The frequency of climate extremes, including droughts and wet periods, is increasing in the United States, and these changes in frequency are expected to have major economic impacts. The overall goal of the research undertaken was to assess changes of forest productivity in the southeastern United States as related to changes in precipitation using the ecosystem simulation model, BGC++;This study used Oak Ridge National Laboratory's Walker Branch Through fall Displacement Experiment (WBTDE) as the basis for BGC++ model validation. The experimental design has the water inputs into a white oak/red maple ecosystem manipulated to artificially induce drought in one large plot and increase the water supply, the WBTDE is unique by including drought in a way which affects little the canopy microclimate. Four major tests were proposed:(1) Comparison of predicted and measured soil water content over time; (2) Comparison of predicted and measured stable carbon isotopic ratios; (3) comparison of predicted and measured carbon fluxes; and (4)comparision of predicted and measured leaf nitrogen contents. Model performance with the measured data suggest the simulations of the hydrologic budget and photosynthesis are the most accurate, whereas simulations of soil process arte the least accurate.