Insights into the role of terrestrial ecosystems in the earth's response to changes in climate and rising atmospheric CO₂ levels rely heavily on the predictions of terrestrial biosphere models. These models contain detailed mechanistic representations of biological processes affecting terrestrial ecosystems; however, their ability to accurately predict field-based measurements of terrestrial vegetation dynamics and canopy carbon and water fluxes has remained largely untested. In this study, we address this issue by developing a constrained implementation of a new structured terrestrial biosphere model, the Ecosystem Demography model version 2 (ED2). Two years of carbon and water flux measurements from an eddy-flux tower are used in conjunction with forest inventory measurements of tree growth and mortality at Harvard Forest (42.5ºN, 72.1ºW) to estimate a number of important but weakly constrained model parameters. Evaluation against a decade of tower flux and forest dynamics measurements shows that the constrained ED2 model yields greatly improved predictions of annual net ecosystem productivity (NEP), carbon partitioning and growth and mortality dynaimcs of both hardwood and conifer trees. The generality of the model formulation is then evaluated by comparing the model's predictions against ecosystem measurements at Howland Forest (45.1ºN, 68.8ºW). Despite the markedly different composition at this site, the optimized model realistically predicted observed patterns of carbon fluxes and tree growth with no further adjustment in model parameters. These results demonstrate how structured terrestrial biosphere models such as ED2, which explicitly track the dynamics of fine-scale heterogeneity in ecosystem structure and composition, can be parameterized and tested against field-based measurements that provide quantitative insight into the underlying biological processes that govern ecosystem composition, structure and function at larger scales.