Beetle-killed forest in Williams Fork Valley, CO. Photo by Jeffry B. Mitton.

Pine-bark beetles are decimating forests throughout the West. At the same time, we’ve had intense forest fires recently in Colorado and Arizona. What is the connection between bark beetles and forest fires?

Most people assume there is one. It seems intuitively obvious: dead wood burns better than green wood, so a dead forest, such as in the photo above, must be more flammable than a live one.

After my recent post on the Flagstaff Fire, an alert reader commented on the supposed link between forests killed by pine bark beetles and our current epidemic of intense fires. I’d heard something quite different from Jeffry Mitton, an ecologist at the University of Colorado, so I responded that beetle kill is likely not a large factor in current wildfires. Mitton had said in a lecture I attended that forest fires owe a lot more to drought than to beetles. Drought is behind both fire and beetle kill, he explained. (The best way to prevent beetle kill is to keep a tree well watered; the tree uses its liquid pressure to expel beetles when they first attack.)

A dry green forest is extremely flammable because each tree is full of sap, which is an explosive fuel. Mitton said that a dry green tree is the most flammable thing in the forest. When beetles hit a green forest and kill the trees, the trees first turn red as their needles die, and then gray after the needles drop. A red forest is also extremely flammable because of all the dead needles on the trees. But once a section of forest dies into gray and the sap is no longer present, trees can no longer explode and send sparks to their neighbors, and a fire has a harder time spreading. Mitton said, “Fire does not propagate through a gray forest.”

Surprised by this info, the reader quoted this study, which suggests that the intensity of the fireline is 4 or 5 times higher in beetle-infested forests and 6 to 8 times higher in beetle-killed forests compared to forests that have not been touched by beetles.

I asked Jeffry Mitton to comment on the study. Mitton looked at the study and noticed three things:

(1) It has not been reviewed by anonymous reviewers.
(2) It is not published.
(3) It is based not on observations but on applying a model to some rather arbitrary data.

Right, the peer-reviewed test—where a paper is sent out to anonymous readers well versed on the topic, who examine the study to see if it followed standard protocols and the most reliable methods. If it didn’t, then the study’s results cannot be trusted and the paper is not published. In effect, it doesn’t give us true enough information about the world. This study was not peer reviewed and so should not be given as much weight as those that have been anonymously reviewed and published.

Mitton’s third point is also important: this is an argument from a model, not from actual measurements. The authors are saying that based on what they know about fuel and treetop mass and how fire acts, a fire in beetle-killed forest should have a certain intensity, and that intensity should be higher than in a noninfested forest.

But is it true? Do forest fires actually act like that? The study doesn’t address this.

Mitton said,

If the results of that paper were correct, why have we not seen any big fires . . . around Steamboat, Vail, and Dillon? If beetle kill is such a threat, then we ought to be able to list the immense fires that have followed the beetle in this epidemic, and in previous epidemics. Those fires have not occurred. The fires in the West occur in green forests suffering drought conditions.

Mitton took the above photo recently in the Williams Fork Valley, in north-central Colorado. There are miles of such beetle-decimated forests at higher elevations stretching from Colorado to British Columbia, which has been hit particularly hard. Mitton said,

In BC, this type of landscape stretches for miles and miles and miles. I have not heard of any fires torching these landscapes. And remember, these landscapes have been around for a while now, for the epidemic began 13 years ago in BC.

Interestingly, while this email conversation was going on, an article in the Boulder Daily Camera quoted researchers who also dispute the conventional wisdom that beetle-killed forests are more flammable. Monica Turner, a forest fire researcher of the University of Wisconsin, said,

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. But in a forest fire it’s those green needles that are extremely flammable.

It’s why you don’t ever want to burn your Christmas tree in a fireplace. Those needles flame and flash like crazy. (I saw the effects above one fireplace—smoke damage up the stone facade on the wall, reaching all the way to the high ceiling. No, it wasn’t my fireplace, thank goodness!)

You might think of a beetle-killed forest as a set of needle-free logs in a fireplace spaced out so they’re barely touching. It’s not easy to keep that fire going even if you try. To feed a fire, we stack the logs up. To kill it, we spread them out. Dead trees in a forest, with no needles and spaced evenly apart, cannot easily conduct fire to one another.

It appears that drought is a much bigger contributor to forest fire than beetle kill, as another recent Daily Camera article shows. And that means higher risks of fire as climate change advances.

Climate Central is a research center that recently released a graphic to show the relation between early snowmelt and forest fires in the West. You can check out the interactive web map here. Alyson Kenward, a senior scientist with Climate Central, summarizes what it shows:

Years with higher temperatures in spring and summer and lower snowpack tend to be the years with the most fires.

So our increasing rate of forest fires has a lot more to do with climate change and drought than they do with beetle kill. Knowing this won’t change the path of the beetles, but it may allay some fears about where the beetles have been.