Elevated methane concentrations in trees of an upland forest

Background/Question/Methods

There is intense debate about whether terrestrial vegetation contributes substantially to global methane emissions. Although trees may act as a conduit for methane release from soils to atmosphere, the debate centers on whether vegetation directly produces methane by an uncharacterized, abiotic mechanism. A second mechanism of direct methane production in plants occurs when methanogens – microorganisms in the domain Archaea – colonize the wood of living trees. In the debate this biotic mechanism has largely been ignored, yet conditions that promote anaerobic activity in living wood, and hence potentially methane production, are prevalent across forests. Given the expectation of widespread and abundant fungal infection of living wood, we selected individuals in lowland and upland habitat of six trees species that vary in their vulnerability to heart rot. To determine in situ trunk-gas CH₄ concentrations, prior to (April) and post (July) leaf-out in 2011, trees were drilled horizontally at breast height to center and immediately plugged with a stopper. A 50-mL gas-syringe was inserted into the cavity to remove 50 mL of trunk-gas from each tree. To confirm that trunk-wood had the potential to produce CH₄, we removed bark-to-pith increment cores from the same trees, and incubated them under an anaerobic headspace.

Results/Conclusions

We find average, growing season, trunk-gas methane concentrations >15,000 mL×L­^-1 in common, temperate-forest species. In upland habitat (where soils are not a significant methane source), concentrations are 2.3-times greater than in lowland areas, and wood cores produce methane in anaerobic, lab-assays. Emission rate estimates from our upland site are 52±9.5 ng CH₄ m^-2 s^-1; rates that are of a similar magnitude to the soil methane sink in temperate forest, and equivalent in global warming potential to ~18% of the carbon likely sequestered by this forest. Microbial infection of one of the largest, biogenic sinks for carbon dioxide, living trees, might result in substantial, biogenic production of methane.