Drivers and patterns of forest above-ground biomass carbon losses in the conterminous USA

Rising tree mortality threatens the forest carbon sink and may destabilize the global carbon cycle. Because tree mortality is driven by diverse disturbance agents (e.g., drought, insects, wind, fire), its impacts on above-ground biomass carbon (AGC) and its controlling factors vary widely and remain poorly constrained. Based on a consistent, multi-decadal record of biomass carbon from radar backscatter, we quantify AGC losses associated with specific disturbance agents in conterminous USA. We then develop agent-specific machine-learning models that relate mortality-driven AGC dynamics to climatic drivers (e.g., temperature, moisture deficits), vegetation characteristics, and soil properties. This framework could reveal how sensitivities and thresholds differ among agents and identify regions and conditions where forests are most vulnerable to mortality-related carbon losses. Our results support improved representation of tree mortality processes and associated carbon fluxes in Earth System Models, strengthening projections of present and future vegetation dynamics and carbon stocks under changing disturbance regimes.