OOS 36-2 - Causes, consequences, and trends of extreme fire spread across western North America

Background/Question/Methods
Increasing wildfire activity across portions of North America poses risks for human communities and ecological systems. Recent fire years are characterized not only by expanding area burned but also explosive fire growth. In 2020, for example, several fires grew by >100,000 acres within a 24-hour period. Extreme fire spread events such as these appear to be on the rise, but their geographic and temporal trends have not been previously quantified. We also do not know the relative contributions of climate, fire weather, fuels, topography and meteorological conditions to initiation of these events, yet this knowledge is critical for developing effective responses and mitigation strategies. Finally, there is a need to draw mechanistic and quantitative links between fire spread and key ecological outcomes, like fire severity. Here, we draw from recent efforts across western North America to address these knowledge gaps and offer a conceptual and analytical framework for characterizing fire spread and its linked drivers. We then provide some initial results of biogeographic patterns and trends of fire spread, modeling results of the drivers of extreme fire growth, and relationships between fire spread and fire severity.

Results/Conclusions
Extreme fire spread events are disproportionately responsible for wildfire impacts: just the top 1% of single-day fire spread events account for 20% of annual area burned. During the impactful fire season of 2020, 441 extreme events together burned 2.2 million ha, in contrast to an average of 168 per year that burned 0.5 million ha annually between 2002 and 2019. Our models predict that the annual number of extreme fire spread events more than doubles under a 2°C warming scenario, with an attendant doubling in area burned.

Statistical models of the drivers of fire spread from different forest types of Canada and the U.S. show general similarities, both identifying forest composition and fire weather variables as key drivers of fire spread. Fire spread, in turn, was a principal predictor of fire severity that exhibited clear threshold behaviors of extreme spread associated with high severity fire. Models of fire spread and severity showed little overlap in key drivers, with severity influenced by fire spread, topography, vegetation structure, and, secondarily, fire weather. From this a conceptual model of nested effects between fire spread, severity and biophysical drivers emerges.