Drivers of intra-individual spatial variability in methane emissions from tree trunks in upland forest

In upland forests, trunk CH₄ emissions exhibit significant intra-individual spatial variability, which was explained by variations in both CH₄ concentration and CH₄ production rates in the sapwood.

Methane (CH₄) emissions from tree trunks in upland forests should be scaled accurately to assess the role of tree trunk in the forest CH₄ budget. As the chambers used to measure emissions cover only a small part of the large surface area of tree trunks, it is necessary to understand the intra-individual spatial variability of trunk CH₄ emissions. To assess this spatial variability, we measured trunk CH₄ flux at nine locations per individual on four trees in a cool-temperate upland forest for which microbial production of CH₄ inside the trunk is likely an important source of the emission. To know the origin of this variability and the underlying processes, we also measured the potential rate of CH₄ production and CH₄ concentrations in both sapwood and characterized wood and bark. Up to 15-fold intra-individual spatial variations in CH₄ fluxes were observed in target trees. The variability in emissions within an individual was primarily shown by the variation of the sapwood CH₄ concentration which was further explained by the variation in potential CH₄ production rate. The radial CH₄ diffusivity calculated from concentration gradients and emissions was not related to the measured characteristics of either wood or bark. We emphasized the importance of sampling trunk CH₄ flux at multiple locations on the surface of a tree trunk in order to capture the spatial variability, a prerequisite for estimating tree-level CH₄ emissions.