Methane (CH4) emissions from tree stems in boreal upland and drained peatland forest

Methane (CH4) is a strong greenhouse gas emitted from wetland ecosystems due to the anaerobic microbial production in the soil. Upland forests are generally considered as a sink of atmospheric CH4 due to microbial CH4 oxidation in the soil. Emissions from trees and the responsible mechanisms are still poorly understood; however, several studies have shown that trees may act as conduits for CH4 produced in the wet soils, and that CH4can also be produced within the tree and emitted through the stem or from the canopy. We measured CH4 fluxes from the tree-stems and forest floor in a boreal upland forest and in a paludified forest in 2014-2015 (Hyytiälä), and in a forestry-drained fen in 2016 (Lettosuo) in southern Finland. In all the sites the tree stands consisted of mature Scots pine (Pinus sylvestris), Norway spruce (Picea abies) and downy and silver birch (Betula pubescens, Betula pendula). Fluxes of CH4 were measured using static chamber systems and quantified by gas chromatography, or laser spectroscopy. Scaffold towers were used for measurements at multiple stem heights. Additionally, soil profile and tree-stem CH4 concentrations were measured at the forestry-drained fen to obtain understanding of the in-situ CH4 production in the soil and within the trees. Soil moisture and ground water table as well as a multitude of meteorological parameters were measured to link the tree-CH4fluxes to environmental conditions. Methane was consistently emitted from the stems of all tree species, while the birches showed higher emissions compared to the spruce and pine trees at both upland forest and forestry-drained fen. The emissions were considerably higher from trees growing on wet soils compared to the trees on upland soils. Also, higher CH4 emissions were measured in the lower parts of the birch stems than upper parts, and these emissions increased during the growing season. Preliminary results from the forestry-drained fen show higher stem CH4 emissions under high water table level. This supports our hypothesis that the CH4 emitted from tree-stems originate from the soil. Further analysis of soil and tree-stem CH4 concentrations, as well as microbial analysis of the presence of CH4 producing or consuming microbes will give more information of the origin of tree emitted CH4.