The North American Arctic Transect: Baseline Vegetation and Active Layer Maps for the IPY

Maps of vegetation and active layer were made at 21 10x10-m grids at 11 localities along an 1800-km bioclimate North American Arctic Transect (NAAT). Locations were chosen in each of the five Arctic bioclimate subzones (Subzones A (cold) to E (warm)) and the northern boreal forest. The primary purpose of the maps was to analyze the relationship of the patchy mosaics of plant communities to patterns of plant biomass, microhabitats, active-layer depth, and snow-accumulation along an arctic bioclimate gradient. Vegetation maps show small patterned-ground features (non-sorted circles, earth hummocks, turf hummocks, and small non-sorted circles). The scale of vegetation patterning decreases with latitude; in subzone A, landscape heterogeneity is best visible at small (decimeter) scales, with major differences in plant communities and biomass associated with the cracks vs. the centers of small non-sorted polygons. Toward the southern end of the gradient (Subzone E), the vegetation within the grids is homogenous tussock tundra with minor variations in plant communities mainly associated with earth hummocks and small non-sorted circles that occur between cottongrass tussocks. Patterns of plant communities in the grids are clearly reflected in maps of active-layer thickness and snow depth. In subzones C and D, well-developed non-sorted circles develop wherever there are peaty soils, creating significant micro-scale gradients in soil temperature and soil moisture, which contribute to large differences in active-layer thickness and to frost heave. Thaw depths, therefore, have the most micro-scale variability in the middle part of the climate gradient where there is maximum contrast in plant cover and biomass between circles and inter-circle areas. This contrast is greatest towards the beginning of summer and decreases as the season progresses. The local topography associated with differential winter heave in the patterned ground features was also reflected in the snow measurements. The shallowest thaw depths were found in the coldest sites (subzone A) and also in the warmest sites (subzone E), where the vegetation, especially moss, is thickest (live and dead moss thickness >15 cm) and hence better insulates the soil. Mean end-of-season active-layer depths varied from 29 cm at Isachsen to 70 cm at Franklin Bluffs. These grids will be monitored in the future as part of the Circumpolar Active Layer Monitoring project. The maps produced by the study can serve as a baseline for long-term monitoring of vegetation, active layer, snow depth, and biomass along the full Arctic bioclimate gradient.