Tree-ring time series provide long-term, annually resolved information on the growth of individual trees. When sampled in a systematic context, tree-ring data can be scaled to estimate the forest carbon capture and storage of landscapes, biomes, and ultimately the globe. A systematic effort to sample tree rings in national forest inventories would yield unprecedented temporal and spatial resolution of forest carbon dynamics, and help resolve key scientific uncertainties, which we highlight in terms of evidence for forest “greening” (enhanced growth) versus “browning” (reduced growth, increased mortality). We describe jump-starting such a tree-ring collection across the continent of North America, given the commitments of Canada, the United States, and Mexico to visit forest inventory plots, along with existing legacy collections. Failing to do so would be a missed opportunity to help chart an evidence-based path toward meeting national commitments to reduce net greenhouse gas emissions, urgently needed for climate repair.