the mountain pine beetle in a changing climate: what does it mean for Montana's Forests?

• Author: Six, Diana L.

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Few days go by without seeing an article addressing some issue related to bark beetles. Even fewer go by without seeing a dead pine. Ideas on what is causing the current outbreak of mountain pine beetle seem to flourish as well as the beetle. Some say the outbreak is within bounds of natural historical variability. In other words, the degree and extent of tree mortality occurring now is similar to that which has occurred in past outbreaks. Others say that this outbreak is different--it is more extensive and severe, and this difference is due either to past logging and management practices, to climate change, or to both. Some say none of this would have happened if we had only managed our forests.

So what is the truth? To answer this question we need to know some basic information about the insect. While there are many species of bark beetles in our forests, including many species of pine beetles, the one that is currently causing widespread mortality of pines across the west is the mountain pine beetle, Dendroctonus ponderosae. Unfortunately, this insect is often incorrectly reported as the pine bark beetle, the pine beetle, or the Rocky Mountain pine beetle. Google these names and you run the risk of getting the wrong insect and incorrect information on behavior and management.

Conditions and Triggers

The mountain pine beetle is a native insect whose natural geographic range extends from just north of Mexico to northern British Columbia. It can develop in all but one native pine species as well as most exotic ornamental pines.

However, outbreaks are usually restricted to lodgepole pine, and to a lesser extent, ponderosa pine. Most of the time, it is present in our forests in very low numbers. Only occasionally does it erupt into outbreaks such as we are seeing now.

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Outbreaks are not cyclic as is often claimed. Cyclic outbreaks are predictable and occur at regular intervals over time. Mountain pine beetle outbreaks are instead highly irregular in occurrence. There is a good reason for this. They only occur if and when two things come together at the same time: appropriate stand conditions and a trigger. Appropriate stand conditions include a predominance of pines greater than 9 inches in diameter. However, forest conditions alone cannot lead to an outbreak. If that was the case, old growth wouldn't exist. A trigger is also needed, and for the mountain pine beetle, the main trigger is warm...