SurEau-Ecos v2.0: a trait-based plant hydraulics model for simulations of plant water status and drought-induced mortality at the ecosystem level

A widespread increase in tree mortality has been observed around the globe, and this trend is likely to continue because of ongoing climate-induced increases in drought frequency and intensity. This raises the need to identify regions and ecosystems that are likely to experience the most frequent and significant damage. We present SurEau-Ecos, a trait-based, plant hydraulic model designed to predict tree desiccation and mortality at scales from stand to region. SurEau-Ecos draws on the general principles of the SurEau model but introduces a simplified representation of plant architecture and alternative numerical schemes. Both additions were made to facilitate model parameterization and large-scale applications. In SurEau-Ecos, the water fluxes from the soil to the atmosphere are represented through two plant organs (a leaf and a stem, which includes the volume of the trunk, roots and branches) as the product of an interface conductance and the difference between water potentials. Each organ is described by its symplasmic and apoplasmic compartments. The dynamics of a plant's water status beyond the point of stomatal closure are explicitly represented via residual transpiration flow, plant cavitation and solicitation of plants' water reservoirs. In addition to the “explicit” numerical scheme of SurEau, we implemented a “semi-implicit” and “implicit” scheme. Both schemes led to a substantial gain in computing time compared to the explicit scheme (&gt;10 000 times), and the implicit scheme was the most accurate. We also observed similar plant water dynamics between SurEau-Ecos and SurEau but slight disparities in infra-daily variations of plant water potentials, which we attributed to the differences in the representation of plant architecture between models. A global model's sensitivity analysis revealed that factors controlling plant desiccation rates differ depending on whether leaf water potential is below or above the point of stomatal closure. Total available water for the plant, leaf area index and the leaf water potential at 50 % stomatal closure mostly drove the time needed to reach stomatal closure. Once stomata are closed, resistance to cavitation, residual cuticular transpiration and plant water stocks mostly determined the time to hydraulic failure. Finally, we illustrated the potential of SurEau-Ecos to simulate regional drought-induced mortality over France. SurEau-Ecos is a promising tool to perform regional-scale predictions of drought-induced hydraulic failure, determine the most vulnerable areas and ecosystems to drying conditions, and assess the dynamics of forest flammability.</p

Optimal stomatal behaviour around the world

This is the author accepted manuscript. The final version is available from Springer Nature via the DOI in this recordStomatal conductance (g s) is a key land-surface attribute as it links transpiration, the dominant component of global land evapotranspiration, and photosynthesis, the driving force of the global carbon cycle. Despite the pivotal role of g s in predictions of global water and carbon cycle changes, a global-scale database and an associated globally applicable model of g s that allow predictions of stomatal behaviour are lacking. Here, we present a database of globally distributed g s obtained in the field for a wide range of plant functional types (PFTs) and biomes. We find that stomatal behaviour differs among PFTs according to their marginal carbon cost of water use, as predicted by the theory underpinning the optimal stomatal model and the leaf and wood economics spectrum. We also demonstrate a global relationship with climate. These findings provide a robust theoretical framework for understanding and predicting the behaviour of g s across biomes and across PFTs that can be applied to regional, continental and global-scale modelling of ecosystem productivity, energy balance and ecohydrological processes in a future changing climate.This research was supported by the Australian Research Council (ARC MIA Discovery Project 1433500-2012-14). A.R. was financially supported in part by The Next-Generation Ecosystem Experiments (NGEE-Arctic) project, which is supported by the Office of Biological and Environmental Research in the Department of Energy, Office of Science, and through the United States Department of Energy contract No. DE-AC02-98CH10886 to Brookhaven National Laboratory. M.O.d.B. acknowledges that the Brassica data were obtained within a research project financed by the Belgian Science Policy (OFFQ, contract number SD/AF/02) and coordinated by K. Vandermeiren at the Open-Top Chamber research facilities of CODA-CERVA (Tervuren, Belgium)

