Novel disturbance on Alaska's North Slope: Wildfire emissions and consequences for ecosystem structure in arctic tundra

Background/Question/Methods: 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 on the north slope of the Brooks Range, Alaska, USA, an area larger than the sum of all know fires 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 loss from MAT.  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.

Results/Conclusions: In MAT, the AR fire consumed1.38 ± 0.27 kg C /m² (mean ± 1SE), ranging from 0.4 to 5.80 kg C /m², and 38 ± 10 g N /m², ranging from 10 to 210 g N /m². We estimate that burning of MAT emitted approximately 828,000 tons C and 22,000 tons N.  Radiocarbon dating of residual soils revealed that while fire consumed as much as 1230 years of accumulated C inputs to soil organic matter in severely burned sites, the average consumption was about 30 years of accumulated C. Concurrent N losses 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 plant and soil pools 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.