Post-fire landscape evaluations in Eastern Washington, USA: Assessing the work of contemporary wildfires

In the western US, wildfires are modifying the structure, composition, and patterns of forested landscapes at ratesthat far exceed mechanical thinning and prescribed fire treatments. There are conflicting narratives as to whetherthese wildfires are restoring landscape resilience to future climate and wildfires. To evaluate the landscape-levelwork of wildfires, we assessed four subwatersheds in eastern Washington, USA that experienced large wildfires in2014, 2015, or 2017 after more than a century of fire exclusion and extensive timber harvest. We compared preandpost-fire landscape conditions to an ecoregion-specific historical (HRV) and future range of variation (FRV)based on empirically established reference conditions derived from a large dataset of historical aerial photoimagery. These four wildfires proved to be a blunt restoration tool, moving some attributes towards moreclimate-adapted conditions and setting others back. Fires reduced canopy cover and decreased overall tree sizeand canopy complexity, which moved them into, or slightly outside, the FRV ranges. Moderate- and low-severityfire generally shifted closed-canopy forest structure to open-canopy classes. Patches of high-severity fire shiftedpatterns of forest, woodland, grassland, and shrubland towards or beyond the HRV ranges and within the FRVranges by increasing the total area and size of non-forest patches. However, large patches of high-severity fire indry and moist mixed-conifer forests homogenized landscape patterns beyond FRV ranges towards simplifiedconditions dominated by non-forest vegetation types. Fires realigned and reconnected landscape patterns withthe topo-edaphic template in some cases, but pre-existing fragmentation and spatial mismatches were compoundedin many others. Patches of large-tree, closed-canopy forest were reduced by high-severity fire, and thepotential to restore more climate-adapted large-tree, open-canopy forest was lost. Re-establishing landscapepatterns with desired patch sizes of forest, in particular patches with large trees, will take many decades tocenturies and may not occur in drier locations or where seed trees are no longer present. While large wildfiresburning during extreme fire weather conditions can move some attributes towards HRV and FRV ranges,intentionally planned mechanical and prescribed-fire treatments that are integrated with strategic wildfire response will better prepare and adapt landscapes for future wildfires and climate.