Recent bark beetle outbreaks influence wildfire severity in mixed-conifer forests of the Sierra Nevada, California, USA

In temperate forests, elevated frequency of drought related disturbances will likely increase the incidence of interactions between disturbances such as bark beetle epidemics and wildfires. Our understanding of the influence of recent drought and insect-induced tree mortality on wildfire severity has largely lacked information from forests adapted to frequent fire. A recent unprecedented tree mortality event in California's Sierra Nevada provides an opportunity to examine this disturbance interaction in historically frequent-fire forests. Using field data collected within areas of recent tree mortality that subsequently burned in wildfire, we examined whether and under what conditions wildfire severity relates to severity of prefire tree mortality in Sierra Nevada mixed-conifer forests. We collected data on 180 plots within the 2015 Rough Fire and 2016 Cedar Fire footprints (California, USA). Our analyses identified prefire tree mortality as influential on all measures of wildfire severity (basal area killed by fire, RdNBR, and canopy torch) on the Cedar Fire, although it was less influential than fire weather (relative humidity). Prefire tree mortality was influential on two of three fire-severity measures on the Rough Fire, and was the most important predictor of basal area killed by fire; topographic position was influential on two metrics. On the Cedar Fire, the influence of prefire mortality on basal area killed by fire was greater under milder weather conditions. All measures of fire severity increased as prefire mortality increased up to prefire mortality levels of approximately 30-40%; further increases did not result in greater fire severity. The interacting disturbances shifted a pine-dominated system (Rough Fire) to a cedar-pine-fir system, while the pre-disturbance fir-cedar system (Cedar Fire) saw its dominant species unchanged. Managers of historically frequent-fire forests will benefit from utilizing this information when prioritizing fuels reduction treatments in areas of recent tree mortality, as it is the first empirical study to document a relationship between prefire mortality and subsequent wildfire severity in these systems. This study contributes to a growing body of evidence that the influence of prefire tree mortality on wildfire severity in temperate coniferous forests may depend on other conditions capable of driving extreme wildfire behavior, such as weather.