Shrub Expansion, Sphagnum Peat Growth, and Carbon Sequestration in Arctic Tundra on the North Slope of Alaska

Abstract The ongoing climate warming in the Arctic has caused rapid and widespread environmental changes, including permafrost soil thaw and shrub expansion. However, carbon (C) sequestration potential and the future trajectory of these ecosystems are still poorly understood. Here I present results from multiple peat cores collected along a hillslope covered by Eriophorum-dominated tussock tundra with widespread Sphagnum patches in upper Imnavait Creek (UIC) on the North Slope of Alaska (68°36'N, 149°18'W) to investigate responses of organic soil development, vegetation composition, and C accumulation to climate change during the last millennium. All cores show a consistent organic soil development sequence from mineral soil to sedge peat to Sphagnum peat. Sedge peat initiated during the cold Little Ice Age, suggesting the importance of low decomposition and preservation of organic matter in initial peat buildup. The onset of Sphagnum peat occurred on average at ca. 1930 AD, likely caused by progressive soil drying as a result of regional climate warming and active layer deepening as well as earlier snowmelt. Fossil pollen analysis shows that the vegetation was dominated by sedges from 650 years ago to 1875 AD, followed by increases in dwarf birches first and then willows since the early 2000s. This suggests that the recent observed shrub expansion was preceded by birch expansion more than a century ago. The results from the UIC tundra site show that Sphagnum patches on this tussock tundra have been a sustained C sink, accumulating C rapidly especially in the last two decades (up to 170 gC/m2/yr), likely in response to the recent accelerated Arctic warming. In contrast, eddy flux measurements from similar tundra sites indicate that these ecosystems as a whole are a net source of C to the atmosphere, due to elevated C release during the snow-covered season. If Sphagnum patches expand on the tundra landscape in the future, the region could become a net C sink of atmospheric CO2, impacting the regional and even global C balance.