OOS 17-1 - Characteristics, drivers, and implications of burn severity heterogeneity in a temperate maritime forest ecosystem

Spatial patterns of burn severity affect forest landscapes in key dimensions (e.g., patterns of tree regeneration, early-seral plant communities, carbon). Insight on the characteristics, drivers, and effects of burn severity patterns is particularly lacking in forests shaped by infrequent and severe fires, as opportunities to study such events are inherently rare. In this study, we developed regional maps of burn severity for forests west of the Cascade Crest in Washington and northern Oregon, USA (‘northwestern Cascadia’) for all fires from 1985 to 2020 using a network of field plots and field-calibrated satellite burn severity maps. We then characterized patterns and drivers of severe fire, building understanding of the spatial signature of the northwestern Cascadia fire regime. Further, we asked if the largest fires, often driven by strong easterly winds (e.g., the historic Labor Day fires of 2020), produced qualitatively different burn severity patch configurations than other fires occurring in the past 3.5 decades. Nearly 250,000 hectares burned between 1985 and 2020 in northwestern Cascadia, with >50% of forest area burning at stand-replacing severity. Stand-replacing percentage was variable for smaller fires (ranging from <20% to >75%); however, fires >10,000 ha in extent were consistently ~50% stand-replacing. Collectively, the largest six fires accounted for >85% of total area burned and >85% of total area burned as stand-replacing. Across all fires, >50% of all stand-replacing patches were smaller than 1 hectare, accounting for <1% of total stand-replacing burned area. Conversely, 12 individual patches of stand-replacing fire, all >1,000 hectares and from large fires driven by easterly winds, accounted for >70% of total stand-replacing burned area regionwide. Although landscape patterns of burn severity were driven by the outsized effect of a few large fires occurring during extreme winds, stand-replacing patches were heterogeneous in configuration. More than 75% of total burned area (and >50% of all stand replacing burned area) was within 120 meters of live tree seed sources post-fire, and <8% of total burned area ( <15% of stand-replacing burned area) was more than 500 meters from live tree seed sources post-fire. These landscape patterns of burn severity, coupled with the landscape heterogeneity in pre-fire forest structure, imprint a diversity of post-fire early-seral forest conditions across the region. Our findings address important knowledge gaps in understanding burn severity patterns characteristic of northwestern Cascadia, and provide insight into how mechanisms of resilience to fire are spatially distributed in post-fire landscapes.