Meteorological responses of carbon dioxide and methane fluxes in the terrestrial and aquatic ecosystems of a subarctic landscape

Abstract. The subarctic landscape consists of a mosaic of forest, peatland and aquatic ecosystems and their ecotones. The carbon (C) exchange between ecosystems and the atmosphere through carbon dioxide (CO₂) and methane (CH₄) fluxes varies spatially and temporally among these ecosystems. Our study area in Kaamanen in northern Finland covering 7 km² of boreal subarctic landscape with upland forest, open peatland, pine bogs and lakes. We measured the CO₂ and CH₄ fluxes with eddy covariance and chambers between June 2017 and June 2019 and studied the C flux responses to varying meteorological conditions. The landscape area was an annual CO₂ sink of -25.9 ± 65.7 and -41.3 ± 64.9 g C m^-2, and a CH₄ source of 2.4 ± 0.7 and 2.3 ± 0.7 g C m^-2 during the first and second study year, respectively. The pine forest had the largest contribution to the landscape-level CO2 sink, -78.3 ± 50.8 and -118.9 ± 26.8 g C m^-2, and the fen to the CH₄ emissions, 7.0 ± 0.2 and 6.3 ± 0.3 g C m^-2, during the first and second study year, respectively. The lakes within the area acted as CO₂ and CH₄ sources to the atmosphere throughout the measurement period, with an organic sediment lake located downstream from the fen showing sixfold fluxes compared to a mineral sediment lake. The annual C balances were affected most by the rainy peak growing season of 2017 and the heatwave and drought event in July 2018. The rainy period increased the ecosystem respiration of the pine forest due to continuously high soil moisture content. A similar flux response to abundant precipitation was not observed for the fen ecosystem, which is adapted to high water table levels. During the heatwave and drought period, similar responses were observed for all terrestrial ecosystems, with decreased gross primary productivity and net CO₂ uptake, caused by the unfavourable growing conditions and plant stress due to the soil moisture and vapour pressure deficits. Additionally, the CH₄ emissions from the fen decreased during and after the drought. However, the timing and duration of drought effects varied between fen and forest ecosystems, as C fluxes were affected sooner and had a shorter post-drought recovery time in the fen than forests. The differing CO₂ flux response to weather variations showed that terrestrial ecosystems can have a contrasting impact on the landscape-level C balance in a changing climate, even if they function similarly most of the time.