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 mu mol CH(4)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 mu mol CH(4)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.