Within mid-to-high elevation wet 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 fire patterns. We tested the hypothesis that increased fire activity may disrupt upper-montane and subalpine forest recovery by quantifying post-fire forest structure and conifer regeneration after severe and rapidly repeated wildfires in the Central Cascade Range. A stratified random sampling design was used to select field sites and drivers of conifer regeneration were modeled using logistic and negative binomial regression models.
Results/Conclusions
Median conifer regeneration was very poor (14-49 seedlings/ha) among plots that experienced either a single high-severity fire or rapid reburn. Distance to seed source primarily drove seedling abundance, with shade-tolerant species being most sensitive to seed source distance. Post-fire years with above average spring snowpack limited the abundance of shade-intolerant species, likely disrupting dispersal and germination mechanisms. Reburns effectively increased the size of high-severity patches by killing live seed source trees spared during an initial fire, with sequence of burn severity selecting all species or primarily fire-adapted pines. Low-seedling densities, a general lack of seed source, and future climate-change impacts suggest these forests affected by expansive high-severity and/or repeated wildfire will transition into a low-density forest state. This early-seral state ecosystem will be dominated by shade-intolerant species and incorporate a patchwork of shrubby grassland that in turn, may be more resilient to warming conditions and frequent wildfire than previous forests. If future wildfire patterns manifest as expected in the Cascade Range, recovering mid-to-high elevation forests may begin resembling their drier, low-elevation counterparts in structure and composition.