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.