Formulation of a consistent multi-species canopy description for
hydrodynamic models in mixed-forests
Abstract
Representation of canopy and tree function in earth system models (ESMs)
is going through rapid advancements in the last decade. The simplistic
big-leaf representation of canopy in model is being expanded and
replaced by more complex canopy representations that include multiple
functional types, species and age/size stages, multiple patches with
different canopy height and leaf area characteristics, and vertically
detailed representations with between 2 (light and shade) to n canopy
leaf layers. Recently, the hydrodynamic approach to modeling stomata
conductance has advanced, and many ESMs include a hydrodynamic version.
Under the hydrodynamic approach, stomata respond to water availability
in the xylem, and not directly in the soil. The result is a model that
include the stem, and xylem dimensions as canopy characteristics for
determining the flow and storage of water in the xylem. However, pipe
diameter, area and volume do not scale linearly, and thus, the
simulations cannot resolve canopy-scale hydrodynamics by pulling all the
stem conductive area and volume to a single virtual patch scale, but
instead, need to consider the hydrodynamics of a single virtual tree and
scale its resulting fluxes to the entire canopy. While that is trivial
in a homogeneous canopy, the difference between the tree-level
description of the hydrodynamic canopy and the horizontally pulled
description of the canopy in the surface-flux and radiation-exchange
modules leads to a conundrum. Imagine a mixed canopy: 50% oaks and 50%
maples. Assume oaks have 20% more LAI and are 1 m taller than the
maples. As far as hydrodynamics are concerned, the evaporative demands
of maple follow from light attenuation and leaf area profiles of a
maple-like canopy. However the roughness length and aerodynamic
resistance, and the resulting wind profile inside the canopy is not
characteristic of maple or oak but is a result of the mixed canopy. I
will present formulation for a consistent scaling of tree-level canopies
to patch-level for mixed forests with multiple species of different
functional types. The formulation scales canopy characteristics and the
resulting canopy fluxes from tree to forest in an energy and
mass-conservative way, and allow a smooth and consistent multi-species
canopy description for hydrodynamic models in mixed-forests.