The role of trees in the CH4 and N2O exchange in boreal forest

Trees have been demonstrated to play a role in the methane (CH₄) and nitrous oxide (N₂O) cycling in forests. Emissions of these two greenhouse gases have been observed from tree stems and shoots. The stem emissions of both CH₄ and N₂O have been suggested to originate from the soil, however, their transportation mechanisms might differ, and furthermore, at least the stem-emitted CH₄ can also be produced within tree tissue. Boreal forests are considered a sink of CH₄ due to predominant soil oxidation, but when CH₄ is taken up by the roots, it bypasses the CH₄-oxidation zone in the surface soil. The stem N₂O fluxes at the boreal zone have been shown to follow seasonal physiological activity of trees. However, studies on tree CH₄ and N₂O fluxes are scarce in the boreal zone.

We studied the tree stem CH₄ and N₂O exchange from the stems of Scots pine, downy birch, and Norway spruce – in total 47 trees, growing at six study plots with naturally different soil moisture and ground vegetation conditions (6–9 trees per plot). The measurements were performed during July–August 2017 at the Hyytiälä SMEAR II (Station for Measuring Ecosystem-Atmosphere Relations) ICOS (Integrated Carbon Observation System) research site, in southern Finland. In addition to the stems, we measured forest floor CH₄ and N₂O fluxes at all the plots, and shoot CH₄ fluxes from birch and pine at one plot. The stem chambers were installed at the tree bases, ca. 30 cm above the soil surface. Additionally, from the trees with the shoot measurements, we measured the stem fluxes from several heights in order to study the flux variation in the stem vertical profile. All the flux measurements were conducted with closed chambers – the stem and forest floor measurements were performed by using manual sampling and gas chromatography, while a portable greenhouse gas analyser was used for the shoot measurements. Soil moisture and soil temperature were monitored at the study plots throughout the measurement period.

The results show that all the studied tree species emit both CH₄ and N₂O from stems. Birches growing at one plot with waterlogging conditions stand out with the highest stem CH₄ emissions. Concerning the N₂O emissions, birch stems showed significantly higher emissions than pine stems. The results of the shoot measurements indicate that both birch and pine emit small amounts of CH₄ from their shoots, but the driving factors of the emissions may be different for the two species. Our aim is to model the spatial variability of the stem CH₄ and N₂O fluxes at the site, and to develop an upscaling method combining the stem and forest floor CH₄ and N₂O exchange, based on an existing modelling work on the forest floor CH₄ fluxes at the site.

 

Acknowledgements: This research was supported by the Academy of Finland (288494, 2884941), National Centre of Excellence (272041), ICOS-FINLAND (281255), 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).