Long term effects of wildfire on permafrost stability and carbon cycling in peatlands

Permafrost soils in boreal and arctic ecosystems store almost twice as much carbon as is currently present in the atmosphere and are therefore important players in the global carbon cycle. Wildfire can act as an important trigger of permafrost thaw (laterally through thermokarst development and vertically though active layer deepening), and due to increasing fire frequency and more severe fires with a changing climate, rates of permafrost thaw are expected to accelerate in the future. However, there remains much uncertainty about the extent and timing to which wildfire affects permafrost stability. This study aims to assess the role of wildfire on future permafrost stability and carbon storage in permafrost peatlands by exploring the hypothesis that wildfire increases ground heat flux which leads to permafrost destabilization and changes in ecosystem respiration that spans decades until vegetation recovery occurs. Research has been conducted in a series of sites that burned 2-48 years ago in the Northwest Territories, Canada. Thaw depths, soil thermal regimes and ecosystem respiration have been monitored. Satellite imagery has been used to determine the area of recent permafrost thaw within fire scars and adjoining unburned areas. We show that for approximately thirty years sites that experienced fire had deeper active layers than their paired unburned sites. Surprisingly, the effect of fire was most pronounced 10-15 years following fire with about a doubling of active layer during this time period. With increased active layer deepening we found that permafrost stability along the edges of peat plateaus was drastically affected. Using remote sensing approaches, the rate of new thermokarst development approximately doubled over 30 years following fire. These results suggest that increasing fire frequency and lengthening of the fire season will lead to increased fire-induced permafrost thaw which has long term implications for carbon cycling and land use.