USA –-As a series of devastating wildfires consumed tens of thousands of acres of Colorado forest land over the last month, the media, the public and even fire managers have been quick to blame the numerous stands of trees killed by voracious pine beetles as one of the factors contributing to the intensity of the blazes.
But scientists aren’t so sure.
Researchers at the University of Colorado’s Biogeography Lab, led by Professor Thomas Veblen, have published a number of studies in recent years showing that the degree to which beetle-killed trees increase wildfire risk — if at all — is related to the makeup of the forest vegetation, the time that has passed since the trees were killed and the prevalence of other weather conditions that favor forest fires, such as high winds and low humidity, among other factors.
In total, 3.3 million acres of Colorado forests have been affected by the current mountain pine beetle epidemic, according to the U.S. Forest Service’s most recent aerial survey. In 2011 alone, pine beetles infested 752,000 acres, 275,000 of which were in lower-elevation ponderosa pine forests instead of the higher-elevation lodgepole pine stands where the epidemic began. And the majority of new ponderosa pine infestations were in Larimer County (254,000 acres) and in Boulder County (18,000 acres).
Since the vast majority of the current mountain pine beetle infestation has been in lodgepole pine forests, scientists have a better understanding of fire behavior in those areas than in the relatively recently affected ponderosas pine forests, Veblen said.
But the fires burning this summer along the Front Range — especially the High Park Fire, which has charred more than 87,000 acres in Larimer County — have tended to be in an areas largely dominated by ponderosas and Douglas firs.
“It’s important to recognize that most of the published research on the relationship between fire activity and mountain pine beetle has been in the lodgepole pine forest at much higher elevations,” said Veblen, who also pointed out that the impact of beetle-killed trees on each individual fire cannot be determined until after the fire is put out.
Lessons of history
But scientist can look to history for clues about the impact of pine beetles on fires in ponderosa forests.
While the pine beetle outbreak in the Front Range’s ponderosa pines has grown over the last couple years, the magnitude of the outbreak is, so far, not unprecedented for local forests. Most recently, a pine beetle epidemic struck ponderosas on the northern Front Range in the 1970s.
“The level of pine beetle outbreak in the ponderosa pine is not exceptional if we look at the long-term record,” Velben said. “And the long-term record does not show that catastrophic fires have followed previous beetle kills.”
This leaves Veblen to conclude that the factors driving the fires that began burning this summer are related to the weather conditions.
Colorado is experiencing an earth-cracking, vegetation-browning drought and an incredibly tenacious heat wave that have conspired to push the fire danger to extreme levels. Add in whipping winds and an ignition source and fires this year have the capacity to be catastrophic — beetle kill or no beetle kill.
“This is a consequence of warm, dry conditions and we have every reason to believe that warming is going to continue into the future and that this will not be an unusual occurrence,” Veblen said.
In lodgepole pine forests — where scientists have a better understanding of the effect of the current beetle outbreak — it is also not clear that beetle kill increases fire danger under all circumstances. About 40 acres of the 300 acres west of Boulder burned by the Flagstaff Fire were covered by an unusually low-elevation lodgepole pine stand that had been infested with pine beetles, said Chris Wanner, a forest ecologist for the city of Boulder.
Studies published by CU’s Biogeogrpahy Lab on fires that burned on Colorado’s Western Slope during the drought of 2002 — including the Hinman Fire and the Big Fish Fire — showed that fires moving through lodgepole pine forests infested with pine beetles did not burn in a significantly different way than those that were relatively beetle-free.
CU scientists also have used computer modeling to predict what fire behavior in pine beetle-ravaged lodgepole pine forests would look like under a variety of conditions.
Tania Schoennagel, who also works in the Biogeography Lab, published a study in February in the journal PloS ONE that reported that, under extreme fire conditions, fires in beetle-killed stands do not appear to behave in a significantly different way than fires in areas without beetle kill.
But Schoennagel’s modeling did find that under more moderate conditions, the risk of crown fires in beetle-kill stands may be higher than in areas without beetle kill.
Another paper published this year by Utah State University reviewed 39 studies that tackled the interaction of beetle-killed trees and fire behavior. That review found that beetle-killed trees can impact fire behavior, but the degree of the impact depends on the conditions. The authors concluded that “generalizations about the effects of beetle-caused tree mortality on fire characteristics are unwarranted.”
The studies reviewed, however, did tend to agree that the potential for crown fires may actually drop a few years after a tree is killed by pine beetles when the dead needles fall off the tree. However, the risk of surface fires tends to increase over the decades as the dead trees fall to the ground and the forest begins to re-grow.
In any case, wildfires such as the High Park Fire that are now burning in Colorado and across the West may provide scientists with more insight into the interaction between fires and pine beetles.
“We can’t really answer questions about whether or not the spread and severity of (the High Park Fire) was influenced by beetle kill until after the fire is over and we can actually analyze exactly where it spread and what the pre-fire conditions were like,” Veblen said. “That’s something that could be done maybe in September or October.”