Spatial and temporal dynamics of subarctic birch forest carbon exchange

High northern latitudes are of special importance for the global carbon budget as they store large amounts of organic matter in the soil and are expected to be the most strongly affected by climate change. Here we investigate carbon exchange in the subarctic mountain birch forest growing on the limit where tree growth is possible. The study sites are located close to the Abisko village in northern Sweden, in the catchment of Lake Torneträsk. The investigated area is under influence of increasing temperature and periodical insect outbreaks causing defoliation of the trees. The last insect outbreak which has happened in 2004 caused at least 90 g C m-2 loss of carbon to the atmosphere and it turned the site from being C neutral in undisturbed years into a source of carbon. The forest was regenerating for 1-3 years after disturbances depending on the level of defoliation during outbreak. The annual cumulative gross primary production in the undisturbed forest was mainly dependent on the length of the growing season and the incoming radiation. Climate change causes an increase in the length of the growing season and expansion of the forest on tundra-heath ecosystem. The annual cumulative respiration is mainly dependent on the thickness of the organic soil layer and temperature. Thickness of the organic soil in the forest is most likely connected with past transformation from a tundra-heath to the forest ecosystem. Respiration in the tundra-heath ecosystem is carbon limited and expansion of birch with litter rich in highly labile carbon cause the newly established forest to be carbon source to the atmosphere. This study suggests that the invasion of deciduous tree species onto the C limited arctic tundra may cause high losses of soil carbon to the atmosphere