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.
Highlights
Plants control their transpiration (T ) and CO2 assimilation by adjusting leaf stomatal apertures in response to environmental variations (Katul et al, 2012; Fatichi et al, 2016)
We aim to (i) verify the transport-limitation spectrum from the minimalist analysis (Sect. 3.1) for a complex plant hydraulic models (PHMs) formulation common to terrestrial biosphere models (TBMs), (ii) identify errors incurred by selecting β over a PHM (Sect. 3.2) for a real transport-limited soil–plant system, and (iii) develop a new dynamic β that approximates a PHM with simple modifications to the existing β
The consistency between the minimalist and complex PHM suggests that the divergence between PHMs and β in transport-limited systems is not sensitive to the linear or nonlinear forms of supply or demand lines but is rather controlled by the existence of a finite conductance itself. These results strongly support the need to use two independent variables, ψs and Tww, to capture the coupled effects of soil water stress and atmospheric moisture demand on transpiration downregulation in transport-limited soil–plant systems. In light of these findings, we have developed a new dynamic β that has an additional functional dependence on Tww (Eq 16) and compared it against four other downregulation schemes in this land surface model (LSM) analysis
Summary
Plants control their transpiration (T ) and CO2 assimilation by adjusting leaf stomatal apertures in response to environmental variations (Katul et al, 2012; Fatichi et al, 2016) In doing so, they mediate the global water, carbon, and energy cycles. Many TBMs represent declining gs and, in turn, transpiration reduction (i.e., downregulation) in response to soil water stress with an empirical function of soil water availability. This method, known as β (Powell et al, 2013; Verhoef and Egea, 2014; Trugman et al, 2018; Paschalis et al, 2020), reduces gs from its peak value under well-watered conditions (gs,ww), i.e., gs = β ·gs,ww, 0 ≤ β ≤ 1.
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