The savanna model is a coupled tree-grass system and uses the forest and
grassland/crop submodels. 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 in the section
on the forest submodel.
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. N competition 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) and the basal area conversion factor (BASFCT) 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]). Tree parameter BASFC2 modifies the site potential N allowing more or less of available mineral N to be taken up by grass. The relationship between parameters BASFC2 and SITPOT is illustrated in the graph Factor for N Availability to Grasses In Savanna.
Plant Production Submodels: Overview