Extreme Winds Flip Influence of Fuels and Topography on Megafire Burn Severity in Mesic Conifer Forests Under Record Fuel Aridity

Abstract

The coupling of unusually hot and dry weather have led to global increases in the occurrence of megafires. Despite the conventional wisdom that extreme heat and aridity overwhelm the controls on burn severity patterns (i.e., vegetation mortality), we hypothesize that wind is the main driver of megafire events in temperate mesic forests with climate-restricted fire regimes, yet that fuels and topography remain important influences on burn severity patterns. The infrequent occurrence of large high-severity wildfire in these forests means that contemporary empirical data (e.g., remote sensing) from past megafires are largely missing. During the extraordinary 2020 fire season, ca. 0.8 million ha burned in the North American Pacific Northwest (PNW) over two weeks under record-breaking fuel aridity and winds, representing the first modern example of megafires that characterize disturbance regimes west of the region’s Cascade Mountains. Considering increasing concern and uncertainty surrounding the drivers of megafire events in temperate mesic forests, our objective was to understand the relative influence of, and potential interactions between, weather, fuels, and topography on high-severity (> 75% tree mortality) fire probability among five synchronous megafires in the western Cascade Mountains. To assess the influence of several potential drivers of high-severity fire and whether these relationships varied with land use and ownership, we developed remotely sensed fire extent and burn severity maps for two periods of the explosive 2020 PNW fire season: (1) during extreme winds and (2) after the extreme winds subsided. The area burned during the windstorm accounted for 90% of the total fire sizes and saw a 2.5-times greater proportion of high-severity fire than during the period without winds. Our results suggest that wind is the major driver of megafires in forests with climate-limited fire regimes, yet that fuels and topography shape burn severity patterns even under extreme fuel aridity and winds. The relative influence of topography on burn severity outweighed fuels during the windstorm, while fuels outweighed the influence of topography after winds subsided. Early-seral forests primarily concentrated on private lands, burned more severely than their older and taller counterparts, regardless of topography, over the entire megafire event. Meanwhile, mature stands burned severely only under extreme winds and especially on steeper slopes. Although climate change and land-use legacies may prime mesic temperate forests to burn more frequently and at higher severities than historically observed, and especially among early-seral forests, our work suggests that future high-severity megafires are only likely to occur during coinciding periods of heat, fuel aridity, and extreme winds.