Greenhouse gas fluxes from Downy Birch stems during the spring sap-run period and their dependence on dissolved gas concentrations in xylem sap

Tree stems are known to emit greenhouse gases CH₄, CO₂ and N₂O to the atmosphere but the processes and drivers behind these fluxes are still contested. Soil water is taken up by tree roots and moves up the xylem due to a negative pressure gradient caused by transpiration through the leaves. Consequently, dissolved gases in the soil water move up the stem and are potentially diffused to the atmosphere through the bark. Periods of soil freeze-thaw in the spring are crucial hot-moments of GHG release from the soil, as well as stems. As birch trees go through a sap running period between the thawing of the soil and bud break, they provide an opportunity to study stem GHG fluxes during the peak time of emissions, together with the concentrations of dissolved gases in the birch sap.

We quantified the fluxes of CH₄, CO₂ and N₂O from Downy birch (Betula pubescens), as well as Norway spruce (Picea abies) for comparison, in a temperate nutrient-rich drained peatland forest in April and May 2023. In addition, we studied the relationship between birch stem fluxes and dissolved gas concentrations inside the xylem sap. Stem fluxes were determined using static chambers attached to the tree stems and automatic LI-COR gas analysers. Dissolved gas samples were extracted from the collected birch sap and soil water after water-atmosphere equilibration, and were analysed in the lab using gas-chromatography. In addition, we analysed the relationships between the chemical and microbiological composition of the soil and soil and stem GHG fluxes.

Birch stem CH₄, CO₂ and N₂O fluxes peaked in the end of April, following the the temporal trend of soil and air temperature, with higher fluxes during warmer days, likely related to increased microbial activity in the soil. Dissolved CH₄ concentrations in the birch sap peaked with a delay in relation to peak stem emissions, indicating that xylem sap flow rate needs to be studied to comprehend the water dynamics inside the stem. Relationships between stem fluxes and dissolved gas concentrations were strongest at the bottom part of the tree. A more detailed analysis together with examination of the underlying soil chemistry and microbiology will be presented to further explain the processes behind soil and tree stem GHG flux dynamics.