Optimal stomatal behaviour around the world

© 2015 Macmillan Publishers Limited. All rights reserved. Stomatal conductance (g s) is a key land-surface attribute as it links transpiration, the dominant component of global land evapotranspiration, and photosynthesis, the driving force of the global carbon cycle. Despite the pivotal role of g s in predictions of global water and carbon cycle changes, a global-scale database and an associated globally applicable model of g s that allow predictions of stomatal behaviour are lacking. Here, we present a database of globally distributed g s obtained in the field for a wide range of plant functional types (PFTs) and biomes. We find that stomatal behaviour differs among PFTs according to their marginal carbon cost of water use, as predicted by the theory underpinning the optimal stomatal model and the leaf and wood economics spectrum. We also demonstrate a global relationship with climate. These findings provide a robust theoretical framework for understanding and predicting the behaviour of g s across biomes and across PFTs that can be applied to regional, continental and global-scale modelling of ecosystem productivity, energy balance and ecohydrological processes in a future changing climate

Rethinking plant's stomatal behavior

It's commonly accepted that the universal trade-off between water saving and CO2 capture driven by stomatal conductance includes two strategies of hydraulic functioning that determine plant vulnerability to drought. Accordingly, isohydric species, that close stomata rapidly to regulate water loss, can be subjected to carbon starvation. By contrast, anisohydric species can maintain significant rates of gas exchange but are predisposed to hydraulic failure because they operate with narrower hydraulic safety margins during drought. Despite a large research effort very few convincing evidence has been reported so far. In this study we developed an ecophysiological model that couple stomatal and hydraulic behaviors to study the response to water deficit. Based on initial soil features, plant transpiration rate, conductance and leaf area, resistance to cavitation and osmotic potential at full turgor; the model computes soil and plant water potential, leaf turgor pressure and plant and soil conductance. Stomatal regulation is a function of leaf turgor potential. The model allows computing the dynamic of dehydration, the number of days with stomata open (i.e. length of the photosynthetically active period) and the plant death by hydraulic failure. The model was tested for a wide range of plant and soil type. Three major outcomes have been addressed: first, any species can behave as isohydric or anisohydric according to environmental conditions. Second, for a given safety margins, species more resistant to cavitation are surprisingly more prone to drought-induced hydraulic failure. Third, to maximize carbon capture while avoiding hydraulic failure; stomatal closure should occur within a narrow range of water potential (before ~-4MPa) whatever the resistance to cavitation of the species. The predictions of the model for an optimal behavior (in terms of avoiding cavitation and maximizing openness of stomata) are congruent with global databases of stomatal closure, osmotic potential and resistance to cavitation. Altogether these results reject the isohydric versus anisohydric behavior of plant stomata and their implication in droughtinduced plant mortality. The model can serve as a basis to improve vegetation models of biogeochemical cycles or plant distribution models in a context of anticipating risk related to increasing drought under climate change

Rethinking plant's stomatal behavior

It's commonly accepted that the universal trade-off between water saving and CO2 capture driven by stomatal conductance includes two strategies of hydraulic functioning that determine plant vulnerability to drought. Accordingly, isohydric species, that close stomata rapidly to regulate water loss, can be subjected to carbon starvation. By contrast, anisohydric species can maintain significant rates of gas exchange but are predisposed to hydraulic failure because they operate with narrower hydraulic safety margins during drought. Despite a large research effort very few convincing evidence has been reported so far. In this study we developed an ecophysiological model that couple stomatal and hydraulic behaviors to study the response to water deficit. Based on initial soil features, plant transpiration rate, conductance and leaf area, resistance to cavitation and osmotic potential at full turgor; the model computes soil and plant water potential, leaf turgor pressure and plant and soil conductance. Stomatal regulation is a function of leaf turgor potential. The model allows computing the dynamic of dehydration, the number of days with stomata open (i.e. length of the photosynthetically active period) and the plant death by hydraulic failure. The model was tested for a wide range of plant and soil type. Three major outcomes have been addressed: first, any species can behave as isohydric or anisohydric according to environmental conditions. Second, for a given safety margins, species more resistant to cavitation are surprisingly more prone to drought-induced hydraulic failure. Third, to maximize carbon capture while avoiding hydraulic failure; stomatal closure should occur within a narrow range of water potential (before ~-4MPa) whatever the resistance to cavitation of the species. The predictions of the model for an optimal behavior (in terms of avoiding cavitation and maximizing openness of stomata) are congruent with global databases of stomatal closure, osmotic potential and resistance to cavitation. Altogether these results reject the isohydric versus anisohydric behavior of plant stomata and their implication in droughtinduced plant mortality. The model can serve as a basis to improve vegetation models of biogeochemical cycles or plant distribution models in a context of anticipating risk related to increasing drought under climate change