Bark Beetles & Forest Fires – A Common Misconception

One of the major ecological issues facing the Rocky Mountain Region, particularly for buyers and sellers of working and recreational ranches, is the current beetle infestation that is taking place in many large forests throughout the west.  As anyone familiar with the western landscape will tell you, it’s becoming very common to see large swaths of forests where once-green pine and spruce trees are now reddish or gray, due to being infested and ultimately killed by a swarm of small insects collectively known bark beetles.

Beetle infested trees

In my conversations with both landowners and ranch buyers, one of the most common concerns I hear over and over is that stands of “beetle-killed” trees drastically increase the risk of severe forest fires in affected areas.  While it seems reasonable to deduce that a forest full of dead trees would be much more flammable than a healthy, green, live forest, recent scientific research is showing that the reality may be the exact opposite.

There is a growing consensus among many scientists that a forest of dead, beetle-killed trees is no more flammable than a healthy, fully functioning, live forest.  In fact, there is even some evidence suggesting a dead forest is less prone to destructive crown fires than a normal healthy forest.

“Such ongoing [beetle] outbreaks have led to widespread public concern about increased fire risk; however, outbreaks of mountain pine beetle and spruce beetle do not appear to substantially increase the risk of subsequent fire under most conditions.”  – Excerpt from “Do Bark Beetle Outbreaks Increase Wildfire Risks in the Central U.S. Rocky Mountains? Implications from Recent Research,” Black et al. 2013

Two or three years after bark beetles have killed a tree, all of the needles fall off, leaving stands of bare, needleless trees where there was once a green, dense forest.  This lack of needles reduces the forest canopy’s “bulk density” (i.e. the amount of fuel, alive or dead, in the trees’ upper branches).  This reduction in bulk density has two main effects on a forest’s flammability: 1) it creates gaps between trees’ canopies that reduce the ability of a crown fire to spread from tree to tree, and 2) it reduces “ladder fuel” which allows a surface fire to climb from the forest floor, up a tree, and transform into a more severe crown fire.  In short, the trees may be dry, but the lack of needles simultaneously reduces the fuel and limits the fire’s ability to spread.

So if bark beetle infestations do not significantly increase forest fire risk, what is the main risk factor for wildfires?  The answer, according to many scientists, is climate.  Warm, dry, severe drought conditions, similar to the ones Colorado has experienced over the last few decades, seem to be the main cause of many of the large, destructive fires we’ve experienced.  For example, during the historically dry summer of 2002, there were widespread forest fires throughout the state.  After examining the fire data from that summer, it became clear to researchers that beetle-affected forests did not fare significantly worse than areas unaffected by beetle infestations.

The current bark beetle infestation is definitely a cause for concern for many reasons, most notably the visual and aesthetic impacts the dead trees are having on the western landscape.  However, when considering a property’s forest fire risk, the data suggests that there is no clear link between beetle infestation and increased forest fire risk.

For more information on bark beetles and forest fire risks, please see the following resources:

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