Influences of an Active Spruce Beetle Outbreak on Fire Severity in Spruce-Fir Forests in Southern Colorado

Recent large and severe outbreaks of native bark beetles has raised concern among the general public and land managers about potential risks of amplified fire activity in western North America. However, there are no field studies of the effects of the severity of spruce beetle (Dendroctonus rufipennis Kirby) infestation on subsequent fire severity in subalpine Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) forests. We collected extensive field data in subalpine forests in the eastern San Juan Mountains, southwestern Colorado, to investigate whether fire severity changed across a range of severity of early, gray-stage spruce beetle infestations. Using correlation and multivariate generalized linear regression analyses, we found that pre-fire spruce beetle severity was not correlated with or a significant predictor of most field measurements of fire severity or a remotely sensed estimate of fire severity under moderate and extreme burning conditions. However, under moderate burning conditions only, the number of fire-killed stems and fire-killed basal area were weakly but positively correlated with the severity of beetle infestation. In comparison to severity of the pre-fire beetle outbreak, we found that topography, live and dead basal area at time of fire, and short-term weather conditions exerted a stronger effect on fire severity. This suggests the beetle outbreak and fire were either independent of one another or factors unrelated to beetle outbreak overrode any effects of the outbreak on fire activity. This finding of no effect agrees with retrospective studies using remotely sensed products to examine fire activity following other spruce beetle outbreaks, which similar to this study, concluded that the overriding influence of climate is essential for creating conditions conducive to large, high-severity fires in the subalpine zone of Colorado