Seasonal variations of net CO2 exchange in European Arctic ecosystems

The carbon dioxide exchange in arctic and subarctic terrestrial ecosystems has been measured using the eddy-covariance method at sites representing th e latitudinal and longitudinal extremes of the European Arctic sea areas as part of the Land Arctic Physical Processes (LAPP) project. The sites inclu de two fen (Kaamanen and Kevo) and one mountain birch ecosystems in subarct ic northern Finland (69 degrees N); fen, heathland, and snowbed willow ecos ystems in northeastern Greenland (74 degrees N); and a polar semidesert sit e in Svalbard (79 degrees N). The measurement results, which are given as w eekly average diurnal cycles, show the striking seasonal development of the net CO2 fluxes. The seasonal periods important for the net CO2 fluxes, i.e . winter, thaw, pre-leaf, summer, and autumn can be identified from measure ments of the physical environment, such as temperature, albedo, and greenne ss. During the late winter period continuous efflux is observed at the perm afrost-free Kaamanen site. At the permafrost sites, efflux begins during th e thaw period, which lasts about 3-5 weeks, in contrast to the Kaamanen sit e where efflux continues at the same rate as during the winter. Seasonal ef flux maximum is during the pre-leaf period, which lasts about 2-5 weeks. Th e summer period lasts 6 weeks in NE Greenland but 10-14 weeks in northern F inland. During a high summer week, the mountain birch ecosystem had the hig hest gross photosynthetic capacity, GP(max), followed by the fen ecosystems . The polar semidesert ecosystem had the lowest GP(max). By the middle of A ugust, noon uptake fluxes start to decrease as the solar elevation angle de creases and senescence begins within the vascular plants. At the end of the autumn period, which lasts 2-5 weeks, topsoil begins to freeze at the end of August in Svalbard; at the end of September at sites in eastern Greenlan d; and one month later at sites in northern Finland.