Wildfire occurrence in the arctic tundra is increasing in frequency and extent due to warmer temperatures, drier fuels, and greater incidence of lightning strikes. In the summer of 2007, the lightning-ignited Anaktuvuk River fire burned an unprecedented area of arctic tundra on Alaska’s North Slope. The wildfire burned over 1000 km2, releasing large amounts of C into the atmosphere while combusting much of the organic soil horizon and fundamentally changing the nature of above- and belowground conditions at the site. Given these major transitions in soil characteristics and vegetation, we investigated the response of above- and belowground food webs to fire 4 and 6 years after disturbance. Our study aimed to answer the following questions across a burn severity gradient on the arctic tundra: (1) How do above- and belowground food web structure and function respond to fire? (2) How do these responses vary with fire severity (non-burned, moderate, high) and microhabitat (canopy, soil surface, organic soil and mineral soil)? We extracted microbes, nematodes and protozoa from soils and invertebrates from above- and belowground microhabitats to estimate biomass. We then applied an energetic food web modeling framework to estimate C flow through the web, and C and N mineralization rates.
Belowground biota biomass recovered between 4 and 6 years after the fire, with a disproportionate increase in consumer biomass at the severely burned site relative to moderately burned and non-burned sites. While non-burned belowground food webs remained stratified by organic and mineral soil horizons, food webs at the moderately and severely burned sites were structurally homogenized across soil horizons. With respect to the aboveground food webs, herbivory in the canopy was greatest at the severely burned site, while the recovery of the surface community was most pronounced at the moderately burned site. Evidence of disproportionate biomass recovery at the severely burned site was confirmed by estimates of above- and belowground C and N mineralization rates. While belowground C and N mineralization rates remained low in moderately and severely burned sites 4 years post fire, consumer contribution to above- and belowground C and N mineralization was 4x greater in the severely burned site relative to the non-burned site by 6 years post fire. While the recovery of different ecosystem components across the microhabitats are asynchronous with one another, our results suggest these arctic food webs are quite resilient to wildfire, with overall recovery expected to occur on decadal time scales.