Study: Southern California Forests With Beetle and Drought-killed Trees Do Not Burn More Severely Than Unaffected Areas

A newly published scientific study finds no evidence that areas with
conifer trees killed by drought or insects will burn at higher severity
in southern California's forests compared to areas with fewer dead
trees. The study directly refutes claims by forestry officials and
timber industry groups that dead trees are a contributing factor to
recent large-scale fires in the region. Such claims formed the basis
for proposals to conduct widespread removal of dead trees in wild lands
far from towns, power lines, and other human infrastructures.

The
peer-reviewed study looked at the San Bernardino Mountains of Southern
California where, in 2002 and 2003, severe drought and an outbreak of
western pine beetles killed a significant number of the conifer trees.
The U.S. Forest Service mapped tree mortality and estimated the number
of dead trees per acre.

In October 2003, the Grand
Prix and Old fires burned a total of 143,000 acres, of which 14,500
acres were conifer forest. The forest areas that burned included a mix
of areas with varying levels of pre-fire tree mortality from drought
and beetles. After the fires, the U.S. Forest Service analyzed the
degree to which the forest burned using satellite imagery that
accounted for trees that were already dead before the fire. Using these
maps, the authors of the study found no correlation between number of
beetle- and drought-killed trees and fire severity.

"The
hypothesis that forests are more likely to burn severely if they've had
recent tree mortality from drought and insects had never been tested
with real data," said Curt Bradley, a GIS specialist with the Center
for Biological Diversity and one of the study's authors. "The Forest
Service, the timber industry, and the media had simply presumed that
the standing dead trees provide the fuel that leads to higher-severity
fires. We used real data and found no evidence to support this
presumption. Wind, moisture, air temperature, and other climactic
factors - not fuels - are likely determining fire severity in Southern
California."

The study also challenged long-held
assumptions that harvesting dead trees is necessary to reduce fire
severity. "Logging dead trees, especially large dead trees, is likely
to be not only ineffective but counterproductive," added Monica Bond,
wildlife biologist and primary author of the paper. "Our study found
that areas with the largest size-classes of trees burned at lower
severities than areas dominated by smaller trees, regardless of
pre-fire tree mortality."

The study was published in the Open Forest Science Journal, a peer-reviewed online journal. The manuscript can be downloaded at https://biologicaldiversity.org/publications/papers/Bond_et_al.pdf.