Influence of Species-Specific Hydraulic Traits on Stress Response: Insights from the Hydrodynamic Canopy Transpiration Model FETCH3.14

Improving the representation of plant hydraulic behavior in vegetation and land-surface models is critical for improving our predictions of the impacts of water stress on ecosystem carbon and surface fluxes. Species-specific hydraulic traits play an important role in determining the response of ecosystem carbon and water fluxes to water stress; however, representation of biodiverse forest canopies remains a challenge in land-surface models. Here, we introduce FETCH3.14, a multispecies, canopy-level, hydrodynamic model which builds upon the previous versions of the Finite-difference Ecosystem-scale Tree Crown Hydrodynamics model (FETCH). FETCH3.14 simulates water transport through the soil, roots, and stem as flow through porous media. The model resolves water potentials along the vertical dimension, and stomatal conductance is controlled by xylem water potential in the stem. A key feature of FETCH3.14 is the implementation of a multi-species canopy formulation, which uses crown and stem dimensional characteristics to allow the model to produce both tree-level and plot-level outputs. Fluxes from representative model trees with distinct hydraulic traits are scaled to the plot level based on the species composition and canopy structure of the plot, allowing the model to be parameterized using observations at both the tree level (sap flux, stem water storage) and plot level (eddy covariance evapotranspiration).

Parameter optimization is performed using the newly developed Bayesian Optimization for Anything (BOA) package, which facilitates parameter optimization using multi-scale and multi-variate observations. This framework allows us to incorporate multiple sources of information, including multi-scale ET observations, soil and stem water potential observations, and carbon flux observations to provide insights about species-specific hydraulic traits. Here, we parameterized the model using both sap flow and eddy covariance measurements from a mixed forest at the University of Michigan Biological Station (UMBS, flux site US-UMB). This approach allows us to utilize ET observations to resolve information about species-specific hydraulic parameters, and provides insights about the interactions among specific traits and water stress, hydraulic strategies, and stomatal regulation.