The savanna model is a coupled tree-grass system and uses the forest and
grassland/crop submodels already described. The fundamental difference in the
savanna submodel is the manner in which total system production is obtained.
Total system production is the sum of forest and grass production. Potential
maximum production of forest is computed in the manner described above.
Grassland/crop production is modified to include the effect of tree canopy
cover on grassland/crop production. A shade modifier is calculated as a
function of the canopy cover and leaf biomass (Figure 3-21) and is multiplied by the
normal grassland/crop production equation (see Grassland/Crop Submodel). Increasing canopy cover
and leaf biomass reduces the potential grass production. Removal of grass or
forest is accomplished independently with the FIRE and TREM management events,
so that user can specify fire intensity and frequency as desired. Fire removal
parameters for grassland/crop vegetation are specified in
fire.100, while forest fire parameters are specified in
trem.100. In this manner, a grass fire can occur at a higher
intensity and/or frequency than fires affecting forest combustion losses. In
the present model, fire does not influence tree distribution and establishment.
Nitrogen competition is the other major interaction between the forest and
grass systems. The interaction is controlled by the amount of tree basal area,
total nitrogen available, and site potential for plant production. The fraction
of N available for tree uptake is calculated as a function of tree basal area
(m2 ha-1) and available mineral N using the function shown in
Figure 3-22. The fraction of N uptake by
grass is one minus the forest fraction and if grass N uptake did not consume
all of the N allocated to it, this amount is added to the pool of N which is
available to the trees. Two important site-specific parameters for the savanna
model are the site potential parameter (SITPOT, tree.100) and the
basal area conversion factor (BASFCT, tree.100) which calculates
tree basal area as a function of large wood C level. Parameter SITPOT controls
how fast trees can dominate grasslands with lower numbers (1200 vs. 2400)
leading to quicker dominance by trees. SITPOT is a measure of the aboveground
herbaceous layer production in kg ha-1 yr-1 in the absence of trees (SITPOT =
2400 * monthly N availability [in gN m-2 yr-1])
Plant Production Submodels: Overview
Grassland/Crop Submodel
Forest Submodel