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

Abstract

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.