Seasonal shifts in the sensitivity of plant hydraulic parameters controlling ecosystem water and carbon fluxes in eCLM

Plant hydraulic traits regulate water transport in the soil–plant–atmosphere continuum and mediate the coupling between soil moisture availability, stomatal regulation, and ecosystem carbon uptake. Mechanistic representations of plant hydraulics in land surface models, such as the Community Land Model (CLM), improve the accuracy of simulated vegetation fluxes, particularly under drying soil conditions¹, but they also introduce additional parameters that can be difficult to constrain and can strongly influence model outputs. Global ensemble perturbation experiments in CLM have shown that plant hydraulic parameters are among the most influential controls on evapotranspiration, although their relative importance varies across regions². Yet, how the sensitivity of these parameters varies across plant functional types (PFTs) and seasons remains largely unexplored.

In this study, we investigated the sensitivity of simulated vegetation water potential and water and carbon fluxes to five key plant hydraulic parameters, including stomatal behavior (medlyn slope), plant and root conductance (k_max and k_rmax), cavitation resistance (psi₅₀) and root distribution (β) using eCLM (https://github.com/HPSCTerrSys/eCLM). Ensemble simulations were performed for 13 ICOS sites across Europe, covering four climate zones and five PFTs, over the period 2009–2018. The selected parameters were varied within PFT-dependent ranges following previous perturbation experiments²^,³, resulting in a total of 336 ensemble members. Variance-based parameter sensitivities (main effects, two-way interaction effects, and total effects) were quantified using the GEM-SA global sensitivity analysis framework based on Gaussian process emulation⁴. Emulators were trained on monthly averages for each station and each output variable individually.

Across simulations, medlyn slope and k_max showed the strongest effects on simulated water and carbon fluxes (ET, Tr, GPP, NEE) with main effects explaining more than 60% of the variance, while two-way interaction effects contributed only marginally. However, parameter sensitivities varied substantially among PFTs, with distinct patterns in the relative importance of dominant parameters for Mediterranean evergreen broadleaf forests, temperate deciduous forests, and evergreen needleleaf forests. Sensitivities also varied seasonally, with the remaining parameters—particularly psi₅₀—becoming increasingly influential under dry summer conditions. Most notably, seasonal shifts in the direction of parameter effects on canopy transpiration were detected at drought-prone Mediterranean sites: higher medlyn slope increased transpiration during spring, but led to reduced transpiration during summer, reflecting earlier stomatal closure under increasing plant hydraulic stress.

Our results show that model sensitivity to plant hydraulic parameters varies across PFTs and seasons, reflecting changes in model behavior across environments. These findings motivate further model development and refinement of plant hydraulic and stomatal process representation to ensure consistent performance across seasons, especially during drought.

References 

• ¹Kennedy et al. (2019). 10.1029/2018MS001500
• ²Kennedy et al. (2025). 10.1029/2024MS004715
• ³Eloundou et al. (2024). 10.5194/egusphere-egu24-16086
• ⁴O’Hagan (2006). 10.1016/j.ress.2005.11.025