The role of the intraspecific variability of hydraulic traits for modelling the plant water use in different European forest ecosystems

Abstract

The drought resilience of forest ecosystems is generally believed to strongly 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 given 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 broadleaved tree species with contrasting phenologies, geographical distribution, degrees of vulnerability to hydraulic failure, and water use strategies. We performed a series of numerical experiments by modifying the parameters of the plant vulnerability curve and the maximum xylem hydraulic conductance to account for the variability within each tree 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, whereas at temporarily dry sites, the model results are counter-intuitive 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. Using the default value for maximum xylem conductance, the model tends to overestimate the spring transpiration at drier sites, forcing the vegetation to experience unrealistic water stress in summer. Our findings suggest that the parameterization of maximum xylem conductance is an important and yet unresolved problem in the CLM and similar land surface models.