Hybrid is a numerical biogeochemical model of terrestrial ecosystem dynamics (Friend et al., in press). Hybrid's critical features include:
By using a set of hypotheses about plant and soil behavior which are independent of location or time, Hybrid is potentially capable of predicting ecosystem behavior and structure under new conditions. Hybrid treats the cycling and exchange of carbon, nitrogen, and water using a daily time step. Grass also grows on a daily time step, while individual tree growth occurs on an annual time step. Individual trees and grass compete with each other for light, water, and nitrogen within a patch or "plot". Competition for light results in shading of smaller plants by larger ones. Larger plants also take up more of the available water and nitrogen. Each plot is divided into 1 m deep layers for calculating irradiance interception. The soil is represented as a single layer, with a daily hydrological budget. Decomposition is calculated using an empirical sub-model. The initial size of each tree seedling is stochastic; mean behavior for a particular boundary condition or change scenario is thus predicted by simultaneously simulating a number of plots.