Dry and wet periods determine stem and soil greenhouse gas fluxes in a northern drained peatland forest.

Greenhouse gas (GHG) fluxes from peatland soils are relatively well studied, whereas tree stem fluxes have received far less attention. Simultaneous year-long measurements of soil and tree stem GHG fluxes in northern peatland forests are scarce, as previous studies have primarily focused on the growing season. We determined the seasonal dynamics of tree stem and soil CH, NO and CO fluxes in a hemiboreal drained peatland forest. Gas samples for flux calculations were manually collected from chambers at different heights on Downy Birch (Betula pubescens) and Norway Spruce (Picea abies) trees (November 2020-December 2021) and analysed using gas chromatography. Environmental parameters were measured simultaneously with fluxes and xylem sap flow was recorded during the growing season. Birch stems played a greater role in the annual GHG dynamics than spruce stems. Birch stems were net annual CH, NO and CO sources, while spruce stems constituted a CH and CO source but a NO sink. Soil was a net CO and NO source, but a sink of CH. Temporal dynamics of stem CH and NO fluxes were driven by isolated emissions' peaks that contributed significantly to net annual fluxes. Stem CO efflux followed a seasonal trend coinciding with tree growth phenology. Stem CH dynamics were significantly affected by the changes between wetter and drier periods, while NO was more influenced by short-term changes in soil hydrologic conditions. We showed that CH emitted from tree stems during the wetter period can offset nearly half of the soil sink capacity. We presented for the first time the relationship between tree stem GHG fluxes and sap flow in a peatland forest. The net CH flux was likely an aggregate of soil-derived and stem-produced CH. A dominating soil source was more evident for stem NO fluxes.