Do beetle-killed forests burn faster?

Posted: July 24, 2013

Source: The Durango Herald

Picture courtesy of the U.S. Forest Service

Mountain pine beetle kill forestColorado’s 4.3 million acres of beetle-decimated forests represent a catastrophe in the making during another devastating wildfireseason.

Or do they?

That is the conventional wisdom as another summer unfolds with destructive blazes that have left skies in Southwest Colorado and along the Front Range choked with smoke, but the reality is not so simple.

“The issue is not will beetle-kill forests burn – they certainly will,” said Monica Turner, a University of Wisconsin professor who has done extensive research of wildfires in the West. “The question is, are they burning worse – more severely – than if the forest was green?”

And the answer to that question is a matter of ongoing scientific debate, wrapped in factors that include the amount of time that has passed since the beetles did their damage, the number of trees that survived the infestation, other species of plants in the area and weather patterns.

“This is a field of study that we just don’t have all the answers for,” said Matt Jolly, a researcher at the Missoula Fire Sciences Laboratory in Montana whose work has looked extensively at the way plants burn in wildfires.

Anyone who has spent any time in the Colorado high country has seen the damage done by mountain pine beetles – vast swaths of formerly green forested hills painted red, or gray, by dying and dead trees.

The most recent count by the Colorado State Forest Service showed 3.35 million acres affected by the mountain pine beetle and 924,000 acres attacked by a different bug, the spruce beetle.

An I-News examination of state maps found that hundreds of thousands of those acres are in the so-called “red zones” – the high-fire danger areas primarily along Colorado’s Front Range and up the Interstate 70 corridor. State officials use a number of factors to determine what constitutes a red zone, including development (primarily homes), the type of vegetation in the area and the slope of the land.

So as the West Fork Complex fires continue to burn in the beetle-kill pocked forests of Southwest Colorado, and as the state reels from blazes such as the ones that destroyed 511 homes in the Black Forest near Colorado Springs, it’s tempting to look at all the dying, dead and decaying trees attacked by beetles and conclude that massive wildfires pose a real threat to all affected areas sooner or later.

Not so fast, said some of those who have dedicated years to studying the ways that trees and other vegetation burn in wildfires.

Today’s forests are vastly different than those of previous centuries. A century of aggressive firefighting efforts have left many areas overgrown and choked with downed and dead trees. Added to that, development has left many forested areas peppered with homes and parts of the West are experiencing prolonged, even historic drought. So the propensity for big, destructive fires is a near constant. Those conditions fuel blazes known as “crown fires,” which burn through the tops of the trees as if they were torches, spreading rapidly and generating tremendous heat.

Those massive, fast-moving fires – like the Black Forest blaze last month, where some beetle-kill trees were present – make for mesmerizing television and are the subject of extensive research.

Turner and researcher Jesse Logan, a former U.S. Forest Service scientist and college professor, are among those who believe that beetle-kill forests go through a predictable cycle – one in which they are at times much less-volatile than green forests. It starts with a beetle infestation, and it will take three or four years for the bugs to inflict all the damage they will on a section of forest. The trees in that stage turn red – and there’s little dispute those needles are highly combustible. But over the next couple years, the needles fall to the ground and begin to decompose.

“The overall trend would be that immediately after trees are killed and they still have all those fine fuels, needles in particular, on the tree, then it’s highly flammable, probably more flammable than a green forest,” Logan said. “But after those needles fall and that can be, like in lodgepole, a couple years after the tree is killed, then the standing forest is actually less likely to lead to a crown fire than a green forest.”

The reason? Green needles contain oils that are highly flammable.

But that strange juxtaposition – that green, seemingly healthy forests might burn with more fury than dead ones – is difficult for many people to comprehend.

“I think that one of the reasons that this seems counter-intuitive to people in terms of its effect on fire is that when we burn a fire in our fireplace, we put dead logs in there – we don’t put green branches,” Turner said. “But in a forest fire, it’s those green needles that are extremely flammable, and that’s what gives you the amounts of fuel up in the canopy in the forest and its conductivity.”

One of the difficulties in getting answers is that it has been difficult to build realistic fire models to examine the effect of beetle-kill trees. At the same time, studies that have looked at actual fires in beetle-kill areas are still in the review process, and the results have not been made public.

Still, Jolly, the Montana researcher, cautioned against assuming that a forest will be less burnable six years after being hit by beetles because the trees no longer have their needles. His research has shown that dead, red needles burn faster and hotter than green ones – but that’s only part of the reason for being circumspect.

“It’s just not that simple,” he said. “A standing gray tree, particularly one like a spruce … will have a lot of really, really fine dead branches. It may not have needles, but it will have those fine branches that will also burn and support a crown fire.”

In addition, even areas with heavy beetle-kill have some trees that survive, and many have other kinds of trees mixed in among those that die.

And then there’s another huge factor: the combination of weather and climate.

Logan pointed to the massive Yellowstone fires of 1988, which took down every kind of forest – beetle-kill and green alike.

“It all burned, just because conditions were so volatile,” he said. “In any situation, what’s driving it is fuel – you’ve got to have fuel. And the fuel can be green, red, gray or gray on the ground, and if the weather conditions are right, and you get a lightning strike or some idiot with a match, it’s going to go. And if the weather conditions are like they’ve been in Colorado these past few years, or like they were in ’88, it’s going to go big, regardless of what anybody can do.

“As humans, we have this idea that we can control nature, and we often can – we turn on the air conditioner and things like that. But these are forces of nature you’re not going to control,” he said.