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Colors of Wildfire Risk: Do Dead Trees Increase the Threat?

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There are many reasons to enjoy living in the West — the large number of sunny days each year and low humidity immediately spring to mind. Driving through the high country is another reason, as I did last week from the Front Range of Colorado to Aspen.  Among the highlights (depending on the route you take) is passing through Leadville, the highest incorporated city in the United States at 10,152 feet. There’s also the drive over Independence Pass, which definitely qualifies as white-knuckle driving with no guardrails for drop-offs that seem to go on forever. Another highlight was always the drop into Summit County (home of Breckenridge and other ski resorts) coming down from the Continental Divide after passing through the Eisenhower Tunnel.

However, what was a stunning vista in years past is now marred by dead trees virtually everywhere you look.  Beyond the aesthetic impacts it also raises the question of whether these mountainsides and valleys are tinderboxes waiting to go up in flames. With the serious wildfire impacts throughout Colorado and the rest of the West, it’s a critical question. Even as I’m writing this a state of emergency has been declared in California for a wildfire in Yosemite, and others are burning in Montana. And our ability to respond to these threats is being challenged as Federal budgets are being drained empty. But the answer to where wildfire risk is highest is a bit more complicated than just locating dead trees.

Are all (dead) trees created equal for wildfire?

Trying to make observations while driving at 65 mph or trying not to ride my brakes too much on descents was neither easy to do nor very safe. But, from what I could tell, I was looking primarily at gray-phase, dead lodgepole pines. Some comments on trees and wildfire I’ve heard are along the lines of, “Of course a dead tree is higher fire risk. You don’t burn live trees in your fireplace do you?” But research is beginning to point to wildfire risk depending on how long past mortality a tree is and what “phase” it’s in.

Research from last year attempted to sift through existing studies on tree mortality and wildfire risk with a focus on bark beetle impacts. They looked at nearly 40 studies and developed a graphical summary of how fire behavior varies with time and forest condition that includes three main phases.

Three main phases – red, gray, and old

The “red” phase is roughly 1-5 years after a mortality event, in this case a pine beetle outbreak. The needles are still on the tree during this period and are dry. The flammability goes up along with the crown fire potential. Crown fires are intense, difficult to manage, and are often stand-replacing in which most or all of the canopy trees are destroyed.  It is thought that during this phase crown fire potential is at its highest.

Red phase lodgepole pines in Washington state.  The dead trees still have their dry, needles on and crown fire potential is thought to be highest during this phase.  Source:  United States Forest Service

Red phase lodgepole pines in Washington state. The dead trees still have their dry, needles on and crown fire potential is thought to be highest during this phase. Source: United States Forest Service

The “gray” phase (which I believe I was seeing with a few red-phase trees mixed in) is the period around 5-10 years after mortality. The needles have now dropped and the crown fire threat goes down. Fuels are now building up on the ground from the dropped needles, so surface fire risk begins increasing. Also, the dead trees themselves can begin falling, thus building up surface fuel but reducing the density of standing trees and crown fire threat.

Gray phase lodgepole pines in Wyoming’s Medicine Bow National Forest.  Crown fire potential is likely lower during this phase as dry needles have dropped and trees begin falling reducing canopy density.  Source:  National Agriculture Imagery Program

Gray phase lodgepole pines in Wyoming’s Medicine Bow National Forest. Crown fire potential is likely lower during this phase as dry needles have dropped and trees begin falling reducing canopy density. Source: National Agriculture Imagery Program

Finally, over much longer periods is the “old” phase, during which the forest begins re-growing. Surface and crown fire potential increases during this phase as ladder fuels begin building up in the understory, which can allow ground fires to move up into the canopy and potentially lead to crown fires. Also, in some cases, the regrowth may change the character of the forest.

Still questions to be answered, but the risk is here and growing

Unfortunately, the “red” phase of peak crown fire threat is the phase we understand the least about, either due to conflicting studies or just a lack of research. As an example, one other recent study looking specifically at crown fire potential in Colorado lodgepoles found dry, gusty conditions to be a primary factor rather than the phase the trees are in.

So there’re still questions to be answered on whether all trees are created equal or not when it comes to wildfire risk (or even tree mortality itself; the subject of a follow-up post), but a basic picture is starting to emerge that may help assess high fire potential areas. This will be critical for triaging limited and dwindling resources even as demands and costs for battling damaging wildfires across the West continue to grow.

But on a larger scale, beyond this or that tree stand, a warming, drying West is providing conditions ripe for wildfire over very large areas of the region. We’ve already experienced an uptick in area burned and number of large wildfires over the past few decades. And looking into the future, some projections under continued warming from heat-trapping gas emissions show that this is a problem that will not soon go away.

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About the author: Todd Sanford is a climate scientist with expertise in the atmospheric chemistry and physics of the climate system. His current work involves the public health impacts of climate change. He holds a PhD in physical chemistry from the University of Colorado. See Todd's full bio.

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