Climate changes resulting from increases in atmospheric CO2 are expected to alter forest productivity and species distributions. But forest response to climate change depends in part on changes in soil water and nitrogen availability which limit tree growth. Here we report an investigation into the possible responses of northeastern North American forests to a warmer and generally drier climate by driving a linked forest productivity/soil process model with climate model predictions corresponding to a doubling of CO2. The greatest changes occurred at the current boreal/cool temperate forest border. Simulated productivity and biomass increased on soils that retained adequate water for tree growth and decreased on soils with inadequate water. Simulated changes in vegetation composition altered soil nitrogen availability, which in turn amplified the vegetation changes. The simulated responses of the forests were results of a positive feedback between carbon and nitrogen cycles, bounded by negative constraints of soil moisture availability and temperature.