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Modeling plant–water interactions: an ecohydrological overview from the cell to the global scale

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Vegetation and the water cycles are inherently coupled across a wide range of spatial and temporal scales. Water availability interacts with plant ecophysiology and controls vegetation functioning. Concurrently, vegetation has direct and indirect effects on energy, water, carbon, and nutrient cycles. To better understand and model plant–water interactions, highly interdisciplinary approaches are required. We present an overview of the main processes and relevant interactions between water and plants across a range of spatial scales, from the cell level of leaves, where stomatal controls occur, to drought stress at the level of a single tree, to the integrating scales of a watershed, region, and the globe. A review of process representations in models at different scales is presented. More specifically, three main model families are identified: (1) models of plant hydraulics that mechanistically simulate stomatal controls and/or water transport at the tree level; (2) ecohydrological models that simulate plot‐ to catchment‐scale water, energy, and carbon fluxes; and (3) terrestrial biosphere models that simulate carbon, water, and nutrient dynamics at the regional and global scales and address feedback between Earth's vegetation and the climate system. We identify special features and similarities across the model families. Examples of where plant–water interactions are especially important and have led to key scientific findings are also highlighted. Finally, we discuss the various data sources that are currently available to force and validate existing models, and we present perspectives on the evolution of the field. WIREs Water 2016, 3:327–368. doi: 10.1002/wat2.1125 This article is categorized under: Water and Life > Nature of Freshwater Ecosystems Science of Water > Hydrological Processes
Plant–water interactions occurr over a broad range of spatial scales from (a) leaf interior (≈ µm), to (b) individual plant (≈ m), to (c) catchments (≈ km), up to (d) the entire Earth. A map of annual evapotranspiration (ET) on the Rietholzbach catchment and the annual global estimate of annual evapotranspiration (ET) from the MOD16 product are shown. (e) The increasing attention that is paid by the scientific community to ecohydrology is reflected in the number of published articles and the citations they received during the last 16 years (Source: ISI Web of knowledge, August, 2015). The average increasing rate of publication in scientific literature is also shown as a benchmark. The MOD16 map is reprinted with permission from Ref . Copyright 2011 Elsevier; the leaf section is reprinted with permission from Ref . Copyright 2013 John Wiley and Sons.
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Ecohydrological and terrestrial biosphere models have components and parameterizations to simulate the (1) surface energy exchanges, (2) the water cycle, (3) the carbon cycle, and (4) soil biogeochemistry and nutrient cycles. Many models do not include all the components presented in the figure.
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Effects on plant physiology caused by a decrease in water potential and turgor. The length of the horizontal lines represent the range of stress levels within which a process becomes first affected. Two different levels of minimum water potential, Ψ, are given: −2 and −12 MPa. These are indicative, and the former corresponds to a value characteristic for drought intolerant plants/crops and the latter for drought‐adapted plants in deserts. Dashed lines signify an incipient or vanishing effect. The figure is inspired by Hsiao et al. and Porporato et al.
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Global view of long‐distance water and carbohydrate transport pathways in a vascular plant. The xylem mediates the net transfer of water from the soil to the atmosphere down a gradient in water potential, ∇Ψ. The phloem carries a flow of sugars and other metabolites down the plant from the leaves to the tissues. Optical micrographs show cross sections of a leaf, stem, and root with the approximate location of the xylem (blue) and phloem (orange). Root nutrient uptake is also shown (green). Typical curves for changes in stem relative water content (RWC) (black line) and xylem cavitation (red line) expressed as percentage loss of conductivity (PLC) as a function of water potential are shown along with the increase in fluid viscosity as a function of sucrose osmotic concentration (blue line). (Reprinted with permission from Ref . Copyright 2014 Annual Reviews)
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Representation of plant‐ and patch‐scale root systems suitable for explicit 3‐D hydraulic models of plant water and nutrient uptake. (a) Spatial distribution of tree stems and their root systems based on measurements at the University of Michigan Biological Station: the central dot is tree stem (diameter ≥ 10 cm); the solid line shows the maximum lateral root extent, while the dashed line delineates distance corresponding to 95% of vertically and radially integrated root length. (b) Plant‐scale properties of root distribution are controlled by using explicit root architecture obtained with the RootBox software. Roots of different order (color‐coded) as well as overlapped areas where competition for soil water and nutrients occurs are shown for three exemplary trees. (c) Patch‐scale property of root distribution with depth is inferred from in situ observations of the bulk biomass density converted to the length density from variations of root diameters and root specific density. For each depth, the median density, 25–75%, and 10–90% ranges of the obtained distribution are shown.
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A leaf is mostly composed of mesophyll and epidermal cells. The mesophyll is subdivided into palisade and spongy mesophyll. The epidermis secretes a waxy substance called the cuticle to separate the leaf interior from the external atmosphere. Among the epidermal cells, there are pairs of guard cells. Each pair of guard cells forms a pore called stoma. Water and CO2 enter and exit the leaf mostly through the stomata. The vascular network of the plant is composed of xylem (blue) that transports water to the leaf cells and of phloem (red), which transports sugars from the leaf to the rest of the plant. Water that exits the xylem is evaporated in the leaf interior (dashed lines). The terms Ψx,v Ψm, Ψe, Ψg, Ψi, and Ψa are the water potential in the xylem of the leaf vein, mesophyll cell, epidermal cell, guard cell, leaf interior, and atmosphere, respectively. Stomatal aperture responds positively to guard cell turgor pressure (Pg) and negatively to epidermal cell turgor pressure (Pe) (hydromechanical feedback). The conductance of the stomatal aperture (gs) decreases with water potential in the leaf because of a combination of hydraulic and chemical factors.
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Science of Water > Hydrological Processes

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