A structure-function-diversity model of grassland ecosystems (Gemini) has been developed. For a potentially unlimited number of clonal plant populations, it explicitly simulates competition for two key resources (light and nitrogen) along vertical canopy and soil profiles. Population turnover, shoot and root morphogenesis, photosynthesis, respiration, transpiration, N acquisition by uptake, allocation of assimilates between structural compartments, and reserve storage and remobilization, are simulated for each plant population. The object-oriented structure of the modeling framework allows to couple, or not, the simulated plant populations to other sub-models describing climate variables, soil functioning, grazing behavior and grassland management. Partitioning of growth between shoot structures, leaf photosynthetic proteins and roots is based on two assumptions: (i) functional balance between root and shoot activity, (ii) coordination of leaf photosynthesis. The model was parameterized from plant functional trait measurements of 13 native perennial pasture grass species grown in monocultures at high N availability and low cutting frequency in a field trial. Predicted and measured annual dry-matter yields were highly correlated without bias across species, N supply and cutting frequency treatments in monocultures and in mixtures of six species. Results show the ability of this mechanistic model to simulate without bias nitrogen and disturbance responses of net primary productivity and of plant community structure.