At the shorter time scales, water transport can be described by water balance equations where fluxes depend on the hydraulic features of the different compartments, encoded in the form of conductances that nonlinearly depend on water availability. Over longer time scales, ontogeny, acclimation, and shifts in species composition in response to environmental changes can impose constraints on these equations in the form of tradeoffs and coordinated changes in the hydraulic (and biochemical) parameters. Quantification of this evolutionary coordination and the related tradeoffs offers novel theoretical tactics to constrain hydrologic and biogeochemical models. [Copyright &y& Elsevier]

• We analyzed daily air temperature and soil moisture time series of 54 forest sites. • Moister soils supported cooler understory microclimates. • Temperature buffering was more sensitive to soil moisture in dry soils and on sunny slopes. • Forests can lose their temperature buffering function when soils become drier. Forest canopies can buffer the understory against temperature extremes, often creating cooler microclimates during warm summer days compared to temperatures outside the forest. The buffering of maximum temperatures in the understory results from a combination of canopy shading and air cooling through soil water evaporation and plant transpiration. Therefore, buffering capacity of forests depends on canopy cover and soil moisture content, which are increasingly affected by more frequent and severe canopy disturbances and soil droughts. The extent to which this buffering will be maintained in future conditions is unclear due to the lack of understanding about the relationship between soil moisture and air temperature buffering in interaction with canopy cover and topographic settings. We explored how soil moisture variability affects temperature offsets between outside and inside the forest on a daily basis, using temperature and soil moisture data from 54 sites in temperate broadleaf forests in Central Europe over four climatically different summer seasons. Daily maximum temperatures in forest understories were on average 2 °C cooler than outside temperatures. The buffering of understory temperatures was more effective when soil moisture was higher, and the offsets were more sensitive to soil moisture on sites with drier soils and on sun-exposed slopes with high topographic heat load. Based on these results, the soil–water limitation to forest temperature buffering will become more prevalent under future warmer conditions and will likely lead to changes in understory communities. Thus, our results highlight the urgent need to include soil moisture in models and predictions of forest microclimate, understory biodiversity and tree regeneration, to provide a more precise estimate of the effects of climate change. [Display omitted] [ABSTRACT FROM AUTHOR]