The CENTURY model is set up to simulate the dynamics of grasslands, agricultural crops, forests, and savanna (tree-grass) systems. The grassland/crop production model simulates plant production for different herbaceous crops and plant communities (e.g. warm or cool season grasslands, wheat and corn). Grassland/crop options are selected from the crop parameters database. Existing crop options may be altered to suit particular varieties or environments or new options created using the FILE100 program. Harvest, grazing, fire and cultivation can all directly effect aboveground biomass, while grazing and fire may also impact root to shoot ratios and nutrient content. The forest model simulates the growth of deciduous or evergreen forests in juvenile and mature phases. Fire, large scale disturbances (e.g. hurricanes), and tree harvest practices may impact forest production. The savanna system is simulated as a tree-grass system, essentially using the existing tree and grassland/crop submodels with the two subsystems interacting through shading effects and nitrogen competition.
Both plant production models assume that the monthly maximum plant production is controlled by moisture and temperature and that maximum plant production rates are decreased if there are insufficient nutrient supplies (the most limiting nutrient constrains production). The fraction of the mineralized pools that are available for plant growth is a function of the root biomass with the fraction of nutrients available for uptake increasing exponentially as live root biomass increases from 20 to 300 gm-2. Most forest or grassland/crop systems are limited by nutrient availability and generally respond to the addition of N and P. The savanna model modifies maximum grass production by a shade modifier that is a function of tree leaf biomass and canopy cover. Additional nutrient constraints on plant production due to nutrient allocation between trees and grasses decrease maximum production rates for the grasses.
Plant growth is affected by the structure of the soil as specified in the site parameters. Water availability over NLAYPG soil layers affects potential production, growth, and root death. Root uptake of nutrients is determined over NLAYER soil layers. The initial values of nutrients in each soil layer are specified in the Site Parameters for Initial Mineral N, P, and S.
Grassland/Crop Submodel
Forest Submodel
Savanna Submodel