LOS ANGELES - September 23 - 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 http://biologicaldiversity.org/publications/papers/Bond_et_al.pdf.