Microbial methane production is affected by secondary metabolites in the heartwood of living trees in upland forests

Methane (CH4) is a potent greenhouse gas that exerts large effects on atmospheric chemistry and the global warming. Living trees in upland forests might contribute to the global CH4 emissions but the contribution is poorly understood and the mechanisms of CH4 production in their heartwood are not completely clear. Using gene sequencing and a series of laboratory incubations, this study addressed whether CH4 produced in the heartwood of living trees in upland forests is microbial in origin and how the CH4 production is affected by secondary metabolites. Both the response of CH4 production to temperatures and the presence of methanogenic archaea indicate that the CH4 produced in heartwood was microbial in origin. Methanobacterium was the dominant methanogens in the heartwood of Populus canadensis which had high concentrations of CH4, and was absent from the heartwood of Pinus tabuliformis and Salix matsudana which produced negligible or very low CH4. Water- and ethanol-soluble extractives enhanced microbial CH4 production in heartwood incubations but acetone-soluble extractives inhibited the production. Since acetone-soluble extractives inhibited the production of microbial CH4, it is assumed that the CH4 production may be suppressed from heartwood with phenolic compounds that act as an antibiotic for methanogenic archaea in most living trees in upland forests. The inhibitory effects of secondary metabolites on microbial CH4 production in heartwood reduce the CH4 emissions from the stems of living trees in upland forests.