Landscape Effects of Wildfire on Permafrost Distribution in Interior Alaska Derived from Remote Sensing

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.

Climate change decreases the cooling effect from postfire albedo in boreal North America

“…Fire is the dominant disturbance mechanism in boreal forests (Stocks et al, 2002), and shifts in future fire regimes are likely to alter forest dynamics including vegetation composition (Johnson, 1992;Johnstone et al, 2010;Payette, 1992;Viereck, 1973) and carbon cycling (Balshi et al, 2009b;Genet et al, 2013). Boreal fires initiate both positive (+) and negative (−) climate feedbacks by emitting greenhouse gases (+; Amiro et al, 2001;Bond-Lamberty, Peckham, Ahl, & Gower, 2007) and carbonaceous aerosols during combustion (+ and −;Flanner, Zender, Randerson, & Rasch, 2007;Mouteva et al, 2015;Ward et al, 2012), reaccumulating carbon during postfire regrowth (−; Alexander & Mack, 2016;Amiro et al, 2010;Goulden et al, 2011), degrading permafrost which releases CO 2 and CH 4 (+; Brown et al, 2016;Jafarov, Romanovsky, Genet, McGuire, & Marchenko, 2013;Jorgenson et al, 2010), and altering the surface energy balance, primarily through albedo (−; Amiro et al, 2006;French, Whitley, & Jenkins, 2016;Randerson et al, 2006;Rogers, Randerson, & Bonan, 2013).…”

Groundwater Controls on Postfire Permafrost Thaw: Water and Energy Balance Effects

“…While the dominance of energy balance changes may seem to contradict the strong sensitivity of modeled thaw dynamics to hydrological parameters (Figures and ), these results are reconciled by noting that heat transport via advection is a function of both the energy content of groundwater (a function of soil temperature) and the magnitude of groundwater flow (a function of recharge and active layer thickness). Therefore, changes in the energy balance can be the dominant driver of permafrost thaw dynamics as observed in previous studies (Brown et al, ), even where groundwater flow is an important process. As warming in high‐latitude regions shifts the timing and magnitude of spring snowmelt, changes in to the water balance may increase in importance (Bring et al, ; Lique et al, ), in particular at sites with finer‐grained mineral soils which are able to hold more unfrozen water even at subzero temperatures, buffering permafrost from changes in air temperature (Nicolsky & Romanovsky, ).…”

“…Increases in active layer thickness following fire occur via two mechanisms: (1) thinning the near‐surface organic soil layer which acts as a thermal buffer between air and the subsurface (Brown et al, , ; Kasischke & Johnstone, ) and (2) decreasing albedo which increases energy input into the subsurface (Rocha & Shaver, ; Smith et al, ; Yoshikawa et al, ). Past modeling efforts studying postfire active layer thickness have primarily concluded that soil thermal properties are the most important control on permafrost response to fire (Jiang et al, ; Jiang, Rastetter, et al, ; Yi et al, ).…”

Burst Misalignment Evaluation for ALOS-2 PALSAR-2 ScanSAR-ScanSAR Interferometry

“…Synthetic Aperture Radar (SAR) has become an essential tool for the rapid analysis of, for example, disaster monitoring [1], maritime monitoring and land cover changes [2,3] including environmental monitoring [4]. Especially for a large-scale deformation, interferometric analysis (InSAR) in a wide-swath observation mode is effective for precisely detecting ground displacement [5].…”