Mountain pine beetle outbreaks do not increase the likelihood of forest fires

Study finds that bare, beetle-killed trees may actually decrease crown fire likelihood

Conventional wisdom about the effects of the mountain pine beetle (MPB) on forest fires across western North America has often assumed that the beetle-induced death of trees has led to an increase in rapidly spreading forest fires. However, in a study published online recently in Ecological Monographs, a journal of the Ecological Society of America, Martin Simard from the University of Wisconsin, Madison and colleagues found that MPBs may actually decrease the likelihood of crown fires in Greater Yellowstone. They report that, comparatively, extreme weather conditions may play a more prominent role in the spread of forest fires.

Within the last 30 years, the extent and severity of MPB outbreaks and forest fires have both increased. MPBs undergo periodic pheromone-mediated mass attacks on host trees, in this case lodgepole pine in the Greater Yellowstone area. The recent increase in these attacks has led to widespread tree mortality and—it was previously thought—may contribute to an increase in active crown fires as well.

“This is not the case,” says Simard, along with co-authors William Romme from Colorado State University and Jacob Griffin and Monica Turner from the University of Wisconsin, Madison. “Our study suggests that, when beetle-killed trees lose their needles, the amount of canopy fuel is reduced by 50 percent, and as a result, fuel connectivity and the probability of active crown fire are also reduced.”

Lodgepole pine trees that have been recently killed by the MPB are classified in two stages: In the red stage, dead dry needles are still hanging on the trees, and in the gray stage, all needles have fallen to the ground. In other words, the needles in the tree canopy are the most important fuel for crown fires—not the dead tree trunks. Even in stands with many trees in the red stage, which visually appear to be so flammable, dead needles already are falling, reducing canopy fuels and the potential for crown fire activity. 

Simard and colleagues sampled surface and canopy fuels—that is, live and dead needles, twigs and branches on trees and on the ground, downed logs and understory plants known as “ladder fuels”—in 35 lodgepole pine stands in Greater Yellowstone that were either undisturbed or that had been attacked by MPBs between one and 36 years ago. They then used computer models to predict potential fire behavior in these stands and considered other factors that could be contributing to active crown fire spread, including moisture conditions and extreme weather.

“Fires in subalpine conifer forests, including in Yellowstone, are mainly driven by climate,” says Simard. “If we look at the fires that occurred in Yellowstone in 1988 as an example, we see that forests burned quickly and readily, regardless of the age or structure of the forests. Hot, dry, windy conditions lead to active crown fires, in which the fire spreads from tree to tree indiscriminately.”

Although the probability of active crown fires is reduced, passive crown fires may be more likely to occur 25 to 35 years after an outbreak of MPB; however, such fires are less intense and spread more slowly than active crown fires. In passive crown fires, flames move from the ground to the tree crown and cause torching as flames climb up ladder fuels to the canopy; however, these types of fires do not directly spread to neighboring tree crowns.

“These findings have important implications for Greater Yellowstone,” said co-author Turner. “This study suggests that management of beetle-killed forests primarily to reduce a perceived increase in fire likelihood would not be warranted. Rather, the data indicate that the mountain pine beetle reduces the probability of active crown fire for up to 35 years by thinning the forest. However, it will be important to pay attention to how both of these key natural disturbances respond to a changing climate.”

For access to the author summary, downloadable photo and in-press paper, visithttp://esa.org/papers/.

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