OOS 26-2 - Old forests and landscape fire: Quantifying fire effects across a gradient of late-successional forests in the eastern Cascade Range, WA

Abstract: As wildfire activity accelerates with climate change, land managers and diverse stakeholders are concerned about vulnerable habitats and ecosystem services. Although recent assessments have documented fire-induced declines of mature and old-growth forests in the US Pacific Northwest, many uncertainties remain regarding fire effects across different forest types. This study evaluates the impacts of a landscape-scale fire event - the Schneider Springs Fire that burned 43,000 ha in 2021 - on mature and old-growth forests spanning a gradient of forest types and historical fire regimes in the eastern Cascades of Washington. Our specific goals were to: (1) quantify the drivers of burn severity across dry, moist, and cold forest types and (2) compare burn severity in late-successional and other forests within and among forest types to estimate fire impacts relative to the distribution of historical fire regimes. We mapped forest composition and structure with satellite, LiDAR, and aerial imagery and burn severity with satellite-based change detection, classifying high-, moderate-, and low-severity fire based on field inventories of tree mortality. We quantified the drivers of burn severity using random forests and a suite of predictor variables encompassing fuels, topography, weather, climate, and management activities. We estimated forest type-specific fire regimes from the literature. The drivers of burn severity varied with forest type; climate and wind were more important in moist and cold forests, whereas road proximity and fuels were more important in dry forests. Across the full fire extent, burn severity proportions were similar to historical estimates, despite several very large patches (≥400 ha) of high-severity fire. Low- and moderate-severity fire accounted for the majority of fire extent, but high-severity, stand-replacing fire affected 2,400 ha (30%) of large tree habitat across all forest types. Although high-severity fire did not disproportionately affect late-successional forests more than other forests, substantial late-successional forest losses underscore their susceptibility to fire and other disturbances. Concurrently, non-stand-replacing fire likely reduced fire risk and bolstered the fire resistance of late-successional forests across the majority of the fire footprint. As forested landscapes become warmer and drier, strategies that prioritize protection and restoration of mature and old-growth forests, while recognizing their geographic variability, will be important for their conservation over time and space. Within disturbance-prone landscapes, long-term monitoring and adaptive management are vital for fostering ecosystem resilience to climate change, wildfires, and other disturbances.