Widespread carbon accumulation in mature forests remains underestimated by satellite observations

Mature forests are often assumed to approach carbon saturation, yet their role in continued carbon accumulation remains uncertain. Here we show that, across the conterminous USA, carbon accumulation in mature forests, covering ~72% of forest area, is systematically underrepresented by current satellite-derived biomass estimates relative to lidar, forest inventories and ecosystem models. Satellite estimates indicate near-neutral biomass changes over recent decades, whereas forest inventories estimate an increase of 102 TgC yr^-1. Repeat lidar and forest inventory measurements reveal widespread structural and carbon growth, including in tall forests. By contrast, satellite estimates fail to capture growth beyond canopy closure, showing little biomass increase once canopy height exceeds ~16 m. This asymmetric bias preferentially detects losses over gains yielding a weaker inferred land carbon sink. These findings reveal a critical observational gap in carbon dynamics in mature forests, suggesting that the land carbon sink may be systematically underestimated and highlighting the need for Earth observations sensitive to forest structure and their integration with forest inventories and ecosystem models.