High-severity and short-interval wildfires limit forest recovery in the Central Cascade Range

Increasing forest fuel aridity with climate change may be expanding mid-to-high-elevation forests' vulnerability to large, severe, and frequent wildfire. Long-lasting changes in forests' structure and composition may occur if dominant tree species are poorly adapted to shifting wildfire patterns. We hypothesized that altered fire activity may lower existing forest resilience and disrupt the recovery of upper-montane and subalpine conifer forest types. We empirically tested this hypothesis by quantifying post-fire forest structure and conifer tree regeneration after spatially large, severe, and rapidly repeated wildfires (<12-yr interval) in the Central Cascade Range in the U.S. Pacific Northwest. Post-fire conifer regeneration was generally very poor among plots that experienced either a single high-severity fire or rapid reburn, driven primarily by lack of proximate seed source. Pre-fire dominant, shade-tolerant species' abundance was highly negatively correlated with increasing seed source distances and dry, exposed post-fire environmental conditions. In rapidly reburned plots, the order of burn severity was critical and promoted establishment of all conifer species, if low-then-high severity, or primarily fire-adapted pines, if high-then-low severity. Our findings suggest that these forests, affected by expansive high-severity and/or short-interval wildfire, may transition into a patchy, low-density, pine-dominated forest state under future warming trends. These emerging, early seral ecosystems will incorporate more fire-adapted tree species, lower tree densities, and more non-forest patches than prior forests, likely expanding their resilience to anticipated increases in fire frequency. If future larger, more severe, and more frequent wildfire patterns manifest as expected in the Cascade Range, previously denser, moist mid-to-high-elevation forests may begin resembling their drier, lower-elevation mixed-conifer counterparts in structure and composition.