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 (CO), methane (CH) and nitrous oxide (NO) in a boreal riparian forest, and investigate their spatiotemporal variability and ecosystem level importance. For two years, we measured CO and CH 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 NO fluxes on three occasions. All tree stems were net emitters of CO and CH, while NO fluxes were around zero. CO fluxes correlated strongly with air temperature and peaked in summer. CH fluxes correlated modestly with air temperature and solar radiation and peaked in late winter and summer. Trees with larger stem diameter emitted more CO and less CH and trees closer to the stream emitted more CO and CH. The CO and CH fluxes did not differ between spruce and birch, but correlations of CO fluxes with stem diameter and distance to stream differed between the tree species. The absence of vertical trends in CO and CH fluxes along the stems and their low correlation with groundwater levels and soil CO and CH partial pressures suggest tree internal production as the primary source of stem emissions. At the ecosystem level, the stem CO, CH and NO emissions represented 52 ± 16 % of the forest floor CO emissions and 3 ± 1 % and 11 ± 40 % of the forest floor CH and NO uptake, respectively, during the snow-free period (median ± SE). The six month snow-cover period contributed 11 ± 45 % and 40 ± 29 % to annual stem CO and CH 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.