Forest Structure, Composition, and Regeneration after High-Severity and Rapidly Repeated Wildfires in the Central Cascade Range

Within mid-to-high elevation conifer forests in the Cascade Range, wildfire extent, severity, and frequency are expected to rise due to increasingly drier forest fuels under climate change. Considering dominant species composition, existing forests may be poorly adapted to absorb stress and recover following altered wildfire patterns. We tested the hypothesis that increased fire activity may disrupt the recovery of upper-montane and subalpine forest types by quantifying post-fire forest structure and conifer regeneration after spatially large, severe, and rapidly repeated wildfires in the Central Cascade Range. A stratified random sampling design was used to select field plots (n=122) and drivers of conifer regeneration were modeled using logistic and negative binomial regression models. Median conifer regeneration was very poor among sample plots that experienced either a single high-severity fire (49 seedlings/ha) or rapid reburn (14-28 seedlings/ha). Distance to seed source primarily drove seedling abundance, with shade-tolerant species abundance being most sensitive to increasing seed source distances and dry, exposed, post-fire environmental conditions. Rapidly repeated fire increased the size of high-severity patches by killing live seed source trees spared during an initial fire, with chronological sequence of burn severity promoting regeneration of all conifer species or primarily fast growing, fire-adapted pines. Low-seedling densities, a general lack of seed source, and future warming trends suggest these forests affected by expansive high-severity and/or repeated wildfire will transition into a patchy, low-density forest state. This early-seral state ecosystem will be composed of fire-adapted pines farther from live seed source and incorporate a patchwork of shrubby grassland that in turn, may be more resilient to frequent wildfire than prior forests. If future wildfire patterns manifest as expected in the Cascade Range, recovering mid-to-high elevation forests may begin resembling their drier, lower-elevation mixed-conifer counterparts in structure and composition