Interindividual variations in methane production in tree trunks in relation to the wood micro-environment and microbial communities

Key message: Methane accumulates in the heartwood of two tree species independently of the presence of wetwood. Moisture or density drives anoxia, while species-specific microbial communities and early decay stages might promote methanogenesis.Methane (CH₄) accounts for approximately 20% of global warming. Recent evidence suggests that CH₄ accumulating in living tree trunks of some species can be produced internally by methanogenic archaea, yet the factors governing microbial community composition and activity within trunks remain poorly understood. This study characterises the microenvironment conducive to methanogenesis within the heartwood of two contrasting species: Cryptomeria japonica, prone to wetwood formation, and Fagus crenata, in which this phenomenon remains rare. In-situ CH₄ molar fractions, ex-situ potential production rates, physical and biochemical properties, and bacterial community composition were assessed. Elevated CH₄ molar fractions were consistently observed in the heartwood of both species, regardless the presence of wetwood. Anoxic conditions were driven by high moisture content in C. japonica, whereas high bulk density promoted oxygen limitation in F. crenata. Microbial communities differed markedly between species and among individuals, reflecting variations in methanogenic activity and internal decay state. Temporal dynamics of microbial communities emerged as key drivers of CH₄ production. A non-linear pattern was identified: early-stage wood degradation promotes CH₄ accumulation, whereas advanced degradation limits production.