Long-Term Perspectives of Shrub Expansions and Peat Initiation in Arctic Tundra on the North Slope of Alaska

Abstract:

The ongoing climate warming in the Arctic has caused rapid terrestrial ecosystem changes, including shrub expansion and permafrost thaw. Here we used results from a peat-accumulating permafrost tundra in upper Imnavait Creek on the Arctic foothills of Alaska (68° 36’ N, 149° 18’ W) to investigate ecological responses to recent climate warming in the context of the last millennium. Six peat soil cores were collected from Sphagnum mosaics along an elevational gradient from 906 m to 950 m on a hillslope covered by Eriophorum-dominated tussock tundra. Macrofossil analysis documents a consistent development sequence among all cores from a mineral soil to a minerotrophic sedge peat and finally to an ombrotrophic Sphagnum peat. The ¹⁴C dating results show the ages of peat initiation range from about 900 to 140 cal BP, but do not follow the elevation gradient, suggesting the dominant control of local factors. The Sphagnum onset begins at 1820 AD near the ridge top, and subsequently propagates downslope to the floodplain at 2008 AD. This transition (ombrotrophication) was likely in response to Arctic warming, and subsequent permafrost thaw and active layer thickening, leading to drying initiating at the ridge top and facilitating Sphagnum colonization. Pollen analysis of the master core UIC13-3 at 916 m elevation (basal age 700 cal BP) shows that the vegetation was dominated by sedges (up to 84%) during the cool Little Ice Age until 1800 AD, followed by increases in shrubs first from dwarf birch (Betula nana) (up to 57%) and then willows (Salix spp.) up to 62% in the 1960s. These results indicate that shrub expansion of willows, due to accelerated warming in recent decades, was preceded by birch expansion over the last two centuries. Our new results provide a long-term perspective on ecological transformations in the Arctic, in particular the history of recent shrub expansions and the process of peatland initiation and expansion across Arctic tundra.