Linkages between Patterned Ground, Alder Shrubland Development, and Active Layer Temperature in the Northwest Siberian Low Arctic

Satellite photo-comparisons indicate extensive expansion of alder (Alnus) shrubs since 1968 within tundra ecotones near Kharp in northwest Siberia. Field observations reveal that nearly all expansion occurred on exposed circles in patterned ground. We mapped the location of alders and circles and measured surface organic thickness, mineral soil thickness, nearsurface soil temperature, shrub height, and Leaf Area Index along transects according to categories of alder stand-age. Young alders occur almost exclusively on circles that lack organic matter and competing vegetation, and are relatively warm in summer. Contrasts in the summer thermal regime of circles and inter-circles diminish during alder colonization and become reversed in older shrublands. We conclude that exposed circles comprise abundant microsites that are susceptible to shrubification in warmer areas of the Low Arctic; summer active layer temperatures decline and circles are inactivated after shrubification, as organic matter buildup and canopy shading reduce the potential for differential frost heave.