What shapes the environments favorable to methane production in the trunks of living trees

Tree CH₄ emissions are high in forested wetland and floodplain forests where tree trunks are viewed simply as pipes transferring CH₄ produced in the soil and emitting it into the atmosphere. Trees in upland forests are also able to transfer CH₄ from deep soil layers where anaerobic conditions prevail. In addition, CH₄ that accumulates in living tree trunks can also be produced by methanogenic archaea in the wood if favourable environmental conditions for CH₄ production prevail.

Inconsistent vertical patterns of trunk CH₄ emission and internal CH₄ concentrations are observed between species or between trees of the same species when CH₄ is produced internally. Large radial variations in CH₄ emissions is also observed, with for example hotspots located on one side of the tree. Furthermore, large variations in CH₄ emissions and internal concentrations from year to year suggest temporal dynamics of methanogenic activity within hotspots. CH₄ is the ultimate waste product of the energy metabolism of methanogens, which requires available substrates provided by a cascade of catabolic reactions breaking down macromolecules present in the wood. The inclusion of a biochemical module of CH₄ production in physical models of CH₄ transport within the trunk requires characterizing what shapes the environments favorable to CH₄ production within the trunk, the biochemical processes producing upstream the substrates necessary for methanogenesis and the microbial communities involved in these processes.

The aim of this presentation is to review current knowledge on internal CH₄ production and highlight challenges to build a comprehensive biogeochemistry of the trunk of living trees. Wetwood in living tree trunks is for example an ideal anoxic environment for methanogens, and this is where they were first found in the 1970s, but neither a necessary nor a sufficient condition. Methanogenic microbes in the trunk of living trees was recently found ubiquitous. However, the starting point of complex biogeochemical processes, supplying substrates and energy, but also inhibitors, to a unique microbiome in a unique ecological niche, is still not well understood.