Data from: Fire severity unaffected by spruce beetle outbreak in spruce-fir forests in southwestern Colorado

Recent large and severe outbreaks of native bark beetles have raised
concern among the general public and land managers about potential for
amplified fire activity in western North America. To date, the majority of
studies examining bark beetle outbreaks and subsequent fire severity in
the U.S. Rocky Mountains have focused on outbreaks of mountain pine beetle
(MPB, Dendroctonus ponderosae) in lodgepole pine (Pinus contorta) forests,
but few studies, particularly field studies, have addressed 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. In Colorado,
the annual area infested by spruce beetle outbreaks is rapidly rising,
while MPB outbreaks are subsiding; therefore understanding this
relationship is of growing importance. We collected extensive field data
in subalpine forests in the eastern San Juan Mountains, southwestern
Colorado, to investigate whether a gray-stage (<5 years from
outbreak to time of fire) spruce beetle infestation affected fire
severity. Contrary to the expectation that bark beetle infestation alters
subsequent fire severity, correlation and multivariate generalized linear
regression analysis revealed no influence of pre-fire spruce beetle
severity on nearly all field or remotely sensed measurements of fire
severity. Findings were consistent across moderate and extreme burning
conditions. In comparison to severity of the pre-fire beetle outbreak, we
found that topography, pre-outbreak basal area, and weather conditions
exerted a stronger effect on fire severity. Our finding that beetle
infestation did not alter fire severity is consistent with previous
retrospective studies examining fire activity following other bark beetle
outbreaks and reiterates the overriding influence of climate that creates
conditions conducive to large, high-severity fires in the subalpine zone
of Colorado. Both bark beetle outbreaks and wildfires have increased
autonomously due to recent climate variability, but this study does not
support the expectation that post-beetle outbreak forests will alter fire
severity, a result that has important implications for management and
policy decisions.