Must estimates of the amount of carbon stored in forest ecosystems rely on the simple assumption that half of woody material is comprised of carbon. Martin and others demonstrate that this value can differ amongst species, and using species-specific wood carbon fractions can change the estimate of carbon stored in large trees by 500 kg or more. Extrapolating these differences to the globe suggests that current global forest carbon stocks may be overestimated by more than 2 Pg. Results create a path forward for integrating species-specific food carbon fraction measurements into carbon cycle science.