Methane (CH₄) is the second most important anthropogenic greenhouse gas, and understanding its global dynamics is crucial for addressing climate change. Stem CH₄ flux exhibits spatial and temporal variability, and ongoing studies aim to clarify its controlling factors and production processes. Regarding the relationship between stem CH₄ flux and methanogens within stems, previous studies have suggested that methanogen distribution and community size are highly localized. This implies that the “micro-niches” where methanogenesis occurs are spatially heterogeneous, and potentially explains the large intra-individual, inter-individual, and inter-specific variability of stem CH₄ flux. In this study, we hypothesized that internal wood decay drives this variability by altering moisture conditions and wood structure, thereby defining the spatial variation of stem CH₄ flux.
Field measurements were conducted at the Ashiu Forest Research Station and the Teshio Experimental Forest. Stem CH₄ fluxes were calculated from temporal changes in CH₄ concentration using a closed-chamber system with a trace gas analyzer (LI-7810). At each site, six individuals were selected from each dominant species (six species at Ashiu and five at Teshio), and five chambers were installed per individual. To assess the progression of decay status while minimizing disturbance, internal wood decay was evaluated non-destructively using acoustic tomography (ArboSonic 3D), which detects differences in sound-wave transmission velocity. Measurements were conducted at four cross-sections for each individual, and three-dimensional decay structures were reconstructed by vertically integrating these data. A chamber-specific decay index was defined based on the mean sound velocity around each chamber.
The results confirmed substantial intra-individual, inter-individual, and inter-specific variability in stem CH₄ flux, with these factors explaining most of the observed variance, whereas site effects were negligible. In contrast, decay status exhibited greater inter-site and inter-specific variability than individual-level variation. Overall, stem CH₄ flux showed a marginal positive relationship with decay progression (p = 0.091). However, the strength and direction of this relationship differed significantly among species (interaction: p < 0.01), with some species showing strong positive responses to decay, while others exhibited weak, inverse, or no relationships.