Patrik Vestin

Understanding greenhouse gas fluxes in semi-arid ecosystems is critical for improving our understanding of biogeochemical cycles, particularly in underrepresented regions like the African Sahel. In these landscapes, greenhouse gas exchange arises from ground, trees, and water ponds, and is further shaped by environmental conditions and grazing. The carbon dioxide, methane, and nitrous oxide fluxes were quantified from these components in a Sahelian savanna in Senegal, while also assessing grazing impacts and environmental drivers (soil water content, temperature, vapor pressure deficit, photosynthetically active radiation). The ground was a net carbon dioxide sink during the rainy season but shifted toward neutrality or weak emission in the dry season, consistently acted as a methane sink, and was a year-round nitrous oxide source. Seasonal ponds were strong methane and nitrous oxide emission hotspots, with methane emissions being high enough to offset the sink of the overall savanna landscape. Trees contributed to carbon dioxide and methane uptake via branches, whereas stems were net methane and carbon dioxide emitters. Both stems and branches emitted nitrous oxide, and the presence of trees enhanced carbon dioxide, methane and nitrous oxide fluxes from soils beneath their canopies. Grazing enhanced ground gross primary production, reduced methane uptake, while no effect was seen on nitrous oxide fluxes. Temporal variability of carbon dioxide and nitrous oxide fluxes was strongly linked to soil water content and temperature, whereas methane fluxes showed no correlations with any of the measured drivers. These results demonstrate that tree- and pond-mediated fluxes, together with grazing, substantially alter the greenhouse gas fluxes of savanna ecosystems and incorporating these effects is essential for accurately representing semi-arid savannas in global greenhouse gas budgets.