Contribution of tree stem and canopy fluxes to the CH4 budget of a boreal birch and spruce forest

Upland forest soils are typically a net methane (CH₄) sink, while trees may act as CH₄ sources. Studies on tree CH₄ exchange in boreal forests, especially regarding canopies, are rare. We aimed to quantify the contribution of trees to the forest CH₄ budget during spring leaf-out period and to reveal the role of microbes in the CH₄ exchange of trees. We measured stem and shoot fluxes of two common boreal tree species at a fen and at an upland site at Hyytiälä, southern Finland, together with soil CH₄ flux, environmental variables and the abundances methanogens and methanotrophs within the forest. Both birch (Betula pubescens) and spruce (Picea abies) trees emitted CH₄ from their aboveground surfaces, with significantly higher stem emissions detected from the birch and higher shoot emissions from the spruce. The shoot CH₄ exchange had no clear link to the vertical profile of the canopy or the progress of the leaf-out. The stem CH₄ emissions from birches at the fen were high (mean 45 µg h⁻¹ m⁻²) and decreased drastically with stem height. Their dynamics followed soil temperature, suggesting the emitted CH₄ originated from the soil. A lack of similar pattern in the fen spruces and in the upland birch indicates other processes behind the stem CH₄ fluxes of these trees. The lack of detection of methanogens or methanotrophs in the aboveground plant tissues suggest that the observed tree-derived CH₄ fluxes were not induced by these microbes. The emitted CH₄ from the tree stems may, however, be produced microbially in the soil indicating that physiological differences in tree anatomy or adaptation to different belowground conditions might be a key factor explaining the differences between the tree species.

Acknowledgements: This research was supported Academy of Finland (288494, 2884941), National Centre of Excellence (272041), ICOS-FINLAND (281255), Helsinki Institute of Life Science (HiLIFE), Czech Science Foundation (17-18112Y) and National Sustainability Program I (LO1415), and the European Research Council (ERC) under Horizon 2020 research and innovation programme, grant agreement No (757695).