Wildfire effects on permafrost and soil moisture in spruce forests of Interior Alaska

Abstract

In the summer of 2015, hundreds of forest fires burned across the state of Alaska. Several uncontrolled wildfires near the town of Tanana on the Yukon River were responsible for the largest portion of the area burned statewide. In July 2017, field measurements were carried out in both unburned and burned forested areas nearly adjacent to one another, all within 15 miles of the village of Tanana. These surveys were used to first visually verify locations of different burn severity classes, (low, moderate, or high), estimated in 2016 from Landsat images (collected before and after the 2015 Tanana-area wildfires). Surface and soil profile measurements to 30-cm depth at these same locations were collected for evidence of moss layer and forest biomass burning. Soil temperature and moisture content were measured to 30-cm depth, and depth to permafrost was estimated by excavation wherever necessary. Digital thermal infra-red images of the soil profiles were taken at each site location, and root-zone organic layer samples were extracted for further chemical analysis. Results supported the hypothesis that the loss of surface organic layers is a major factor determining post-fire soil water and temperature changes and the depth of permafrost thawing. In the most severely burned forest sites, complete consumption of the living moss organic layer was strongly associated with both warming at the surface layer and increases in soil water content, relative to unburned forest sites. Soil temperatures at both 10-cm and 30-cm depths at burned forest sites were higher by 8–10 °C compared to unburned sites. Below 15 cm, temperatures of unburned sites dropped gradually to frozen levels by 30 cm, while soil temperatures at burned sites remained above 5 °C to 30-cm depth. The water content measured at 3 cm at burned sites was commonly in excess of 30% by volume, compared to unburned sites. The strong correlation between burn index values and depth to permafrost measured across all sites sampled in July 2017 showed that the new ice-free profile in severely burned forest areas was commonly 50-cm deeper than in unburned soils.