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 pre-fire tree mortality in Sierra Nevada mixed-conifer
forests. We collected data on 180 plots within the 2015 Rough Fire and
2016 Cedar Fire footprints. Our analyses identified pre-fire 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). Pre-fire 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 pre-fire mortality on basal area killed by fire was
greater under milder weather conditions. All measures of fire severity
increased as pre-fire mortality increased up to pre-fire 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 pre-fire mortality and subsequent wildfire severity
in these systems. This study contributes to a growing body of evidence
that the influence of pre-fire tree mortality on wildfire severity in
temperate coniferous forests may depend on other conditions capable of
driving extreme wildfire behavior, such as weather.