Characterizing wildfire activity across northeastern Siberia 2001-2020

Abstract:

Wildfire is an integral, natural disturbance in boreal forest ecosystems across northeastern Siberia, contributing to forest regeneration, stability of underlying permafrost soils, and ultimately, carbon cycling. Understanding wildfire dynamics in this region is challenging because an annual fire perimeter database does not exist, and MODIS satellite imagery underestimates the area burned. Mapping annual area burned and characterizing fire frequency and severity are crucial for elucidating wildfire contribution to carbon cycling in these systems. Using long-term satellite data from Landsat, we mapped area burned, generated a fire perimeter database, and characterized annual area burned to better understand wildfire extent, severity, and frequency over the last 20 years.

Our study area covered northeastern Siberia from 60-72°N and 118-172°E, a region characterized by fire-adapted larch (Larix spp.) forests that are underlain by continuous permafrost. We used the cloud computing power of Google Earth Engine to access the Landsat archive. We generated composite images for the annual growing season (May - September), which allowed us to mitigate missing data from snow-cover, cloud-cover, and the Landsat 7 scan line error. We used annual composites to calculate the difference Normalized Burn Ratio (dNBR) for each fire year using the extended assessment. The annual dNBR images were converted to a binary burned or unburned imagery used to vectorize fire perimeters. We mapped over 10,000 fire perimeters that ranged in size from 400 to 1,627,295 ha. We found that the total area burned between 2001-2020 is estimated at over 51 million ha with an average annual area burned of 4824 ha. Most fires that burned annually were <1000 ha and 66 fires exceeded 100,000 ha. Years in which the annual area burned exceeded three million ha included 2001, 2002, 2010, 2014, 2018, 2019, and 2020. As fire activity increases in extent, severity, and frequency with continued climate warming, Siberian larch forest ecosystems may be less resilient to disturbance, with increased likelihood of irreversible permafrost thaw or conversion of forests to shrublands.