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

Abstract

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 CH₄, N₂O and CO₂ 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 CH₄, N₂O and CO₂ sources, while spruce stems constituted a CH₄ and CO₂ source but a N₂O sink. Soil was a net CO₂ and N₂O source, but a sink of CH₄. Temporal dynamics of stem CH₄ and N₂O fluxes were driven by isolated emissions' peaks that contributed significantly to net annual fluxes. Stem CO₂ efflux followed a seasonal trend coinciding with tree growth phenology. Stem CH₄ dynamics were significantly affected by the changes between wetter and drier periods, while N₂O was more influenced by short-term changes in soil hydrologic conditions. We showed that CH₄ 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 CH₄ flux was likely an aggregate of soil-derived and stem-produced CH₄. A dominating soil source was more evident for stem N₂O fluxes.

Keywords: Carbon dioxide; Downy birch; Methane; Nitrous oxide; Sap flow; Stem fluxes.

Graphical abstract

Fig. 1

Temporal dynamics of soil and stem CH₄, N₂O and CO₂ fluxes and environmental parameters in the drained peatland forest. (A) Daily mean air and soil temperatures (°C), soil water content (SWC, m³ m⁻³), water table depth (WTD, cm), and daily sum precipitation (mm) (October 2020–December 2021); daily mean soil (December 2020–August 2021) and stem (October 2020–December 2021) (B) CH₄ (μg C m⁻² h⁻¹), (C) N₂O (μg N m⁻² h⁻¹) and (D) CO₂ (mg C m⁻² h⁻¹) fluxes with standard error as the shaded area. Stem fluxes are expressed in units per m² of stem bark surface area. Soil fluxes are expressed in units per m² of soil surface area. Drier and wetter periods of the year are emphasised by grey vertical lines. The drier period was defined by SWC being continuously <0.3 m³m⁻³. Drier period: 13 July 2021–20 October 2021; wetter period: 22 October 2020–12 July 2021.

Fig. 2

Vertical profile of (A) CH₄, (B) N₂O and (C) CO₂ stem fluxes at 0.1 m (N = 349), 0.8 m (N = 174) and 1.7 m (N = 174), averaged across all plots and throughout the study period. Different letters above bars indicate statistically significant differences between fluxes at different heights within species, according to a Kruskal-Wallis one-way analysis of variance followed by a post-hoc Dunn test (p < 0.05). The solid line within each box marks the median value, circles the mean value and dotted lines the 25th and 75th percentiles.

Fig. 3

Relationship between sap flow density and birch (N = 39) and spruce (N = 39) stem (A) CH₄, (B) N₂O and (C) CO₂ fluxes during the growing season (04 June 2021–06 September 2021). Adjusted R² and p-values of the relationships have been calculated according to the linear regression model.

Fig. 4

Contributions of average tree stem (birch and spruce) and soil (A) CH₄, (B) N₂O and (C) CO₂ fluxes during the drier and wetter periods of the year, scaled to a unit of ground area of forest. The drier period was defined by SWC being continuously <0.3 m³m⁻³. Wetter period: 4 December 2020–12 July 2021; drier period: 13 July 2021–19 August 2021. Note that both stem and soil flux data from these date ranges were used for contributions' calculations as soil flux data was only available between 4 December 2020–19 August 2021. Positive flux values indicate gas emission, negative values uptake. The boxes represent mean fluxes across all measurement points.