Abstract
Climate change coupled with an intensifying wildfire regime is becoming an important driver of permafrost loss and ecosystem change in the northern boreal forest. There is a growing need to understand the effects of fire on the spatial distribution of permafrost and its associated ecological consequences. We focus on the effects of fire a decade after disturbance in a rocky upland landscape in the interior Alaskan boreal forest. Our main objectives were to (1) map near-surface permafrost distribution and drainage classes and (2) analyze the controls over landscape-scale patterns of post-fire permafrost degradation. Relationships among remote sensing variables and field-based data on soil properties (temperature, moisture, organic layer thickness) and vegetation (plant community composition) were analyzed using correlation, regression, and ordination analyses. The remote sensing data we considered included spectral indices from optical datasets (Landsat 7 Enhanced Thematic Mapper Plus (ETM+) and Landsat 8 Operational Land Imager (OLI)), the principal components of a time series of radar backscatter (Advanced Land Observing Satellite—Phased Array type L-band Synthetic Aperture Radar (ALOS-PALSAR)), and topographic variables from a Light Detection and Ranging (LiDAR)-derived digital elevation model (DEM). We found strong empirical relationships between the normalized difference infrared index (NDII) and post-fire vegetation, soil moisture, and soil temperature, enabling us to indirectly map permafrost status and drainage class using regression-based models. The thickness of the insulating surface organic layer after fire, a measure of burn severity, was an important control over the extent of permafrost degradation. According to our classifications, 90% of the area considered to have experienced high severity burn (using the difference normalized burn ratio (dNBR)) lacked permafrost after fire. Permafrost thaw, in turn, likely increased drainage and resulted in drier surface soils. Burn severity also influenced plant community composition, which was tightly linked to soil temperature and moisture. Overall, interactions between burn severity, topography, and vegetation appear to control the distribution of near-surface permafrost and associated drainage conditions after disturbance.
Highlights
Permafrost is vulnerable to thawing with continued climate warming, in the boreal forest region where the mean annual permafrost temperature is close to 0 ◦ C [1]
The nonmetric multidimensional scaling (NMDS) diagrams show the variation in plant community composition across sites, with vector length and direction indicating the relationships with vegetation, environmental, and remote sensing variables (Figure 2)
We found a weak association of differenced normalized burn ratio (dNBR) with organic layer thickness, but strong associations with post-fire soil temperature, moisture, and vegetation composition, site characteristics that are strongly impacted by burn severity
Summary
Permafrost is vulnerable to thawing with continued climate warming, in the boreal forest region where the mean annual permafrost temperature is close to 0 ◦ C [1]. Permafrost degradation has begun in some areas, and widespread thawing over the century is predicted under future climate scenarios [2,3]. The loss of this protective organic layer through wildfire combustion is a major positive feedback to permafrost degradation [6]. Fire is a widespread disturbance of the boreal forest, and the impacts of fire in Alaska appear to have recently intensified as a result of climate warming, with increased fire frequency, extent, and severity [9,10]. Climate change coupled with the intensifying wildfire regime is becoming a significant driver of permafrost loss and ecosystem change in the northern boreal forest [6,7,8,11]
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