BIOME4 is a coupled bio-geography and bio-geochemistry model which simulates the equilibrium distribution of 28 major potential natural vegetation types (biomes) from latitude (for the calculation of incoming short-wave and photosynthetically active solar radiation), atmospheric CO2 concentration, mean monthly climate (i.e. mean monthly precipitation, temperature, and percent sunshine) and soil physical properties (water holding capacity and percolation rate). Photosynthesis, stomatal behaviour and hydrology are simulated as in BIOME3 and the ecosystem dynamics model LPJ.
BIOME4 implicitly simulates competition between plant functional types (PFTs) as a function of relative net primary productivity (NPP) and uses an optimisation algorithm to calculate the maximum sustainable leaf area (LAI) of each PFT and its associated NPP. Empirical rules are used to classify these outputs and assign biomes based on the identity of the most successful and second-most successful PFT and their sustainable LAI.