Dynamic aspects of soil water availability for isohydric plants: Focus on root hydraulic resistances

Couvreur, Valentin; Vanderborght, Jan; Draye, Xavier; Javaux, Mathieu

doi:10.1002/2014wr015608

Cited by 80 publications

(61 citation statements)

References 90 publications

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“…The strong correlation between DBH and δ 18 O xylem and the isotopic mixing model results (Figures c and c) supports the idea of larger trees using relatively large amounts of water below 1 m (and likely down to 13 m). While we are aware that our results only provide a picture of water use dynamics during one dry period of a single year (the extreme drought of 2015), we also believe there is a substantial plasticity of root water uptake to allow for shifts in effective rooting depth in response to changes in soil dryness conditions (Couvreur, Vanderborght, Draye, & Javaux, ; Doussan, Pierret, Garrigues, & Pagès, ; Fan et al., ; Schröder, Javaux, Vanderborght, Körfgen, & Vereecken, ).…”

Section: Discussionmentioning

confidence: 92%

Hydrological niche segregation defines forest structure and drought tolerance strategies in a seasonal Amazon forest

et al. 2018

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The relationship between rooting depth and above‐ground hydraulic traits can potentially define drought resistance strategies that are important in determining species distribution and coexistence in seasonal tropical forests, and understanding this is important for predicting the effects of future climate change in these ecosystems. We assessed the rooting depth of 12 dominant tree species (representing c. 42% of the forest basal area) in a seasonal Amazon forest using the stable isotope ratios (δ18O and δ2H) of water collected from tree xylem and soils from a range of depths. We took advantage of a major ENSO‐related drought in 2015/2016 that caused substantial evaporative isotope enrichment in the soil and revealed water use strategies of each species under extreme conditions. We measured the minimum dry season leaf water potential both in a normal year (2014; Ψnon‐ENSO) and in an extreme drought year (2015; ΨENSO). Furthermore, we measured xylem hydraulic traits that indicate water potential thresholds trees tolerate without risking hydraulic failure (P50 and P88). We demonstrate that coexisting trees are largely segregated along a single hydrological niche axis defined by root depth differences, access to light and tolerance of low water potential. These differences in rooting depth were strongly related to tree size; diameter at breast height (DBH) explained 72% of the variation in the δ18Oxylem. Additionally, δ18Oxylem explained 49% of the variation in P50 and 70% of P88, with shallow‐rooted species more tolerant of low water potentials, while δ18O of xylem water explained 47% and 77% of the variation of minimum Ψnon‐ENSO and ΨENSO. We propose a new formulation to estimate an effective functional rooting depth, i.e. the likely soil depth from which roots can sustain water uptake for physiological functions, using DBH as predictor of root depth at this site. Based on these estimates, we conclude that rooting depth varies systematically across the most abundant families, genera and species at the Tapajós forest, and that understorey species in particular are limited to shallow rooting depths. Our results support the theory of hydrological niche segregation and its underlying trade‐off related to drought resistance, which also affect the dominance structure of trees in this seasonal eastern Amazon forest. Synthesis. Our results support the theory of hydrological niche segregation and demonstrate its underlying trade‐off related to drought resistance (access to deep water vs. tolerance of very low water potentials). We found that the single hydrological axis defining water use traits was strongly related to tree size, and infer that periodic extreme droughts influence community composition and the dominance structure of trees in this seasonal eastern Amazon forest.

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Section: Discussionmentioning

confidence: 92%

Hydrological niche segregation defines forest structure and drought tolerance strategies in a seasonal Amazon forest

et al. 2018

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“…When the total resistance to flow in the axial direction is small (large axial conductance and/or small transport distance), then the 3-D version of the C model reproduces the flow in the root system exactly that is predicted by solving the flow equations in the root system for boundary conditions that correspond with the soil water potentials (Couvreur et al, 2014b(Couvreur et al, , 2012. In these conditions, K comp should be equal to K rs .…”

Section: Model Descriptionmentioning

confidence: 95%

“…When this condition does not hold, then the model is not an exact solution anymore but the impact of the resistance to flow in the axial direction can be represented by using a smaller K comp (Couvreur et al, 2014b). This was shown by comparing detailed simulations using a coupled soil-root model (Javaux et al, 2008) against simulations with the C model (Couvreur et al, 2014b). The second assumption is related to the upscaling.…”

Section: Model Descriptionmentioning

confidence: 99%

Root growth, water uptake, and sap flow of winter wheat in response to different soil water conditions

Cai¹,

Vanderborght²,

Langensiepen

et al. 2018

Hydrol. Earth Syst. Sci.

