Observations of Post-Wildfire Land Cover Trends in Boreal Alaska Using Geospatial Analyses

Abstract:

Global mean temperature increased at a rate of ~0.02 °C yr^-1 since 1980, with high-latitude areas warming at nearly twice the global rate. Amplified warming can result in frequent boreal wildfires, and enhanced burning may catalyze profound ecosystem change. Specifically, frequent wildfires may promote a shift from coniferous- to deciduous-dominated forests. However, paleofire records suggest this can result in a negative feedback to fire frequency by reducing landscape flammability. In this study, we use geospatial datasets from Alaskan boreal forest to assess how modern wildfires have impacted vegetation recruitment. To test the relationship between modern wildfire activity and vegetation change, we compare wildfire perimeters from the past 50 years (Alaska Interagency Coordination Center) with land cover classifications from 2001, 2011, and 2016 (National Land Cover Database).

Over 25.5% of Alaskan boreal forest burned over the past 50 years, with preferential fire occurrence in coniferous-dominated areas with abundant fire-adapted Picea mariana, compared to deciduous and mixed-forest areas. Specifically, 49.4% and 44.2% of total area burned 2001-2011 and 2011-2016, respectively, occurred in coniferous areas. Between 2001 and 2016, the percentage change for all vegetation classes in burned areas was -50.0% (coniferous for­est), -19.3% (deciduous forest), -27.7% (mixed forest), +48.5% (early-successional), documenting the large-scale conversion of forest to early-successional vegetation associated with fire. Although all forest classes decreased in total area, the ratio of deciduous to evergreen forest cover increased from 23.4% to 37.8% in burned areas, while landcover types in non-burned areas showed <0.1% change. While this ratio in part reflects the lower flammability of deciduous relative to evergreen vegetation types, the rate of conversion from coniferous to deciduous vegetation 2001-2016 was 3.3 times higher in burned relative to unburned areas. Our results suggest that modern high-frequency burning may already be driving coniferous to deciduous change in Alaska. Given the low flammability of deciduous relative to coniferous forest in the modern record, these trends highlight the potential for large-scale vegetation limitations to boreal burning despite ongoing warming.