An ecosystem process model is described that calculates the carbon, water and nitrogen cycles through a forest ecosystem. The model, FOREST-BGC, treats canopy interception and evaporation, transpiration, photosynthesis, growth and maintenance respiration, carbon allocation above and below-ground, litterfall, decomposition and nitrogen mineralization. The model uses leaf area index (lai) to quantify the forest structure important for energy and mass exchange, and this represents a key simplification for regional scale applications. FOREST-BGC requires daily incoming short-wave radiation, air temperature, dew point, and precipitation as driving variables. The model was used to simulate the annual hydrologic balance and net primary production of a hypothetical forest stand in seven contrasting environments across North America for the year 1984. Hydrologic partitioning ranged from 14/86/0% for evaporation, transpiration and outflow, respectively, in Fairbanks, AK (annual precipitation of 313 mm) to 10/27/66% in Jacksonville, FL (annual ppt of 1244 mm), and these balances changed as lai was increased from 3 to 9 in successive simulations. Net primary production (npp) ranged from 0.0 t C ha-1 year-1 at Tucson, AZ, to 14.1 t C ha-1 year-1 at Knoxville, TN and corresponded reasonably with observed values at each site. The sensitivity of ecosystem processes to varying lai in different climates was substantial, and underscores the utility of parameterizing this model at regional scales in the future with forest lai measurements derived from satellite imagery.