Natascha Kljun

High latitude mires are key ecosystems in the context of climate change since they store large amounts of carbon while constituting an important natural source of methane (CH₄). However, while a growing number of studies have investigated methane fluxes (FCH₄) at the plot- (~1 m²) and ecosystem-scale (~0.1–0.5 km²) across the boreal biome, variations of FCH₄ magnitudes and drivers at the mesoscale (i.e., 0.5–20 km²) of a mire complex are poorly understood. This study leveraged a network of four eddy-covariance flux towers to explore the spatio-temporal variations in ecosystem-scale FCH₄ across a boreal mire complex in northern Sweden over 3 years (2020–2022). We found a consistent hierarchy of drivers for the temporal variability in FCH₄ across the mire complex, with gross primary production and soil temperature jointly emerging as primary controls, whereas water table depth had no independent effect. In contrast, peat physical and chemical properties, particularly bulk density and C:N ratio, were identified as significant baseline constraints for the spatial variations in FCH₄ across the mire complex. Our observations further revealed that the 3-year mean annual FCH₄ across the mire complex ranged from 7 g C m⁻² y⁻¹ to 11 g C m⁻² y⁻¹, with a coefficient of variation of 16% that is similar to the variation observed among geographically distant mire systems and peatland types across the boreal biome. Thus, our findings highlight an additional source of uncertainty when scaling information from single-site studies to the mire complex scale and beyond. Furthermore, they suggest an urgent need for peatland ecosystem models to resolve the mesoscale variations in FCH₄ at the mire complex level to reduce uncertainties in the predictions of peatland carbon cycle-climate feedbacks.