Bark beetles and wildfires: How does forest recovery change with repeated disturbances in mixed conifer forests?

Increased wildfire activity and recent bark beetle outbreaks in the western United States have increased the potential for interactions between disturbance types to influence forest characteristics. However, the effects of interactions between bark beetle outbreaks and subsequent wildfires on forest succession remain poorly understood. We collected data in dry mixed conifer forests across Idaho and western Montana to test whether vegetation responses differ between sites experiencing single and repeated disturbances. We compared tree seedling density and age, surface fuel loading, and stand structure characteristics in stands that experienced either high severity wildfire, large-scale tree mortality from bark beetles, or stands that experienced high bark beetle mortality followed by severe wildfire within 3–8 years of attack. Tree seedling density was 300–400% higher in gray bark beetle-affected stands than burned sites, but there was no evidence that a beetle and wildfire interaction affected seedling densities. The age distribution of Douglas-fir and grand fir seedlings in stands with repeated disturbances differed from those that only experienced wildfire, suggesting that seed availability varies between these stands. Though both bark beetle outbreaks and wildfires resulted in the death of numerous large trees and surface woody fuel loads 100–200% greater than control sites, the creation of large snags and higher fuel loads across the landscape may have ecological benefits. Compounding effects of bark beetle activity and wildfires were not observed in surface fuel loadings or stand densities. Overall, the effects of high severity wildfire drove post-disturbance fuel complexes and succession whereas the effects of Dendroctonus pseudotsugae and Dendroctonus brevicomis outbreaks before wildfires resulted in minimal post-wildfire differences. We conclude that although seedling age structure is responsive to bark beetle and fire interactions, in terms of fuel complexes and tree densities these disturbances are non-additive and compounding effects on forest trajectory of dry mixed conifer forests of the northern Rockies were not supported.