Adam R Martin, Grant M. Dimke, Mahendra Doraisami, Sean C. Thomas

Alarming increases in tree mortality due to environmental change suggest that contributions of dead wood to global carbon (C) cycles are rapidly increasing 1-3, with dead wood C flux estimates already approximating total annual anthropogenic C emissions 4. Quantifying C in dead wood critically depends on accurate estimates of dead wood C fractions (CFs) to convert dead woody biomass into C. Most C accounting protocols, including those recently revised by the IPCC 5, utilize a default dead wood CF of 50%, but live tree studies suggest this assumption results in substantial bias in forest C estimates 6. Here we compile and analyze a global database of dead wood CFs in trees, showing that dead wood CFs average 48.5% across forests worldwide, deviating significantly from 50%, with systematic variation among biomes, plant phyla, tissue types, and decay classes. Accounting for data-driven dead wood CFs corrects systematic overestimates in global dead wood C stock estimates of ~1.6 Pg C, an estimate approaching annual C flux estimates from land-use change globally 7. Our analysis provides, for the first time, robust empirical CFs for dead wood globally to inform global terrestrial C accounting protocols, and revise estimates of forest C stocks and fluxes.