Beetle-Killed Trees and Wildfire: What the Research Says

What are the green, red and gray phases?

Researchers describe a beetle-killed conifer in stages. In the green or attacked phase, the tree has been colonized but its needles have not changed color yet. In the red phase, the needles fade to red; Montana DNRC's landowner guide notes that trees killed by bark beetles turn red the year following the attack. In the gray phase, the needles and fine twigs have fallen and the crown is bare. The 2015 study by Hart and colleagues in the Proceedings of the National Academy of Sciences (PNAS) puts the gray stage at about three years after attack.

A Forest Service study by Evelyn Bull, which followed ponderosa and lodgepole pines killed by mountain pine beetle in northeastern Oregon, gives a sense of how fast that happens: the needles turned brown in the first year but mostly stayed on, about 12 percent of the original needles remained the next year, and less than 3 percent the year after. After that the dead trees begin to fall, which is the fourth stage and the one most people forget about.

Do red needles really burn faster?

Yes. The clearest evidence comes from the Forest Service's Fire Sciences Laboratory in Missoula. In a 2012 study, W. Matt Jolly and colleagues measured lodgepole pine needles from healthy, recently attacked and red trees. Foliar moisture averaged about 109 percent in green needles and about 12 percent in red needles, and time to ignition ranged from 41 seconds for green needles to as little as 11 seconds for red ones. Moisture in recently attacked trees was the most variable, meaning some trees were already drying before they changed color.

The authors also found chemical changes (more fiber, less starch and sugar) that increased flammability, and concluded that less heat would be required to ignite the foliage of attacked trees, so crown fire potential may be higher in attacked stands as long as foliage is retained on the tree. That last clause is the important one: the extra flammability lasts only while the dead needles are still on the branches.

What changes once the needles drop?

The fuel moves from the crowns to the ground. The PNAS paper by Hart and colleagues summarizes the expectation in the gray stage: forest floor fuels increase from falling needles and branches, and shrubs and understory trees grow quickly into the new light and can become ladder fuels. Many stand-level fire behavior models predict lower crown fire risk in the gray stage because the canopy is thinner and less continuous. The same paper notes that field studies suggest the understory trees may carry surface fire into the remaining crowns.

Later still, the dead trees fall. Bull's Oregon study found that no snags fell until the third year after death, and that eight years after being killed, 50 percent of the ponderosa pine snags and 38 percent of the lodgepole pine snags were still standing, with larger trees standing longer. Once down, they become heavy surface fuel. The Colorado State Forest Service notes that larger, denser woody material generates more heat than small fuels and is a problem when it burns near structures, and the Forest Service's Fire Effects Information System notes that woody debris may smolder long after the flaming front passes, killing even large ponderosa pines.

So do beetle outbreaks make wildfires worse?

At the scale of the West, the evidence says not in the way people expect. Hart and colleagues overlaid the areas burned in the three biggest fire years between 2002 and 2013 (2006, 2007 and 2012) on mapped mountain pine beetle infestation across the western United States. They found no difference between observed and expected area burned in red-stage or gray-stage stands. They note that the largest area of burned infestation occurred in 2012, in areas of extensive outbreaks in Colorado, Idaho and Montana, but that other heavily infested areas did not burn. Their conclusion was that both beetles and fire increased with recent warming and drought, and that area burned has not increased in direct response to beetle activity.

A 2012 review led by Jeffrey Hicke, which synthesized 39 studies, explains why researchers had disagreed. It found that some fuel and fire characteristics are enhanced after an outbreak while others are unchanged or reduced, with time since the outbreak the key factor, and that part of the disagreement came from studies asking different questions: beetle-killed stands compared with unattacked stands, or beetles compared with other drivers such as climate. The review rated confidence lowest for the early postoutbreak phases and for crown fire behavior.

Why does the answer differ for the trees near my house?

Because a homeowner is not asking whether a region will burn more acres. The question is what happens in the 100 feet around one building when a fire arrives. Weather, slope and the overall stand drive whether and how a fire reaches the property; the Hart paper notes that retrospective field studies tie fire occurrence and severity more strongly to elevation, slope, cover type and stand structure than to beetle kill. Once fire is there, a red tree 20 feet from the deck is a column of fine fuel that the Missoula lab measured igniting in seconds.

That is why defensible space guidance treats dead trees near homes simply. The Colorado State Forest Service's Home Ignition Zone guide lists removing stressed, diseased, dead or dying trees and shrubs in the zone 5 to 30 feet from the house, and NFPA's Firewise guidance says to remove dead plant and tree material in the zone out to 100 feet. Neither waits for the research debate to settle. The zone system is explained in the home ignition zone, part of our Montana Wildfire Guide.

Which beetle-killed trees should come out first?

Start with the ones close to buildings and still holding red needles, because that is when they are most ignitable and when their needles are about to rain onto the roof and into the gutters. Next are trees that can reach a house, driveway, power line or road if they fall; the falling hazard and the wildlife trade-off are covered in dead standing trees near homes. Gray trees farther out are a lower priority for crown fire but will become down logs, so plan for them.

Gutters and roof valleys under dying pines need clearing more often while the needles come down; see roofs, gutters and valleys. On a larger property, the remaining live trees usually need thinning too, because the stand that grew dense enough for an outbreak will grow dense again; thinning conifers for defensible space covers spacing.

Does removing infested trees stop the beetles?

It can help, if the wood leaves with the beetles in it. DNRC's landowner guide says identifying and removing infested trees can reduce the beetle population, but that it is imperative the infested logs and firewood are removed from the stand. A red tree is usually past that point: the beetles that killed it have typically already flown. The trees worth catching are the recently attacked green ones with pitch tubes and boring dust.

Timing matters for the pines left standing. The Forest Service's 2025 pine engraver guide rates green pine slash created from April through July as extreme risk for engraver beetles and slash created August through November as low to moderate. Removing dead trees creates slash and opens wounds on neighbors, so we schedule that work in late summer through winter where possible. Our pine engraver guide has the details, and slash disposal options covers what happens to the branches.

Is beetle-killed wood safe to keep for firewood?

Dead, dry pine burns well, and many owners keep it. Two cautions apply. If any of it is freshly infested, DNRC's guidance is that infested wood has to leave the stand; and the Forest Service's engraver guide says not to stack fresh pine logs or green pine firewood against live, desirable pines. Wherever it comes from, a woodpile is fuel, so it belongs well away from the house; see where to stack firewood in fire country.

Where HJ fits

Removing dead and beetle-killed trees near buildings is part of our hazard and dead tree removal work and our fire mitigation service. We look at the live trees around them at the same time, because protecting the survivors is usually the bigger long-term gain. If you are unsure whether a fading tree is dead, dying or just shedding old needles, a tree health and risk assessment will sort it out.

Call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate. One of our owners, Justin Johnson, is an ISA Certified Arborist (RM-8889A), and we will walk the property with you before any work is quoted.

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