Formulation of a Consistent Multi-Species Canopy Description for Hydrodynamic Models Embedded in Large-Scale Land-Surface Representations of Mixed-Forests

The plant hydrodynamic approach represents a recent advancement to land surface modeling, in which stomatal conductance responds to water availability in the xylem rather than in the soil. To provide a realistic representation of tree hydrodynamics, hydrodynamic models must resolve processes at the level of a single modeled tree, and then scale the resulting fluxes to the canopy and land surface. While this tree-to-canopy scaling is trivial in a homogeneous canopy, mixed-species canopies require careful representation of the species properties and a scaling approach that results in a realistic description of both the canopy and individual-tree hydrodynamics, as well as leaf-level fluxes from the canopy and their forcing. Here, we outline advantages and pitfalls of three commonly used approaches for representing mixed forests in land surface models and present a new framework for scaling vegetation characteristics and fluxes in mixed forests. The new formulation scales fluxes from the tree to canopy level in an energy- and mass-conservative way and allows for a consistent multi-species/multi-type canopy description by hydrodynamic models. Plain Language Summary Current land surface models represent vegetation canopies as a simplified column of a unit ground area. A new generation of models, hydrodynamic models, that resolve the role of the amount of water stored in a plant in the regulation of water loss from plant leaves must resolve individual model trees. In mixed forests where multiple types of trees need to be represented, this tree-level representation may lead to an inconsistency with the current ground-area representation of the canopy, the fluxes it produces, and their physical drivers. We discuss the potential pitfalls in current approaches to representation of mixed forests, and present a formulation that scales fluxes from the tree-to canopy-level in an energy- and mass-conservative way and allows for a consistent multi-species canopy description for hydrodynamic models.