Beetlemania: Is the bark worse than the bite? Rocky Mountain subalpine forests recover differently after spruce beetle outbreaks and wildfires

Due to the shifting global climate, the frequency and severity of disturbances are increasing, inevitably causing an increase in disturbances overlapping in time and space. Bark beetle epidemics and wildfires have historically shaped the disturbance regimes of Western North American forests. Their interactive effects on stand dynamics and recovery are inadequately studied in Picea engelmannii (Engelmann spruce)-Abies lasiocarpa (subalpine fir) dominant forests; understanding these interactions is imperative to the management and health of forested ecosystems. This study focuses on the effects of epidemic Dendroctonus rufipennis (spruce beetle) outbreaks, high-severity fires, and the subsequent species and structural diversity of subalpine forest regeneration and structure in Northern Colorado and Southern Wyoming. We compared tree seedling densities and species composition, surface fuel loading, and stand structure characteristics across 80 sites that experienced either high tree mortality from epidemic spruce beetle outbreaks (>50% affected basal area), high-severity wildfire, post-outbreak high-severity wildfire (1-3 years post-outbreak), or no disturbance (control). The beetle outbreak sites span multiple years post-outbreak from 1996 to 2017, ultimately comprising a chronosequence of beetle-affected stands. Analyses indicate a significant increase in fuel loading over time-since-outbreak, as aerial fuels are transferred to the forest floor following high tree mortality. Tree seedling densities among outbreak and control sites differ significantly from burned areas, indicating that wildfires override the effects of repeated disturbances on regeneration. There was consistent Engelmann spruce seedling survival following beetle outbreaks, providing evidence for stable forest recovery following a single disturbance. However, fire was a dominate force in determining post-disturbance species composition, indicating continued prevalence of high severity fire may prove detrimental for the persistence of spruce-fir species, while promoting shifts toward more drought and fire tolerant tree species (e.g., Pinus contorta). It is critical to understand post-disturbance fuel dynamics and stand recovery to identify hazards for subsequent fire suppression, implement treatments to enhance forest resilience, and to understand the potential consequences of climate-induced shifts in disturbance regimes on forest health.

Automated summary

This study focuses on the effects of epidemic spruce beetle outbreaks, high-severity fires, and subsequent species and structural diversity of subalpine forest regeneration and structure. The research found that wildfires override the effects of repeated disturbances on regeneration, while spruce seedlings show stable survival following beetle outbreaks. High severity fire may prove detrimental for the persistence of spruce-fir species, promoting shifts towards more drought and fire tolerant tree species.