It has been nearly two weeks since a tongue of lightning touched down in the foothills of the Colorado Rockies, sparking the biggest wildfire in Larimer County history and the most destructive -- with almost 200 buildings damaged to date -- in the state's memory.

The High Park fire comes as a kind of second death for this stretch of the Arapaho-Roosevelt National Forest, where mountain pine beetles have held at epidemic levels for almost half a decade. Vast stretches of rust-red canopy, dry and primed for fire, are a testament to an infestation that has affected some 70 percent of the trees in the region.

Thanks to the beetles, there has been no shortage of ready fuel and the fire has spread quickly, whipped by sporadic winds from the southeast.

Since the blaze was set on June 9, fire crews have been working ceaselessly, and have reined in the fire at its eastern and northern edges. But converging forces are working against them.

Hot, dry conditions have fanned the flames, while beetle kill and a long-standing policy of fire suppression have left forests thick with dry, available fuel. In the rugged, forested mountains to the west, the fire has continued to advance, claiming 59,500 acres as of Tuesday evening.

Without rain, forest managers say, the blaze could continue throughout the summer.

Agents of change
When pine beetles take over a forest, fire is typically not far behind, said Cal Wettstein, incident commander with the Rocky Mountain division of the Beetle Incident Management Organization.

"When trees die, they go through a natural process of drying" -- leaving them vulnerable to fire -- "and eventually falling," he said. "When beetles get into a forest, they essentially shortcut that first stage."

Mountain pine beetles attack en masse, coordinating by pheromone signal to converge in the hundreds on a single pine tree. They carry in their mouths a potent blue stain fungus -- a kind of natural, viral weapon -- which, once it has infected a tree, pulls moisture from the wood, weakening it.

The tree's only defense against the intruding beetles is to fight back with secretions of sap, further draining it of moisture.

If beetles attack in great enough numbers, the tree is eventually sapped of life, and dies of dehydration.

A forest in the grip of a pine beetle epidemic resembles the aftershock of a severe drought -- reddish-brown canopies of tinder-dry needles, waiting only for the touch of a spark to set them ablaze.

Those trees burn hotter -- and the fire spreads faster -- than they would in a forest untouched by beetle blight, said Rocco Snart, a fire behavior analyst trainee who has been working on the ground with crews battling the High Park fire.

"In the Arapaho-Roosevelt forest, you've got pine trees in just about every stage of beetle kill," he said. "You've got the greenish yellow of last year's killed trees, the red-needled trees that were killed before, and in a few places you'll see what we call the 'ghost forests'" -- trees killed long ago that have since lost their needles.

While the yellowing needles can ignite with enough temperature, it's the reddish-brown canopy you most have to worry about, he said. Those dry, brittle needles can ignite with explosive force, hurling embers up into the air to be carried by the wind to set other fires.

"It's the difference between gasoline and a pile of grass," he said. "Those flammable compounds can induce more fire behavior in the crowns, they're more flashy and they have more energy."

An old story, often retold
Fire is as much a part of Western forest ecology as the trees themselves, clearing old growth and allowing new seeds to take root and grow. So intricately linked are the forces of destruction and rebirth that the pine cones of some trees release their seeds only under extreme heat.

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.

The periodic cycle of destruction allows forests of the lodgepole, like its cousin the ponderosa, to regenerate, he added.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500