Beetles are killing trees in the Western United States and it seems only logical that the dead, dry trees would increase risk of wildfires, but recent scientific studies suggest the issue is more complicated.

So far this year 4,636 wildfires in California have burned more than 200,000 acres. That’s more fires than this time last year and more fires than the five-year average. In fact, in the last few decades, the number of large fires are on the rise across the Western United States and the length of the fire season continues to expand.

One of the biggest reasons for this is warming temperatures, which are impacting snowpack and ushering in an earlier spring. California has an added challenge of dealing with a five-year drought. The drought and climate change have allowed bark beetles to get the upper hand in swathes of forest in the Sierra Nevada, creating what ecologists call “pulses” of tree mortality. To the average person it looks like a lot of red needles on dead trees.

The Forest Service last estimated some 66 million dead trees in the Sierra Nevada based on aerial surveys – it’s not a huge number compared to the entire region, but the dead trees aren’t spread out evenly across the mountain range. Instead, there are some areas, particularly in parts of the southern Sierra Nevada, where up to 85 percent of the large trees are dead or dying. This rings alarm bells.

One such bell is the role that these beetle-killed trees play in increasing fire danger. Lots of dead trees surely means there is an increased risk of severe wildfires, right? U.S. Agriculture Secretary Tom Vilsack said, “Tree dies-offs of this magnitude are unprecedented and increase the risk of catastrophic wildfires that puts property and lives at risk.” The media have also repeatedly reported that beetle-killed trees are creating a tinder box.

It makes logical sense, but a growing body of scientific research being conducted in the field is revealing otherwise.

Fire Probability

In considering whether beetle-killed trees affect fire risk, researchers often consider two different dimensions. The first is the probability that a fire will occur. So, are we likely to see more wildfires in places with pulses of tree mortality from beetles? And second, when fires do occur in an area with beetle-killed trees, will it burn more severely?

When it comes to the first area of study, the science conducted in the mountains in the American West in recent years is growing in consensus.

Tom Veblen, a professor of geography at the University of Colorado at Boulder, has been involved in studying the effects of fire and mountain pine beetle outbreaks in the Rocky Mountains for 25 years. He is also the co-author of a study led by Sarah J. Hart, also of U.C.-Boulder, that was published last year in the Proceedings of the National Academy of Sciences.

The Hart study looked at the effects of mountain pine beetles across forests in the Western U.S. (excluding Alaska) and found definitively that the area burned has not increased because of beetle-killed trees. This includes whether the trees were in the “red stage,” which means the dead needles are still on the trees, or the “grey stage,” in which the needles have fallen off.

“This is one of several studies that have reached the same conclusion,” said Veblen. “There are no studies that I know that have shown the opposite, even though my intuitive expectation and that of many other researchers at the outset was that we should find either an increase in likelihood of fire, meaning probability of fire occurrence after bark beetle outbreaks, or we should see an increase of fire severity.”

Harold Zald, now at Humboldt State University, found similar results in the Pacific Northwest. He co-authored a study that looked at data beginning in the 1970s to determine the effects of two insects – mountain pine beetles and western spruce budworm – on fires in the mountains of Oregon and Washington. “You just don’t see the probability of fire increasing after a lot of tree mortality due to these insects,” said Zald. In fact, in some cases, fire likelihood was lower where western spruce budworm, a “defoliator” that consumes needles, had killed trees.

Fire Severity

When you move beyond fire probability to whether or not these dead trees increase fire severity, things get a bit more complicated. Some studies using computer modeling have shown that fire severity could increase, especially shortly following tree mortality when red needles are still on the trees. The Forest Service maintains that fire risk also increases when dead trees fall to the forest floor. The service states on its website, “Observation of recent wildfires has shown that dead trees ignited easily and burned hotter and faster than the living trees containing minimal moisture.”

Brandon Collins, a research fire scientist with the Forest Service and University of California at Berkeley, said that since green trees contain more moisture than dead trees, it’s easier for combustion to take place. “In a beetle-killed tree there is much less of a threshold to get over in terms of driving off moisture,” he said. And if surrounding trees also contain low moisture, there is the potential of the fire to spread at a higher rate as a burning tree throws off embers.

But recent field studies found different results.

