Vegetation and climate impacts on active layer thickness in palsas: insights from 13 years of field course data

Abstract

Global climate warming has resulted in the circumpolar degradation of palsa mires. While climate is a key driver of palsa distribution and stability, local factors such as vegetation also influence permafrost dynamics. However, studies on the effects of vegetation on active layer thickness (ALT) in palsas remain limited. To address the logistical constraints of long-term annual monitoring, we drew on 13 years of field course data on ALT and vegetation height from a palsa mire in northwestern Finland. Specifically, we explored: (i) the occurrence of a temporal thickening trend in ALT, (ii) the influence of vegetation height on ALT, and (iii) the correlation between ALT and climate variables. Results revealed a complex relationship between ALT, climate, and vegetation height, which was measured at two distinct vegetation types: moss, lichen, and dwarf shrub-dominated low vegetation and shrub-dominated tall vegetation. No statistically significant long-term trends in ALT were detected, but a slight deepening was observed under both low (0.28 cm a-¹) and tall (0.49 cm a-¹) vegetation. Significant correlations (p < 0.05) between ALT and climate variables were found only at tall vegetation patches, specifically with snow depth. While not statistically significant, ALT showed positive correlations with several snow-related variables and summer air temperatures, and negative relationships with snow cover onset, autumn precipitation, and winter wind speed. ALT was statistically significantly thicker under low vegetation compared to tall vegetation. During thaw season, shrubs may reduce the thickening of the active layer by providing shade and increasing evapotranspiration, compared to low vegetation patches. These findings highlight the complex interplay between climate, vegetation, and ALT in palsas, emphasizing the need for long-term monitoring to better understand permafrost degradation patterns and changes in palsa mires. Recurring annual field courses provide an approach with educational benefits for collecting scientifically valid ALT observations.