Mon, Aug 02, 2021:On Demand
Background/Question/Methods
The 2020 wildfires in western Oregon and Washington (the “westside”) were historic. Multiple ignitions driven by a dry, east wind event burned almost as much area in a few days as in the previous five decades. We use an extensive collection of fire perimeters dating back to the mid-1800s and assess their correspondence with regional patterns of dry, east wind events. We then use remotely-sensed maps of forest structure and burn severity (1985 to 2020) to compare how burn severity in stands with different structural conditions differ between recent contemporary fires (1985 to 2019) and the 2020 fires. Leveraging both datasets, we address recent fire effects in the context of the historical range of variability (HRV) for two stages of forest development of interest to regional planning and conservation of biodiversity – fire-created, early seral conditions and old-growth forests. We conclude by synthesizing existing knowledge of westside fire and identifying outstanding knowledge gaps that can inform communities, forest managers, and policy makers on adapting to similar events and potential future shifts in fire regimes with climate change.
Results/Conclusions Historical fires reached much larger sizes than the 2020 fires, and often occurred in regional hot spots of dry, east wind events. The biggest 2020 fires burned over or in close proximity to large fires in 1902 in the Oregon Cascades. The 2020 fires were larger and had much greater proportions and larger patches of high-severity fire than recent fires. Contrary to recent fires, there was little difference in patterns of high-severity fire among forests with different structural conditions in 2020. Open conditions, late-successional, and old-growth forests experienced twice as much high-severity fire in 2020, while stands dominated by small trees (e.g. plantations) experienced similarly high proportions of high-severity fire. Large losses (42%) of old-growth occurred in late successional reserves and high-severity fire created regionally rare, early seral conditions that are now within the historical range of variability in the Oregon Cascades. While fuel treatments are unlikely to mitigate fire on the westside during dry, east wind events, forest management may play a viable role under moderate conditions. A warmer, drier climate in the future will likely increase fire activity, but little is known about changes in dry, east wind events. While available scientific knowledge provides a basis for putting these fires in historical and contemporary context, many gaps and uncertainties remain on the future potential of such events.
Results/Conclusions Historical fires reached much larger sizes than the 2020 fires, and often occurred in regional hot spots of dry, east wind events. The biggest 2020 fires burned over or in close proximity to large fires in 1902 in the Oregon Cascades. The 2020 fires were larger and had much greater proportions and larger patches of high-severity fire than recent fires. Contrary to recent fires, there was little difference in patterns of high-severity fire among forests with different structural conditions in 2020. Open conditions, late-successional, and old-growth forests experienced twice as much high-severity fire in 2020, while stands dominated by small trees (e.g. plantations) experienced similarly high proportions of high-severity fire. Large losses (42%) of old-growth occurred in late successional reserves and high-severity fire created regionally rare, early seral conditions that are now within the historical range of variability in the Oregon Cascades. While fuel treatments are unlikely to mitigate fire on the westside during dry, east wind events, forest management may play a viable role under moderate conditions. A warmer, drier climate in the future will likely increase fire activity, but little is known about changes in dry, east wind events. While available scientific knowledge provides a basis for putting these fires in historical and contemporary context, many gaps and uncertainties remain on the future potential of such events.