Palsa Growth and Decay in Northern Sweden : Climatic and Environmental Controls

Sammanfattning: This thesis outlines the development and decay of palsas in northern Sweden in relation to climatic and environmental factors. Palsas were investigated on morphology, vegetation, peat porosity, ground temperature and local climate in four bogs situated on a north-south gradient in northern Sweden. A new classification for palsa stages (embryo, young, mature, degrading and remnant stage), based on morphological and vegetational characteristics, is proposed and described in the thesis. The start of palsa growth seems to be favoured by a low vegetation height and a high cover of Sphagnum mosses. Very high porosities were measured in this peat type, resulting in a high insulation capacity preserving the frozen ground in summer. A decrease of porosities in the surface peat layer from the embryo palsa stage to the mature and degrading palsa stage was found. This contributes to the degrading of palsas, due to higher thermal diffusivities in palsas with lower porosities. The aggradation of the ice core can also cease due to the observed change in vegetation from low vegetation on embryo and young palsas to vegetation with tall Betula nana shrubs on degrading palsas. This was found to cause thicker snow cover that prevents frost penetration into the palsa core. The present climate conditions (with mean annual temperature of –1.5°C and annual precipitation of 737 mm) at the boundary of palsa distribution in Sweden were found to be unsuitable for palsa development. Palsa growth started here during a cold period in the last part of the Little Ice Age, with estimated mean annual and mean winter temperatures of –2.3°C and –10.0°C, respectively. The major conclusions are that besides air temperature, depth of snow cover, summer precipitation and hydrology, also vegetation cover and peat characteristics are important for palsa growth and decay.