Whizzing along the I-70 corridor into the Rocky Mountains, it’s hard not to notice widespread patches of dry, dead, red trees dispersed throughout the steep green hillsides. Whether they are ponderosa pines or lodgepole pines, many of these trees have been killed by insect infestation, most notoriously by pine beetles.
For many head-shaking observers, there is a next logical thought: All those dead trees are going to provide highly combustible fuel for a wildfire at some point. And, the thinking often goes, that’s going to translate into more — and more catastrophic — fires.
But that conventional wisdom might be wrong. That’s the conclusion of research by three University of Colorado Boulder scientists recently published article in PLoS One, an online peer-reviewed research journal.“When we started seeing mountain pine beetle kill in the lodgepole pine forests in the late 1990s, there was a kneejerk reaction among many fire managers and policy makers that there should be a huge increase in the likelihood of catastrophic fire,” says Tom Veblen, professor of geography and head of CU’s biogeography lab. “But the conventional wisdom is not supported.”
Using data from past fires in lodgepole forests in west-central Colorado and computer modeling developed by Tania Schoennagel, adjunct assistant professor in geography and research scientist at CU’s Institute of Arctic and Alpine Research, the researchers found that under extreme fire conditions, there was no significant difference in fire behavior between beetle-kill stands and those unaffected by infestation.
“In lodgepole pine forests, it was business as usual,” she says, referring to the severity of the fire, which was predicted to be high, leaping from treetop to treetop in a “crown fire”
However, the research also found that the probability of such “crown fires is predicted to be somewhat higher in beetle-kill stands. Specifically, the chance of such catastrophic fires occurring was greater primarily in “red phase” timber, where many standing trees still bristle with dead needles.
The study focused only on lodgepole pines, which occupy a higher subalpine zone than the ponderosa pines covering the foothills and lower mountain elevations. With or without insect infestation, lodgepole forests experience wildfire much less frequently than lower zones, in part because they are closer to snowmelt and temperatures are lower, which results in lower flammability in a typical year. The long-term tree-ring record over the past 400 years shows that there are only one to two years every century with extreme drought conditions necessary for widespread fires, Veblen says.“Fires in lodgepole and spruce fir forests depend on extreme drought. Any contribution by dead fuels from beetle kill appears to be relatively modest,” he says. Indeed, there have been no catastrophic fires in vast swaths of lodgepole pine killed by beetles in the early 2000s, he says.
And if there were ever a year conducive to wildfire in subalpine lodgepole forests, it’s this one. Extreme dry conditions led to early snowmelt in late spring and early summer, reducing fuel moisture. But, Veblen says, a wetter July might have turned the situation around.
Nobody’s unhappy about that. Still, scientists may have to wait for a big lodgepole burn to validate their research.
“We now need some of those beetle-kill stands to burn, so we can study them after or during burning,” Schoennagel says.
There is little research regarding beetle kill effects on fire in other tree species. However, one study on jack pines in British Columbia found that beetle-kill forests did not burn dramatically different from green forests.
“By no means do we have the definitive answer yet,” Schoennagel says.
Fire managers were quick to surmise that beetle kill stands of ponderosa pines may have contributed to severe wildfires along the Front Range this summer. No extensive studies on the effect of beetle kill on actual fire behavior ponderosa forests have yet been conducted.
“What I can say by simple observation is that the High Park fire” — which burned more than 87,000 acres and 259 homes west of Fort Collins in June — “didn’t start in a beetle-kill area. They let it burn into the beetle-kill area,” Veblen says.
Whatever their role in wildfire incidence and severity, pine beetle infestation may be a bellwether of a much larger problem: climate change. Beetle outbreaks are historically associated with warmer temperatures, and there is a broad scientific consensus that warming temperatures are the only explanation for current, synchronous outbreaks from Alaska to Colorado.
“The beetles are sort of a canary in a coal mine,” Veblen says.