Forest managers, scientists, and policy makers have long relied on the assumption that 50 percent of wood biomass in trees is carbon. In the United States, where there are an estimated 1.4 trillion trees, even small deviations in wood carbon fractions can have big implications on estimates of forest carbon stocks and stock changes.

Most carbon estimation and reporting protocols use generic approximations of woody tissue carbon concentrations that assume 50 percent of wood biomass in trees is carbon, which has led to substantial systematic errors in forest carbon estimates. This assumption has become increasingly important as trees and forests are being proposed as nature-based solutions to climate change and large investments are being made in support of reforestation, restoration, and other forest management activities. In a global analysis of more than 3,600 wood carbon observations, a team of Northern Research Station scientists and partners at the University of Toronto, Scarborough, found that wood carbon fractions ranged from 18.4 to 75.1 percent and the mean wood carbon fraction for tree species in the USDA Forest Service Forest Inventory and Analysis (FIA) program was 47.4 percent, not 50 percent. With nearly 1.4 trillion live trees in the United States alone, this small but important deviation in the wood carbon fraction has substantial implications on estimates of forest carbon stocks and stock changes. Based on this analysis, more specific estimates of wood carbon fractions are being adopted by the FIA program and will be available as part of the public FIA database. The Global Woody Tissue Carbon Concentration Database (GLOWCAD) was also established by University of Toronto partners and contains 3,676 individual records of woody tissue carbon concentrations from 864 tree species. Woody tissue carbon concentration data—i.e., the mass of carbon per unit dry mass—were obtained from live and dead woody tissues from 130 peer-reviewed sources published between 1980 and 2020.