Wildfire disturbance is an important factor contributing to ecosystem and landscape changes. The impact of fires on permafrost-influenced terrain in boreal forest regions is well documented; however, the role of fires in initiating thermokarst development in arctic tundra regions is poorly understood. Rapid climate change at high latitudes has increased interest in the spatial and temporal dynamics of thermokarst and other permafrost thaw-related features in diverse disciplines including landscape ecology, hydrology, engineering, and biogeochemistry. As a result, there is an urgent need to develop new techniques and tools to observe and quantify changes to near-surface permafrost terrain.
Remote sensing provides a means for documenting and quantifying many of the changes now occurring on arctic landscapes. In particular, application of multitemporal airborne lidar allows for the detection of terrain subsidence caused by thermokarst. Lidar elevation model differencing provides a direct measure of land surface elevation changes over time. This study compares two airborne lidar datasets covering ~400 km2 acquired in the aftermath of the large and severe Anaktuvuk River tundra fire that occurred in 2007 in northern Alaska. Digital terrain models (DTMs) at 1-m spatial resolution were developed from the lidar datasets that were acquired 2 years and 7 years postfire. These datasets were differenced using the Geomorphic Change Detection tool to quantify thermokarst development in response to the tundra fire disturbance.
Results show permafrost thaw subsidence (more than 0.2 m) occurring across 34% of the burned tundra area studied, compared to less than 1% in similar undisturbed, ice-rich tundra terrain. Postfire thermokarst development as detected in the airborne lidar data shows a relationship with trends in a dense time series of different multispectral indices (Tasseled Cap, NDVI, Normalized Difference Moisture Index (NDMI)) derived from multispectral Landsat satellite data. These relationships allow scaled-up mapping of thaw-affected terrain area across the entire ~1,000-km2 area impacted by the Anaktuvuk River tundra fire; where sufficient cloud-free Landsat data are available, they may also allow for the assessment of thermokarst impacts across other tundra fire disturbances since the mid-1980s.
These new methodologies will enable assessment of the vulnerabilities of ice-rich permafrost terrain to changing disturbance regimes in northern high latitude landscapes. A better understanding of the processes controlling thermokarst initiation and development in Arctic regions is important because of the resulting impacts on the land-atmosphere exchange of water, energy, and greenhouse gases, along with the influence on surface hydrology, snow accumulation, and vegetation dynamics.
Postfire thermokarst following an arctic tundra fire in northern Alaska. (a) QuickBird image acquired the summer after a large and severe tundra fire showing burned versus unburned tundra. (b) Repeat lidar-derived subsidence image indicating degradation of ice-rich permafrost 7 years postfire.