Observations of boreal forest fire seasonality and interannual variability from MODIS active fire registrations

Seasonal and interannual timing of fire events in the boreal forest has a strong effect on fire severity. In general, fires are more intense during the late season than earlier in the year. The MODIS active fire data product (MCD14ML) provides daily global observations of burn locations, as well as an estimate of the amount of energy released by the fire (Fire Radiative Power or FRP) at the time of the overpass. We used the Active Fire Product to compare the seasonal timing of fires and FRP between large and small fire years, early and late season burning, front burning versus remnants of fronts that moved on or were extinguished, and among vegetation types. The daily and seasonal distribution of active fire observations from small fire years were resampled to match the distribution observed in large fire years. Active fire detections for Alaska over the last decade show distinctive patterns in both large and small fire years. Late season fires, especially those that occur during years with a large area burned, occur when the upper duff layer of the forest floor has become dry and warm and highly susceptible to deep burning. Late season fires account for 60% of burning in large fire years, and just 40% of burning in small fire years. Remnant fires that remain after a front has moved through a stand result in more severe burning by consuming more of the surface organic material that is essential to ecosystem function in the boreal forest. These remnant fires account for a maximum of 35% of the cumulative front activity in large fire years, and just 25% in small fire years. Vegetation type sometimes appears to be highly important in controlling fire activity (e.g., in remnant fires during small fire years), whereas at other times it is not a factor. The differences in the pattern of active fire detections between large and small fire years indicate that large fire years are likely to be more severe.