Tree stem-atmosphere greenhouse gas fluxes in a boreal riparian forest

The cycling of greenhouse gases in forest ecosystems is significantly influenced by tree stems. Yet, little is known about the variability and drivers of stem-atmosphere greenhouse gas fluxes, especially in managed boreal riparian ecosystems where environmental conditions vary substantially at small spatial scales and throughout the year. Here, we report magnitudes and drivers of tree stem-atmosphere fluxes of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) in a riparian buffer zone of a Swedish boreal forest that has been subject to recent forest clearcutting and historic ditching. For two full years, we conducted CO₂ and CH₄ flux chamber measurements on a monthly basis in 14 spruce trees (Picea abies) and 14 birch trees (Betula pendula) that grew between one and fifteen meters from a headwater stream. We also performed N₂O flux measurements during three occasions. All trees were net emitters of CO₂ and CH₄ over the majority of the year, while N₂O fluxes were close to zero. CO₂ fluxes correlated strongly and positively with air temperature and followed distinct seasonal cycles peaking in summer. CH₄ fluxes correlated modestly with air temperature and solar radiation and peaked in late winter and summer. Trees with larger stem diameter released more CO₂ and less CH₄, and trees that were nearer the stream released more CO₂ and CH₄. The CO₂ and CH₄ fluxes did not differ between spruce and birch in general, but correlations of CO₂ fluxes with stem diameter and distance to stream differed between the tree species. The absence of distinct vertical trends in the CO₂ and CH₄ fluxes along the stems and their lack of correlation with groundwater levels and groundwater greenhouse gas concentrations point to tree internal production as the primary source of the tree stem gas emissions. Upscaled to the ecosystem, the tree stem CO₂, CH₄ and N₂O emissions represented 52% of the forest floor CO₂ emissions and 2.5% and 11.3% of the forest floor CH₄  and N₂O uptake, respectively, during the snow-free season. The snow cover season contributed 15% and 35% to annual tree stem CO₂ and CH₄ emissions, respectively. In contrast to other riparian zone studies, the stem gas fluxes in our study generally exhibited characteristics of an upland rather than a wetland ecosystem, likely because of historical ditching and subsequent groundwater level declines.