Post-fire delayed tree mortality in mesic coniferous forests reduces fire refugia and seed sources

Ecological functions provided by fire refugia are critical for supporting conifer forest resiliency under increased fire activity across the western United States. The spatial distribution and persistence of fire refugia over time are uncertain as fire-injured trees continue to die over subsequent years post-fire. We examined how post-fire delayed tree mortality affects the spatial distribution and attributes of fire refugia at patch and landscape scales following high-severity wildfires. To explore changes in fire refugia patch size, isolation, and fragmentation over time, we used high-resolution satellite imagery (3 m pixel size) following high-severity fires in Oregon’s western Cascades to map annual changes in the extent of live tree cover up to 3 years post-fire. Delayed mortality decreased live forest cover across all fire perimeters by 8.5% between 1 month and 3 years post-fire. Though prevalent across all forest types, adult to mature and fire-sensitive conifer species were the most vulnerable to delayed mortality. The number of refugia patches decreased by ca. 20%, and most (ca. 77%) were small, non-core patches (< 60 m from the patch edge). In response to delayed mortality, which increased the extent of high-severity burned areas by 9% (ca. 12,000 ha), the area with little to no seed sources based on refugia distance2-weighted density increased by 375% (7632 ha). Delayed mortality altered the size and spatial configuration of fire refugia across landscapes. Considering species-specific fire adaptations may help improve post-fire management strategies and a framework of conifer forest resiliency under novel fire regimes.