Spatial patterns of burn severity in an archetypal infrequent-severe fire regime: Insights from the Western Cascades

Brian J. Harvey and Michele S. Buonanduci, School of Environmental and Forest Sciences, University of Washington, Seattle, WA, Michele S. Buonanduci, Quantitative Ecology and Resource Management, University of Washington, Seattle, WA, Daniel C. Donato, Washington State Department of Natural Resources, Olympia, WA, Joshua S. Halofsky, Washington Department of Natural Resources, Olympia, WA

Background/Question/Methods
As fire activity increases in forested ecosystems worldwide, a key knowledge gap has emerged regarding potential changes to spatial patterns of burn severity (the ecological effect of fire, often characterized by fire-caused tree mortality). This knowledge gap is particularly challenging in forests characterized by infrequent and severe fires, as opportunities to study events are inherently rare. In this study, we characterized one key component affecting forest resilience to fire in the western Cascade Range, a global archetype of the infrequent-severe fire regime. Specifically, for all fire events from 1984 through 2020, we used field-validated satellite indices of burn severity to characterize several metrics related to the spatial configuration of stand-replacing (e.g., most or all above-ground vegetation killed) patches. We examined how measures of spatial configuration for stand-replacing fire patches (area-weighted mean patch size and core area > 150 m from patch edges) related to the proportion of stand-replacing fire, and fire size. We further asked if the historic Labor Day fires of 2020, which burned under extreme conditions associated with a regional synoptic wind event, had qualitatively different burn severity patch configurations than other fires that occurred in the past 3.5 decades.
Results/Conclusions
For all fires that burned between 1984 and 2019 in the western Cascades, the stand-replacing proportion was variable for smaller fires (ranging from 5 to 73%), but was consistently 40 to 50% for fires >2,500 ha in extent. Area-weighted mean patch size and total core area for stand-replacing fire scaled strongly with fire size, with the area-weighted mean patch size accounting for ~10-20% of larger fire extents. This configuration led to a sharp increase in total core area as fire size increased. Initial burn severity maps from the 2020 Labor Day fires indicate stand-replacing burn severity exceeded 50% of the fire extent in some cases, with area-weighted mean patch size typically comprising 10-20% of fire extents. Analyses of pre-2020 fires where initial burn severity maps were quantitatively compared to extended (1-yr post-fire) burn severity maps suggest the 2020 Labor Day fires were approximately 15% greater in severity proportions and more homogeneous in their patterns of stand-replacing fire than initial severity maps suggest. Our findings address an important knowledge gap in understanding the nature of burn severity patterns in high-severity low-frequency fire regimes, and suggest that large stand-replacing burn patches are inherent to this disturbance regime.