Representation of Plant Hydraulics in the Noah‐MP Land Surface Model: Model Development and Multiscale Evaluation

Abstract

Plants are expected to face increasing water stress under future climate change. Most land surface models, including Noah-MP, employ an idealized "big-leaf" concept to regulate water and carbon fluxes in response to soil moisture stress through empirical soil hydraulics schemes (SHSs). However, such schemes have been shown to cause significant uncertainties in carbon and water simulations. In this paper, we present a novel plant hydraulics scheme (PHS) for Noah-MP (hereafter, Noah-MP-PHS), which employs a big-tree rather than big-leaf concept, wherein the whole-plant hydraulic strategy is considered, including root-level soil water acquisition, stem-level hydraulic conductance and capacitance, and leaflevel anisohydricity and hydraulic capacitance. Evaluated against plot-level observations from a mature, mixed hardwood forest at the University of Michigan Biological Station and compared with the default Noah-MP, Noah-MP-PHS better represents plant water stress and improves water and carbon simulations, especially during periods of dry soil conditions. Noah-MP-PHS also improves the asymmetrical diel simulation of gross primary production under low soil moisture conditions. Noah-MP-PHS is able to reproduce different patterns of transpiration, stem water storage and root water uptake during a 2-week dry-down period for two species with contrasting plant hydraulic behaviors, i.e., the "cavitation riskaverse" red maple and the "cavitation risk-prone" red oak. Sensitivity experiments with plant hydraulic capacitance show that the stem water storage enables nocturnal plant water recharge, affects plant water use efficiency, and provides an important buffer to relieve xylem hydraulic stress during dry soil conditions.Plain Language Summary Plants regulate transpiration dynamically through the stomatal aperture, which, in many cases, is governed by plant water status and hydraulic properties. Plant hydraulics describes the mechanics of water movement through plant vascular systems, which is the culmination of emergent phenotypical hydraulic functional traits at the leaf, stem, and root levels. Such physiological mechanisms are excluded in most land surface models, which typically represent plant water stress through empirical soil hydraulics schemes (SHSs) based on either soil water content or soil water potential. In this study, we present a novel plant hydraulics scheme (PHS) to represent plant water stress and the regulation of stomatal conductance for use in the Noah-MP land surface model. Our results show Noah-MP-PHS performs better in its water and carbon simulations than the default Noah-MP with traditional SHSs, especially under dry soil conditions. Noah-MP-PHS also successfully captures the different plant hydraulic behaviors between the "cavitation risk-averse" red maple and the "cavitation risk-prone" red oak. Sensitivity experiments also highlight the vital role played by plant water storage in water and carbon simulations in terms of buffering xylem hydraulic stress during soil moisture dry-down periods. The ...

SurEau: a mechanistic model of plant water relations under extreme drought

“…It is now widely recognized that improving predictions of drought impacts requires coupling ecohydrological principles with hydraulic theories and trait-based plant strategies (Tramblay et al 2020). Consequently, much research is currently focused on improving vegetation models by including the hydraulic framework and hydraulic traits in these models (e.g., Christofforsen et al 2016;Xu et al 2016;Tuzet et al 2017;Venturas et al 2018;Sperry et al 2019;De Cáceres et al 2021;Morin et al 2021;Li et al 2021).…”

“…There are different categories of plant hydraulics modelling strategies, as recently synthesized (Mencuccini et al 2019;McDowell et al 2019;Li et al 2021). According to the overview provided by Li et al 2021, SurEau belongs to electrical analogy models (EAMs) that conceptualize water flow through plants as being analogous to the current through an electric circuit with series of resistance and/or capacitance (Sperry et al 1998).…”

“…There are different categories of plant hydraulics modelling strategies, as recently synthesized (Mencuccini et al 2019;McDowell et al 2019;Li et al 2021). According to the overview provided by Li et al 2021, SurEau belongs to electrical analogy models (EAMs) that conceptualize water flow through plants as being analogous to the current through an electric circuit with series of resistance and/or capacitance (Sperry et al 1998). Within the EAM category, SurEau belongs to the resistor-capacitor model type (RCM) as it considers both resistance and water storage (capacitance) as opposed to more simple schemes that only consider resistance, such as the initial version of the model TREE (Loranty et al 2010;MacKay et al 2012), the SOX scheme developed by Eller et al (2018), and the PHS scheme in CLM5 (Kennedy et al 2019) or the Venturas et al (2018) approach.…”

“…Within the EAM category, SurEau belongs to the resistor-capacitor model type (RCM) as it considers both resistance and water storage (capacitance) as opposed to more simple schemes that only consider resistance, such as the initial version of the model TREE (Loranty et al 2010;MacKay et al 2012), the SOX scheme developed by Eller et al (2018), and the PHS scheme in CLM5 (Kennedy et al 2019) or the Venturas et al (2018) approach. The RCM frameworks, which also include plant internal store, have been developed by Sperry et al (1998), Steppe et al (2006), MacKay et al (2012; Gentine et al (2016), Xu et al (2016), Christoffersen et al (2016, Tuzet et al (2017), andLi et al (2021). The hydraulic capacitance has indeed been demonstrated to play a critical role in regulating transpiration (Matheny et al 2015;Huang et al 2017;Yan et al 2020), delaying water potential drop (Huang et al 2017) and may improve desiccation tolerance (Tyree and Yang 1990;Hölttä et al 2009;Blackman et al 2016).…”

Stressors Reveal Ecosystems' Hidden Characteristics

“…Gleason et al, 2016;Mursinna et al, 2018]. As it stands, these difficulties limit our current plant-hydraulics enabled models to only modest improvements over their empirical counterparts [Li et al, 2020;Venturas et al, 2017]. Therefore, it should be a key research priority to focus on novel parameterization…”