Study: Beetle-killed trees don't increase fire danger

BOULDER, Colo. — The devastation wrought on Western forests by the tiny mountain pine beetle appears to have little impact on the size and severity of wildfires, a new study concludes.

The study refutes fears by firefighters that the millions of acres of beetle-killed trees are a tinderbox waiting to explode with usually ferocious fires. The University of Colorado-Boulder study instead suggested that wildfires burn equally in live and dead forests, and said drought and climate change are more significant factors.

"The bottom line is that forests infested by the mountain pine beetle are not more likely to burn at a regional scale," Sarah Hart, the lead study author and a postdoctoral researcher, said in a statement. "We found that alterations in the forest infested by the mountain pine beetle are not as important in fires as overriding drivers like climate and topography."

Over the past 15 years, mountain pine beetles have killed more than 24,700 square miles of forest across the Western U.S., an area nearly as large as Lake Superior. Beetles kill the trees by burrowing through their bark and laying eggs. The eggs hatch into larva, which eat their way out of the living wood, transform into beetles, and fly away to infest another tree.

The dead trees then turn red as their needles dry out. Beetle-killed forests then take on a purple-grey hue after the needles fall off and branches dry out. The beetles are native to North America, but scientists say they've infested new areas because so many forests are unhealthily dense. Wildfires are a part of the natural ecosystem in the West, but large catastrophic wildfires pose risks for homes built nearby, along with drinking-water reservoirs.

Monica Turner, an ecology professor at the University of Wisconsin-Madison, said a similar study she and her colleagues published last year found similar results to the CU-Boulder study. She said it's easy to think the dead trees would burn better — after all, we build campfires with dry, not wet, wood.

But large forest fires often spread when burning sap-filled needles blow into the tops, or crowns, of other trees, or when flames climb up into the crown from what are known as ladder fuels. Ladder fuels are usually grasses, small branches and needles. When the needles fall off, the dead trees just can't catch fire as easily, Turner said.

"Once those trees drop those needles, you reduce those light fuels and then you end up with what are essentially telephone poles,"" she said. "The amount of fuel available is diminished and they don't burn as well. The fire doesn't carry well from one tree to the next."

Climate change is being blamed for lengthening the nation's wildfire season, with scientists predicting larger and more frequent fires. The U.S. Forest Service says the wildfire season now averages 78 days longer than it did in the mid-1980s.

The CU-Boulder study is being published this week in the journal Proceedings of the National Academy of Sciences.