We use a mechanistic model of tree-ring formation to simulate regional patterns of climate-tree growth relationships in the southeastern United States. Modeled chronologies are consistent with actual tree-ring data, demonstrating that our simulations have skill in reproducing broad-scale patterns of the proxy's response to climate variability. The model predicts that a decrease in summer precipitation, associated with a weakening Bermuda High, has become an additional control on tree ring growth during recent decades. A nonlinear response of tree growth to climate variability has implications for the calibration of tree-ring records for paleoclimate reconstructions and the prediction of ecosystem responses to climate change.