Research published by Veblen and Clark University’s Dominik Kulakowski in 2015 found that “the best available science indicates that outbreaks of bark beetles do not increase the risk of high-severity fires in lodgepole pine and spruce-fir forests of the Rocky Mountains.” It’s possible that in some cases the loss of needles would reduce the risk of crown fires, which spread from the canopy of trees and are associated with severe fires.

A 2011 study led by Martin Simard of the University of Wisconsin looked at lodgepole pine forests in the Greater Yellowstone Ecosystem, which encompasses parts of Wyoming, Montana and Idaho, and found that when it comes to beetles and fire, “contrary to conventional wisdom, the interaction was a negative feedback in which the probability of active crown fire appeared to be reduced.”

A 2009 study looked at conifer forests in the San Bernardino Mountains in Southern California after significant tree mortality, stemming from a pine beetle outbreak in 2002, was followed by fires in subsequent years. The study also did not find evidence of increased severity in pre-killed trees.

“The forests with a lot of snags [dead trees] including with a lot of snags with orange needles still on the trees, they did not burn more severely than the areas with few or no snags,” said Chad Hanson, an ecologist with the John Muir Project and one of the study’s co-authors.

In that study the researchers also found that larger trees, those over 61 centimeters (24 inches) in diameter, burned less severely than areas with trees that were 28 to 60 centimeters in diameter. “This result suggests that harvesting larger-sized trees for fire-severity reduction purposes is likely to be ineffective and possibly counter-productive,” the researchers wrote.

But Zald, whose work has focused on the Pacific Northwest, said that one of the biggest findings from looking at the research is that context matters – findings can change based on location, the kind of insect, how long ago the tree mortality occurred and whether studies were done on individual fires or across regions. What may be true for lodgepole pine forests in the Rocky Mountains may not be true for mixed conifer forests in the Sierra Nevada, he said.

And Veblen said that not much research has been done yet in California. “There has been a disconnect between the researchers in California, who are sometimes quoted as saying [that] because of bark beetle mortality we are very likely to see an increase in fire severity, versus a large body of research on the Rocky Mountain region which does not support that assertion,” said Veblen.

Research location is key. “We have pretty clear answers to these questions for high elevations, subalpine forests,” said Veblen. This include forests with Engelmann spruce, subalpine fir and lodgepole pine. But at lower elevations with chaparral and patches of conifer, there has been little research, and changes in vegetation type can result in radical changes in fuels, affecting fire behavior.

Fire Drivers

These findings may seem counterintuitive if you’ve ever made a campfire, as dead wood clearly burns more readily than green wood. But the magnitude of forest fires can change the equation. Findings suggest that under certain conditions, it actually doesn’t matter if the trees are alive or dead. “During extreme fire weather that promotes high fire activity in the western United States, fuels are likely dry enough to promote extensive burning regardless of alterations to fuels due to [mountain pine beetle] infestation,” the Hart study found.

Collins concurred, and said that under dry and windy conditions, tree moisture may not matter. But he added, “Under anything less than those extreme examples, then yes, the moisture does matter.”

There’s also another explanation. “The increase in bark beetle activity hasn’t fundamentally changed the potential for fire because fire is being driven by climate,” said Veblen. “It’s a tinderbox because of global warming – it’s really that simple.”

Veblen’s work points out that the overwhelming conclusion of the growing body of research highlights a much larger problem that is driving both fires and tree mortality from beetles: climate change.

“Research conducted thus far has consistently indicated that weather and climate are more important than the effects of outbreaks in determining fire risk and behavior in Rocky Mountain subalpine forests,” wrote Veblen and Kulakowski. “Although thoroughly considering the nuanced links between outbreaks and wildfires is important, recognizing that the increase in both outbreaks and wildfires is primarily driven by climatic warming may be as, or perhaps even more, important.”

Just because the findings may not implicate beetle-killed trees in increased fire severity or probability in some places, there may still be other ways in which they affect fire behavior. And, if the trees are in places near homes and other infrastructure, they do still require mitigation, said Veblen, because they can be a danger.

“If we focus on protecting homes, it gives us the potential to reintroduce fire, including larger, mixed intensity fires, into our forests in remote areas and allow those fires to burn without trying to suppress them,” said Hanson. “For the next decade or more, the bulk of our resources need to be spent making sure homes are protected.”