Vegetation and carbon exchange across snow-free seasons in a low-arctic fen in Kalaallit Nunaat (Greenland) : environmental controls of carbon fluxes

Northern mires are important carbon sinks while also being significant natural sources of methane (CH₄), with fluxes that vary depending on mire type, vegetation composition, and environmental conditions. However, these dynamics remain understudied in Arctic regions. This thesis investigates the vegetation, carbon exchange and their drivers in the low-arctic Naqinnera Silarlermi Masarsuk (NSM) fen in Kalaallit Nunaat (previously known as Kobbefjord fen, Greenland), with particular emphasis on less studied methane. The study addresses three key knowledge gaps: (1) the classification and bryophyte species composition of the fen; (2) vegetation composition and carbon fluxes during the spring shoulder season; and (3) the environmental controls of carbon dioxide (CO₂) and CH₄ fluxes. The NSM fen likely represents a low-arctic poor-to-intermediate flark-spotted fen (arktinen rimpilaikkuinen aaparahkasammalneva) dominated by S. lindbergii and S. stramineum mosses followed by vascular plants such as E. angustifolium, C. rariflora and T. cespitosum. Bryophyte composition of the fen was described here for the first time. However, uncertainties in mire classification remain, as the classification framework is largely based on Boreal regions, while mire types and species niches are different in the Arctic. Measurements conducted during 24th –26th June 2025 show that spring shoulder season vegetation was dominated by aboveground plant litter and S. lindbergii moss, and that the fen functioned as a net carbon source, with positive median fluxes of both CO₂ and CH₄. Analysis of long-term data indicates that CH₄ emissions were primarily driven by water level and soil temperature, both positively correlated with the fluxes. Moreover, CO₂ fluxes were negatively related to photosynthetically active radiation (PAR), normalized difference vegetation index (NDVI), and water level. Importantly, after including CH₄ into the net carbon balance calculations as CO₂-equivalents, the fen acted as a carbon source across the snow-free season (June-October) and growing season (July-August) years (2008-2021), altering previous interpretations of the NSM fen being a net carbon sink during the growing season. These results highlight the critical role of methane in the Arctic carbon budgets and emphasize the need for detailed vegetation and species-level characterization, as well as improved mire classification framework, to reduce uncertainty in assessments of arctic ecosystem carbon dynamics.