Are trees source or sink of methane in a cool-temperate mountain forest?

Although the presence of methanogens in living tree trunks was reported more than 50 years ago, it has recently been suggested that trees in upland forests constitute a net sink for atmospheric methane (CH4), which contradicts other recent or older studies. To clarify the role of tree trunks as net emitters or consumers of CH4, we measured trunk CH4 fluxes of 11 upland species, up to 12 m above ground for some trees, and estimated their ex-situ potential CH4 oxidation capacity. Trees from seven species emitted CH4 from their trunks, some at height well-above 2 m above ground, whereas little CH4 was emitted from the trunks of the other four species. The average rate of CH4 oxidation was an order of magnitude lower than the average trunk CH4 fluxes measured on the same individuals, consistent with the very weak net uptake of CH4 occasionally measured on some trees. CH4 oxidation in the bark could nevertheless mitigate CH4 emissions from tree trunks. Trees in our mountain forest were likely a net source of CH4 to the atmosphere rather than a net sink of atmospheric methane, suggesting that it is premature to conclude that tree surfaces could be a significant sink for atmospheric CH4 globally.