Novel Disturbance on Alaska’s North Slope: Effects of a Large Wildfire on Ecosystem Carbon and Nitrogen Dynamics

A predicted consequence of human-caused climate warming at high latitudes is an increase in the frequency, intensity and aerial extent of wildfires. This could feedback positively to climate warming by transferring carbon (C) stored in terrestrial ecosystems to the atmosphere and altering ecosystem structure and function. Between July 16 and October 1, 2007, the Anaktuvuk River (AR) fire burned 1000 km2 of arctic tundra the North Slope of the Brooks Range, Alaska, USA, effectively doubling the cumulative area burned on the North Slope since 1950. Here we report the results of a 2008 field campaign to estimate the impacts of this novel disturbance on ecosystem C and N pools. We focused our efforts on moist acidic tundra (MAT), the vegetation type that comprised 60% of the burned area. We use biometric relationships developed in unburned sites combined with direct sampling of residual plants and soils in burned sites to estimate fire-driven C and N losses and residual pools. For a subset of burned sites, we measured the radiocarbon age of burned soils to estimate the time increment of C storage consumed by the fire. In MAT, the AR fire consumed 2216 ± 483 gC/m2 (mean ± 1SE), ranging from 412 to 10235 gC/m2, and 60 ± 18 gN/m2, ranging from to 9 to 373 gN/m2. We estimate that the AR fire emitted approximately 2.2 Tg C and 0.006 Tg N to the atmosphere. Radiocarbon dating of residual soils revealed that fire consumed, on average, 25 years of vertically accumulated soil C. Concurrent N losses, by contrast, represented 300 to >1000 years of N accumulation as estimated from current rates of atmospheric deposition and biological N fixation. In lightly and moderately burned sites, >30% of C loss came from aboveground vegetation, suggesting that these sites may recover lost C relatively rapidly. In severely burned sites, however, nitrogen constraints on the C cycle and the slow rate of soil organic matter accumulation make it unlikely that C pools will recover to pre-fire levels over the next millennia. Our results suggest that in MAT, intensification of the fire regime is likely to lead to threshold changes in ecosystem functions that feedback to climate and impact the wellbeing of humans and other animals that inhabit Alaska’s North Slope.