The Role of the Intraspecific Variability of Hydraulic Traits for Modeling the Plant Water Use in Different European Forest Ecosystems

Abstract

The drought resilience of forest ecosystems is generally believed to depend on the dominant tree species' hydraulic traits. These traits define the maximum water transport capacity and the degree of vulnerability to hydraulic failure of a tree species. This work evaluates the effect of the intraspecific variability of hydraulic traits on the simulated tree water use in the Community Land Model (CLM, version 5.0). We selected two contrasting broadleaved tree species and performed a series of numerical experiments by modifying the parameters of the plant vulnerability curve and the maximum xylem hydraulic conductance accounting for the variability within each species. Our prescribed parameter sets represent vulnerable and resistant tree responses to the water deficit. At sites with an ample water supply, the resistant configuration simulates reduced water stress and increased transpiration compared to the vulnerable configuration. Meanwhile, the model results are counter‐intuitive at temporarily dry sites when water availability is the limiting factor. The numerical experiments demonstrate the emergent role of the maximum xylem conductance as a modulator of the plant water use strategy and the simulated transpiration within the model. Using the default value for maximum xylem conductance, the model tends to overestimate the early summer transpiration at drier sites, forcing the vegetation to experience unrealistic water stress later in the year. Our findings suggest that the parameterization of maximum xylem conductance is an important yet unresolved problem in the CLM and similar land surface models.

Detecting vegetation stress in mixed forest ecosystems through the joint use of tree-water monitoring and land surface modeling

“…While some assume that CLM5's PHT parameterization represents ecosystems correctly (e.g., Raczka et al, 2021;Wu et al, 2020), present study shows differences in PHT leading to an overestimation of E T in the GMP compared to PAP or the SSP. Some have improved the PHT parameterization of CLM5, enhancing the representation of the tree species in several forest stands dominated by a single tree species (e.g., Ali et al, 2022;Jiménez-Rodríguez et al, 2024).…”

“…The PHS uses the plant vulnerability curve (PVC) to characterize the hydraulic stress in the vegetation by describing plant hydraulic conductivity as a function of the water potential in a given plant organ (i.e., percent loss of hydraulic conductivity, PLC) (Venturas et al, 2017). The implementation of the PHS in LSMs allows to mimic the interspecific variability of plant transpiration response across forest stands and sites (e.g., (Jiménez-Rodríguez et al, 2024;Kennedy et al, 2019;Wu et al, 2020). Furthermore, the explicit simulation of the water potential in different plant segments (i.e., roots, stem, and leaves) provides the opportunity to compare the simulated plant water stress with novel measurements such as the TWD by exploiting experimental evidence on the link between tree diameters and water status (Dietrich et al, 2018;Schäfer et al, 2019).…”

“…Surprisingly, no improvement in E T simulations resulted from species-specific PHT parameterization. This suggests potential deficiencies in other aspects of the model, such as the representation of water use strategies(Jiménez-Rodríguez et al, 2024), or competition for light and water resources between the species.manuscript submitted to Water Resources Research 4.2 The stressed vegetation and the hidden roots…”

“…Rodríguez et al (2024) to determine the c k used in the plant vulnerability curve of CLM5 for the mean Ψ p50 values of PAP and SSP (Table1). FiguresS2 and S3show the differences in the PVCs of the different forest stand classification (i.e., BDT, NET, Oak, Beech, Spruce, Douglas fir) per parameterization (i.e., GMP, PAP, SSP).…”