Climate change impacts in Montana's forests

Bark beetles

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It seems like nearly everywhere we look in Montana these days, we find ourselves gazing at dead trees that have been killed in the recent Great Bark Beetle Outbreak.

In some of the hardest hit areas - around Helena, Anaconda, and Butte - homeowners who used to live on the edges of forests are now surrounded instead by a sea of red trees, while federal agencies have had to close some campgrounds to protect the public from the possibility of falling trees.

Photo: iStock.

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Amazingly, the culprit - the mountain pine beetle - is barely the size of a grain of rice.

When attacking a tree - normally a lodgepole or ponderosa pine - bark beetles bore through the bark and into the soft tissues of the tree.

Within a few short weeks, adult beetles have damaged these tissues enough to cut off the flow of nutrients and water, which soon kills the tree.

In addition to feeding on the inner part of the tree, female bark beetles lay approximately 75 eggs in the tree's soft tissues after boring into the interior.

Photo: Richard Hofstetter.

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As the current outbreak has continued and worsened, we have heard more and more about a connection between bark beetles and climate change.

So what does climate change have to do with the current bark beetle outbreaks?

The key turns out to be warmer, and often drier, conditions throughout our forests compared with a century ago.

Since 1895, average annual temperatures in Montana have increased by 2-3 degrees Fahrenheit, while precipitation levels have dropped.

As average temperatures have increased, we've lost the extremely cold winter temperatures (-40's F.) that used to kill bark beetle populations in Montana every few years.

At the same time, warmer temperatures at other times of the year are allowing bark beetles to breed from May until October now instead of the more normal pattern of two weeks per year, says Diana Six, professor of forest entomology at the University of Montana.

Photo: iStock.

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At the same time, the trees in our forests have become more stressed, and less able to fend off attacks by bark beetles, as our climate has become warmer and drier.

Normally, lodgepole and ponderosa pines that are under attack by bark beetles defend themselves by secreting sap in pitch tubes (pictured above), which often kills beetles before they can burrow into the tree.

Trees that are stressed by warmer, drier conditions, however, are less able to produce pitch tubes, or have become overwhelmed by the sheer numbers of bark beetles attacking them as the insect population has exploded.

Photo: Gary Chancey, U.S. Forest Service.

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The result of all of these changes has been a bark beetle outbreak unprecedented in both size and severity, according to Diana Six, who estimates that the recent outbreak is ten times larger than past outbreaks.

As of early 2010, the bark beetle outbreak in Montana was estimated to cover 2.7 million acres of forest, while British Columbia has lost more than 10 million acres of lodgepole pine alone.

Map: Montana Department of Natural Resources.

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As the current bark beetle outbreak finishes running its course, Montanans are slowly coming to grips with the fact that our forests are going to look very different in the future.

Impacts from the outbreak on forest ecology and function will take years to assess, but may include larger and less predictable fires, as well as effects on large numbers of wildlife species that have traditionally used the affected forests for habitat and food.

In the end, the biggest question that remains is what we, as Montanans, can do to better address climate change in the coming years and begin to minimize the impacts of climate change on our public lands, wildlife species, and way of life in this state.

Photo: iStock.