Vegetation recovery in Alaskan tundra following an unusual fire

Large fires are uncommon in Alaskan arctic tundra. However, the frequency of lightning strikes on the North Slope of Alaska has increased greatly in the past two decades as climate has warmed. The Anaktuvuk River Fire (ARF) of 2007 burned 1000 km2 on the Alaskan North Slope, and was unprecedented in terms of area burned and severity for tundra fires north of 68o N, with more than 80% of the area burned with moderate to high severity. In 2008, we established 24 permanent transects in burned areas, covering a range of vegetation types and burn severities. In 2008 and 2010, we established an additional 17 reference transects in unburned areas in the vicinity of the ARF, matched to vegetation types sampled within the ARF. In 2008, 2009, and 2010, we followed vegetation cover along these transects, active layer thickness, and N availability using ion exchange resin bags (in a subset of transects). We also measured soil pH in organic soils in 2010. Despite the unusually high severity of the fire, vegetation cover has recovered substantially over the first three years. By 2009, relative cover of live vascular plant cover in the ARF transects was approximately two-thirds of that found in the reference transects, while non-vascular plant cover was approximately 1/3 of that in the reference transects. Most of the vascular plant cover is comprised of resprouting biomass from individuals present before the fire, which were damaged but not completely killed. Tussocks of Eriophorum vaginatum resprouted vigorously in the first year following the fire, and tussock mortality was estimated at only 10%. Shrub mortality appeared to be much higher, >50%, but was hard to estimate due to consumption of stems and roots. Resprouting of shrubs was more evident in 2009 and 2010 than in 2008. At present, graminoid cover is greater than shrub cover, but shrub cover increased more than graminoid cover between 2008 and 2010. No non-native vascular plant species have colonized in our ARF transects so far, although certain grass species found at low abundance in the reference transects are more abundant in severely burned transects. In contrast, much of the cover of non-vascular vegetation is comprised of species that were not abundant in tundra before the fire, but are common following fire in boreal forest. These include the liverwort Marchantia polymorpha, and the mosses Ceratodon purpureus and Pohlia nutans. Soil inorganic nitrogen and phosphorus availability, as measured with resin bags, was approximately 3 times higher in the burned transects than in reference transects in 2009. Thawing and subsidence occurred in a minority of the transects, where there was evidence of previous frost-scarring, and numerous thermokarst features have been documented in the ARF by other researchers. However, over most of the landscape that we sampled, the terrestrial ecosystem appears to be resilient, and vegetation recovery is rapid.