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Abstract. How much water can be taken up by roots and how this depends on the root and water distributions in the root zone are important questions that need to be answered to describe water fluxes in the soil-plant-atmosphere system. Physically based root water uptake (RWU) models that relate RWU to transpiration, root density, and water potential distributions have been developed but used or tested far less. This study aims at evaluating the simulated RWU of winter wheat using the empirical Feddes-Jarvis (FJ) model and the physically based Couvreur (C) model for different soil water conditions and soil textures compared to sap flow measurements. Soil water content (SWC), water potential, and root development were monitored noninvasively at six soil depths in two rhizotron facilities that were constructed in two soil textures: stony vs. silty, with each of three water treatments: sheltered, rainfed, and irrigated. Soil and root parameters of the two models were derived from inverse modeling and simulated RWU was compared with sap flow measurements for validation. The different soil types and water treatments resulted in different crop biomass, root densities, and root distributions with depth. The two models simulated the lowest RWU in the sheltered plot of the stony soil where RWU was also lower than the potential RWU. In the silty soil, simulated RWU was equal to the potential uptake for all treatments. The variation of simulated RWU among the different plots agreed well with measured sap flow but the C model predicted the ratios of the transpiration fluxes in the two soil types slightly better than the FJ model. The root hydraulic parameters of the C model could be constrained by the field data but not the water stress parameters of the FJ model. This was attributed to differences in root densities between the different soils and treatments which are accounted for by the C model, whereas the FJ model only considers normalized root densities. The impact of differences in root density on RWU could be accounted for directly by the physically based RWU model but not by empirical models that use normalized root density functions.

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“…For this study an H scenario is not computed using the numerical model. It is, however, a commonly used approach and results can be found e.g., in Couvreur et al (2014b), Javaux et al (2008), or Huber et al (2014. Thus the numerical model is capable of simulating different stomatal control mechanisms.…”

Section: Chemical Signallingmentioning

confidence: 99%

Simulating transpiration and leaf water relations in response to heterogeneous soil moisture and different stomatal control mechanisms

et al. 2015

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Aims Stomata can close to avoid cavitation under decreased soil water availability. This closure can be triggered by hydraulic ('H') and/or chemical signals ('C', 'H + C'). By combining plant hydraulic relations with a model for stomatal conductance, including chemical signalling, our aim was to derive direct relations that link soil water availability, expressed as fraction of roots in dry soil (f dry ), to transpiration reduction. Methods We used the mechanistic soil-root water flow model R-SWMS to verify this relation. Virtual split root experiments were simulated, comparing horizontal and vertical splits with varying f dry and different strengths of stomatal regulation by chemical and hydraulic signals. Results Transpiration reduction predicted by the direct relations was in good agreement with numerical simulations. For small enough potential transpiration and large enough root hydraulic conductivity and stomatal sensitivity to chemical signalling isohydric plant behaviour originates from H + C control whereas anisohydric behaviour emerges from C control. For C control the relation between transpiration reduction and f dry becomes independent of transpiration rate whereas H + C control results in stronger reduction for higher transpiration rates. Conclusion Direct relations that link effective soil water potential and leaf water potential can describe different stomatal control resulting in contrasting behaviour.

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Dynamic aspects of soil water availability for isohydric plants: Focus on root hydraulic resistances

Cited by 80 publications

References 90 publications

Hydrological niche segregation defines forest structure and drought tolerance strategies in a seasonal Amazon forest

Hydrological niche segregation defines forest structure and drought tolerance strategies in a seasonal Amazon forest

Root growth, water uptake, and sap flow of winter wheat in response to different soil water conditions

Simulating transpiration and leaf water relations in response to heterogeneous soil moisture and different stomatal control mechanisms

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