Temperate forest methane sink diminished by tree emissions

Summary Global budgets ascribe 4–10% of atmospheric methane ( CH 4 ) sinks to upland soils and have assumed until recently that soils are the sole surface for CH 4 exchange in upland forests. Here we report that CH 4 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 CH 4  m −2  stem h −1 (mean ± 95% confidence interval ), while soils adjacent to the trees consumed atmospheric CH 4 at a rate of −4.52 ± 0.64 μmol CH 4  m −2  soil h −1 ( P  <   0.0001). High‐frequency measurements revealed diurnal patterns in the rate of tree‐stem CH 4 emissions. A simple scaling exercise suggested that tree emissions offset 1–6% of the growing season soil CH 4 sink and may have briefly changed the forest to a net CH 4 source.