Pine beetles have evolved as a kind of auxiliary agent in this process, sweeping through every decade or so to accelerate the cycle.
Modern humans, however, have thrown a wrench in the gears. Human beings have successfully suppressed most low-level burns that have erupted in the region over the past few decades, in the process allowing the forests to thicken and brushy "fuel" to accumulate.
On top of that, the pine beetle epidemic gripping the West is larger than any in recent memory. Many entomologists point to the record warmth of the last 10 years as a cause. The beetle's life cycle is temperature-dependent. It is killed by frosts, but its life cycle is accelerated by warmth, and in some cases higher temperatures have been shown to allow the beetles to reproduce twice a year rather than once (ClimateWire, March 19).
Add to those factors a dry winter and hot spring of the kind Colorado witnessed this year and last, and you have a perfect storm of conditions for a blaze on the scale and severity of the High Park fire, said Reghan Cloudman, public information officer for the Arapaho-Roosevelt National Forest.
"Usually, you expect to see fuel moisture at around 90 percent in healthy trees," she said. "This year, we were seeing levels of closer to 60 percent before the fire." And that's in trees untouched by beetle blight.
Fighting fire with fire
There's only one real solution that firefighters can fall back on when conditions get this severe, said Greg Kujawa, a senior staff assistant with the U.S. Forest Service -- they have to let it burn.
"You want to burn it in a controlled way -- do a light-intensity burn in the understory, clear the canopy without setting a ground fire" that would be more intense, he said. That clears out much of the most flammable fuel and can be better contained, he said.
Those options get much more complicated when you add people, houses and infrastructure into the mix, he said.
"We've got this dilemma on our hands," he said. "We can't let fire play its natural role when you've got a community or a private development nearby -- especially when conditions are as bad as they are" in the area of the beetle kill, he said. "It'd be too easy for [the fire] to get out of control."
That has left forest managers with a second, much more labor-intensive option -- manually clearing and removing excess fuel from regions where humans and forests come into contact.
For the past 10 years, the Forest Service has been engaged in an aggressive campaign to thin forests and create "fire breaks" -- swaths of empty land -- between tree lines and private land. Last week, Forest Service Chief Tom Tidwell called for an acceleration of the effort, saying an additional 20 percent of land would need to be returned to its pre-accumulation state each year.
Fire hazards ominous in other areas
But forest managers worry that the worst may be yet to come. On the western slopes of the Rocky Mountains, ponderosas give way to lodgepole pine -- a slower-growing tree unaccustomed to regular, low-intensity fires.
"It's a different kind of forest ecology," said Wettstein, of the Beetle Incident Management Organization. "You'd usually only see a high-intensity fire here every 200 to 300 years, when all the forces converge just right."
This year, those stands are even drier than the forests to the east -- where the High Park fire is burning -- with only a light dusting of snow feeding them this winter. And here, too, the bark beetle has been at work, leaving vast swaths of rust-colored forest in its wake.
"Following a bark beetle epidemic, you see, throughout history, all the stars line up, and you end up with the kind of high-intensity fires that hit Yellowstone [National Park] in '88," Wettstein said.