Following the COP21 meeting in Paris, an international consensus emerged: the protection and expansion of forests is necessary to limit global warming to less than 2'C. The fact that trees emit methane (CH4), however, remains underappreciated, despite the potential for these emissions to offset the climate benefits of forest CO2 sequestration. We assessed the magnitude of microbial-based CH4 production in the trunks of living trees in the eastern U.S., and estimated the extent to which CH4 emissions reduce this carbon sink. We determined CH4 concentrations in the trunks of >3,000 trees across 23 sites, finding CH4 concentrations as high as 67.4% (i.e. ~375,000-times atmospheric). Diffusion modeling suggests that the trunks of mature broadleaved trees, in particular, are a net source of CH4 to the atmosphere, a result confirmed using chamber methods to directly measure CH4 efflux through the bark of living and newly-felled trees. Using geospatial analysis and the 100-year global warming potential (GWP) for CH4 vs. CO2, we estimate that this CH4-production pathway reduces the climate mitigation value of the eastern forest by 7%, with the benefits of some forests reduced by as much as 25%. On shorter timescales (e.g. 20 yr), the GWP of CH4 is higher and so modeled effects are greater, offsetting CO2 sequestration benefits by 20% overall and for some forest stands by >65%. Our findings highlight the need to consider CH4 fluxes if the mitigation potential of forests is to be accurately determined and effectively employed to set allowable emissions targets.