<p>Most climate change forecasts assume that longer growing seasons increase carbon storage through increased tree growth, but recent findings have challenged this assumption. Here we highlight divergent findings across studies, spanning diverse methods and disciplinary perspectives. Current hypotheses for why longer growing seasons may not always increase tree growth include drought-related effects and internal constraints. These hypotheses, however, are generally tested in different ways by different fields on different species, and rarely consider how external drivers and internal constraints interact. We outline how bridging these divides while integrating evolutionary history and ecological theory could help build a unified model across species for when longer seasons will—or will not—lead to greater tree growth, with major forecasting implications.</p>