Methane concentration in the heartwood of living trees in a cold temperate mountain forest: variation, transport and emission.

Forest soils are the largest terrestrial sink of methane, but CH4 produced in tree trunks by methanogenic archaea and emitted into the atmosphere can significantly offset CH4 oxidation in the soil. However, our mechanistic understanding of CH4 accumulation in tree trunks, in relation with CH4 emission from the trunk surface, is still limited. We characterised temporal variations in the molar fraction of CH4 in the heartwood of trees ([CH4]HW) of four different species in a mountain forest and addressed the relationship between [CH4]HW and emission from the surface of the trunk (${F}_{CH_4}$), in connection with the characteristics of the wood. [CH4]HW were measured monthly for 15 months using gas-porous tubes permanently inserted into the trunk. [CH4]HW were above ambient CH4 molar fraction for all trees, lower than 100 ppm for seven trees, higher for the nine other trees and greater than 200,000 ppm (> 20%) for two of these nine trees. [CH4]HW varied monthly but were not primarily determined by trunk temperature. Heartwood diffusive resistance for CH4 was variable between trees, not only due to heartwood characteristics but probably also related to source location. ${F}_{CH_4}$were weakly correlated with [CH4]HW measured a few days after. The resulting apparent diffusion coefficient was also variable between trees suggesting variations in the size and location of the CH4 production sites as well as resistance to gas transport within the trunk. Our results highlight the challenges that must be overcome before CH4 emissions can be simulated at the tree level.