Plant hydraulic transport controls transpiration sensitivity to soil water stress

Abstract

Abstract. Plant transpiration downregulation in the presence of soil water stress is a critical mechanism for predicting global water, carbon, and energy cycles. Currently, many terrestrial biosphere models (TBMs) represent this mechanism with an empirical correction function (β) of soil moisture – a convenient approach that can produce large prediction uncertainties. To reduce this uncertainty, TBMs have increasingly incorporated physically based plant hydraulic models (PHMs). However, PHMs introduce additional parameter uncertainty and computational demands. Therefore, understanding why and when PHM and β predictions diverge would usefully inform model selection within TBMs. Here, we use a minimalist PHM to demonstrate that coupling the effects of soil water stress and atmospheric moisture demand leads to a spectrum of transpiration responses controlled by soil–plant hydraulic transport (conductance). Within this transport-limitation spectrum, β emerges as an end-member scenario of PHMs with infinite conductance, completely decoupling the effects of soil water stress and atmospheric moisture demand on transpiration. As a result, PHM and β transpiration predictions diverge most for soil–plant systems with low hydraulic conductance (transport-limited) that experience high variation in atmospheric moisture demand and have moderate soil moisture supply for plants. We test these minimalist model results by using a land surface model at an AmeriFlux site. At this transport-limited site, a PHM downregulation scheme outperforms the β scheme due to its sensitivity to variations in atmospheric moisture demand. Based on this observation, we develop a new “dynamic β” that varies with atmospheric moisture demand – an approach that overcomes existing biases within β schemes and has potential to simplify existing PHM parameterization and implementation.

Intra‐Specific Variability in Plant Hydraulic Parameters Inferred From Model Inversion of Sap Flux Data

“…Here, we have demonstrated that PHM hydraulic parameters can be inferred at the individual level using a MCMC inversion approach using measured sap flow. Reliable and simultaneous inference of multiple hydraulic model parameters has great potential to assist model parametrization, which remains a major impediment to the adoption of PHMs (Feng, 2020; Paschalis et al., 2020; Sloan et al., 2021). The inferred hydraulic traits—including the whole‐plant effective embolism vulnerability and maximum xylem conductance—are subject to a number of uncertainties related to model structure and input data availability but capture well the inter and intra‐specific variability in plant water use and hydraulic vulnerability.…”

“…The recent implementations of PHMs into terrestrial biosphere models have shown promising corrections of previous prediction biases in gross primary productivity and evapotranspiration (Bonan et al., 2014; Eller et al., 2020; Kennedy et al., 2019; L. Li et al., 2021; Powell et al., 2013; Sabot et al., 2020; Xu et al., 2016) as well as soil water balance (Kennedy et al., 2019), especially under drought conditions. However, the use of PHMs in models is still hindered by the need to acquire plant hydraulic traits for parameterization (Feng, 2020; Paschalis et al., 2020; Sloan et al., 2021).…”

“…conditions. However, the use of PHMs in models is still hindered by the need to acquire plant hydraulic traits for parameterization (Feng, 2020;Paschalis et al, 2020;Sloan et al, 2021).…”

Consistent responses of vegetation gas exchange to elevated atmospheric CO2emerge from heuristic and optimization models

“…The decrease in transpiration during drying is often included in soil-plant-atmosphere models through a piecewise linear function, representing water-stress-induced reductions in E (Federer, 1979;Sloan et al, 2021). These observations motivate the inclusion of a further constraint in the optimization relative to OPT1 and OPT2, in the form of a soil-moisturelimited transpiration rate under dry conditions that effectively constrains the allowable range of stomatal conductance (Manzoni et al, 2013),…”

Probabilistic soil moisture dynamics of water- and energy-limited ecosystems

“…A n , and consequently T , can be limited by many external factors such as PAR, vapor pressure deficit, atmospheric pressure, soil moisture, and air temperature (Sloan et al, 2021;Cong et al, 2022). Under no external limitations, vegetation transpires as much as its physiology allows.…”