How much more carbon could be protected in mature and old-growth forests of the United States?

Mature and old-growth (MOG) forests are superior to younger forests for mitigating the climate and biodiversity crises. MOG forests store far more carbon per hectare than younger forests, and continue to accumulate carbon for centuries if not logged or severely disturbed. However, forest and climate policies have not fully protected older forests and large trees for these benefits despite public concerns. Here, we document our novel approach to link remote sensing of MOG structural stages with the USDA Forest Service's Forest Inventory and Analysis (FIA) data to estimate forest area and carbon stocks by conterminous U.S. regions, landowner class, protection status, and maturity class. We found the greatest stock of carbon and area in mature forests (31.5B Mg, 156.5 M ha), followed by young forests (12.8B Mg, 71.9 M ha), and old-growth forests (7.9B Mg C, 35.3 M ha). Old-growth forests had the greatest carbon density (224 MgC ha −1 ), followed by mature (201 MgC ha −1 ), and young (178 MgC ha −1 ). Only about 18 % of the current stock of carbon in MOG forests is protected from logging. Protecting MOG from commercial timber harvest, while allowing mature forests to grow into old-growth forests, could increase accumulated forest carbon stocks by 10.8B Mg this century, comparable to 8 years of fossil fuel emissions in the United States. Stricter prohibitions from logging on Inventoried Roadless Areas of National Forests would account for 2.9B MgC increase. Avoiding logging of old-growth forests and large trees in mature forests would protect and increase this valuable carbon stock that cannot be replaced in the near term. • Satellite and ground data were combined to estimate C stock of mature/old forests. • National Forests have the most protected and partially protected MOG C (4.0B Mg). • 84 % of the C in all structural classes and 74 % of C in old growth are unprotected. • MOG forests could increase C stocks by 15.6B Mg, comparable to 9 years of fossil CO 2 emissions.