The potential impact of changing vegetation on thawing permafrost : effects of manipulated vegetation on summer ground temperatures and soil moisture in Abisko, Sweden

Abstract: The Arctic region has experienced significant warming in the past decades and according to predictions further increases in air temperatures and precipitation will occur during the 21st century. Recent changes in climate have had widespread implications for the permafrost around the Arctic. Shorter periods of snow covering the Arctic regions significantly reduce the average annual albedo which dramatically increases the amount of energy being absorbed by the ground surface. In addition, the amount of snow has increased which results in even further warming of the ground through insulation during the winter. Warmer ground temperatures increase the available amount of organic material stored in the formerly frozen ground when the active layer (i.e. the upper layer that thaws and refreezes annually) becomes thicker. This can potentially lead to additional releases of greenhouse gases resulting in even further regional warming and permafrost degradation with implications for the global climate. The Arctic vegetation has also changed due to higher air temperatures and altered precipitation patterns. Future predictions suggest more productive vegetation over the Arctic. This will likely increase the dominance of trees and shrubby vegetation leading to reductions in moss vegetation. Sub-arctic palsa mires in northern Sweden are experiencing a different shift in vegetation with higher graminoid abundance and declining shrub coverage due to increasing wetness. A change in the type of vegetation i.e. a change in plant functional types can alter the albedo together with the shading-, the evapotranspiration-, the snow trapping- and insulation potential. All these factors have the potential to change the ground temperature and the soil moisture content which affect the underlying permafrost. Changes like this have already been observed at the margin of where permafrost can exist, for example in the Abisko area in northernmost Sweden. This report presents results from an experiment where different plant functional types (dwarf shrubs, graminoids, mosses and lichens) have been removed at the Storflaket mire in the Abisko area. During a five year period the vegetation has been manipulated in different plots to extract information on how soil moisture content and ground temperature (10 cm depth) respond to changes in vegetation. Ground temperatures were significantly warmer (0.2°C) where dwarf shrubs had been removed compared to the control plots. Removal of mosses and lichens significantly decreased the ground temperature by 0.7°C. Soil moisture was only significantly higher (6.6%) where mosses and lichens had been removed. It is likely that ongoing vegetation changes on the mires i.e. a shift from dwarf-shrub dominated to graminoids dominated vegetation in the Abisko area will continue also in the future. The results suggest that the potential decrease of dwarf shrubs on the mires will lead to an additional increase in ground temperatures making the permafrost more sensitive to additional future changes in climate.