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

Tree stems exchange greenhouse gases with the atmosphere but the magnitude, variability and drivers of these fluxes remain poorly understood. Here, we report stem fluxes of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) in a boreal riparian forest, and investigate their spatiotemporal variability and ecosystem level importance. For two years, we measured CO₂ and CH₄ fluxes on a monthly basis in 14 spruces (Picea abies) and 14 birches (Betula pendula) growing near a headwater stream affected by historic ditching. We also measured N₂O fluxes on three occasions. All tree stems were net emitters of CO₂ and CH₄, while N₂O fluxes were around zero. CO₂ fluxes correlated strongly with air temperature and peaked in summer. CH₄ fluxes correlated modestly with air temperature and solar radiation and peaked in late winter and summer. Trees with larger stem diameter emitted more CO₂ and less CH₄ and trees closer to the stream emitted more CO₂ and CH₄. The CO₂ and CH₄ fluxes did not differ between spruce and birch, but correlations of CO₂ fluxes with stem diameter and distance to stream differed between the tree species. The absence of vertical trends in CO₂ and CH₄ fluxes along the stems and their low correlation with groundwater levels and soil CO₂ and CH₄ partial pressures suggest tree internal production as the primary source of stem emissions. At the ecosystem level, the stem CO₂, CH₄ and N₂O emissions represented 52 ± 16 % of the forest floor CO₂ emissions and 3 ± 1 % and 11 ± 40 % of the forest floor CH₄ and N₂O uptake, respectively, during the snow-free period (median ± SE). The six month snow-cover period contributed 11 ± 45 % and 40 ± 29 % to annual stem CO₂ and CH₄ emissions, respectively. Overall, the stem gas fluxes were more typical for upland rather than wetland ecosystems likely due to historic ditching and subsequent groundwater level decrease.