Degrading palsa mires in northern Europe : changing vegetation in an altering climate and its potential impact on greenhouse gas fluxes

Abstract: The on-going global warming is projected to have large impacts on the arctic and subarctic environment. Especially vulnerable are ecosystems with narrow climatic envelopes and with small possibilities of adaptation. An example of this is the palsa mires, subarctic mire complexes with patches of permafrost. Palsas are peat mounds with a frozen core, formed by the expansion and upheaval of the segregating ice. Due to the dryer and more nutrient poor conditions on top of the mounds, the palsa vegetation differs from the wet surroundings, creating a mosaic of different microhabitats within the mire. The occurrence of palsas is determined by several environmental and climatological factors, such as air temperature, precipitation, snow depth and peat thickness. Hence, increasing precipitation and air temperature may induce thawing of the frozen peat and subsidence of the peat surface, resulting in a thicker active layer and wetter conditions. Consequently the vegetation may change, adapting to the increased wetness. This study investigates changes in the vegetational patterns associated with degradation of palsas on a regional scale, and its impacts on greenhouse gas fluxes (GHG). Field observations from fifteen sites in northern Sweden, Finland and Norway showed that the observed vegetational changes related to palsa degradation at the Stordalen mire (northern Sweden) can be considered a general phenomena on a regional scale. Thus the GHG flux measurements from the Stordalen mire may be used to quantify the potential change in GHG fluxes associated with climate induced degradation of palsa mires in the region. A simple model was built to estimate the areal change in palsa vegetation with a future climate change. The modeling results showed a rapid decrease in areas suitable for palsas within the first 30-60 years: by 2020 the area suitable for palsas was reduced by 50%, and by 2050 no areas suitable for palsas were left. The model projected areas dominated by the dryer palsa vegetation to decrease to the benefit of sphagnum mosses and graminoids, due to the expected increased wetness. The projected impact on GHG fluxes is an increase in both carbon dioxide uptake and methane emissions, mainly due to the expansion of tall graminoid vegetation. Together with similar studies these results may help to indicate what potential impacts a changing climate may have on the distribution of palsa mires. However more research is needed for better understanding the long term vegetational succession in degrading palsa mires and its impacts on GHG fluxes with an altered climate.