As one travels northward and encounters lower sun angles and more nitrogen-poor soils, there is an increased dominance of plant communities by evergreens with conical canopies and a more favorable carbon balance in photosynthesis per unit nitrogen in leaves compared with deciduous species from the same environments. Despite the observed correlations between these biogeographic patterns and plant traits, there is no quantitative model that accounts for them. We derive such a model from coupled ordinary differential equations of carbon balance and nitrogen uptake, comparing qualitative equilibrium predictions of correlations among plant traits with experimental findings. The model suggests that many of the experimentally observed correlations among plant traits (e.g., photosynthesis and nitrogen content) are best viewed as constraints on plant growth rather than cause and effect. We use the model to account for some large-scale biogeographic trends in plants morphology and physiology. We also provide suggestions for experiments to test the model predictions.