Local methanogenesis drives significant methane emissions from upper tree trunks in a cool-temperate upland forest

Accurately quantifying methane (CH4) emissions from tree trunks is essential, but emissions from the upper part of trunks have been overlooked. Vertical emission patterns within individual trunks and the processes driving them, including CH4 production and transport, remain unknown. We investigated vertical patterns of CH4 emissions, internal CH4 molar fraction and production, and the methanogenic archaeal gene mcrA up to 12 m aboveground in six trees of three species (Aesculus turbinata, Cryptomeria japonica, and Fagus crenata) in a cool-temperate upland forest. We also modeled CH4 transport inside trunks using a diffusion-advection equation. Of the total CH4 emissions from the main trunk (1.1-44.5 nmol per tree s-1), 44-89% originated from the part of the trunk above 3 m. In certain trees, emissions peaked as high as 4-6 m aboveground. CH4 production and the mcrA gene were consistently detected up to 12 m, with active production also in the upper trunk, linked to wood degradation or limited oxygen diffusion. The simulation of CH4 transport suggested that long-distance CH4 transport from the lower to the upper part of the trunk is unlikely. The results suggest that the upper part of the trunk can emit CH4 produced at the same height in the trunk, and we propose including the upper trunk in the CH4 emission measurements.