Temperate forest methane sink diminished by tree emissions

Global budgets ascribe 4-10% of atmospheric methane (CH₄ ) sinks to upland soils and have assumed until recently that soils are the sole surface for CH₄ exchange in upland forests. Here we report that CH₄ is emitted from the stems of dominant tree species in a temperate upland forest, measured using both the traditional static-chamber method and a new high-frequency, automated system. Tree emissions averaged across 68 observations on 17 trees from May to September were 1.59 ± 0.88 μmol CH₄  m^-2  stem h^-1 (mean ± 95% confidence interval), while soils adjacent to the trees consumed atmospheric CH₄ at a rate of -4.52 ± 0.64 μmol CH₄  m^-2  soil h^-1 (P < 0.0001). High-frequency measurements revealed diurnal patterns in the rate of tree-stem CH₄ emissions. A simple scaling exercise suggested that tree emissions offset 1-6% of the growing season soil CH₄ sink and may have briefly changed the forest to a net CH₄ source.