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Interviewee: Tom DeGomez, University of Arizona
The giants of the forest are being felled in numbers not normally seen by a pest that, while native to the forest, may be getting a very un-natural boost from climate change.
Tiny beetles are attacking pine and other kinds of trees from Arizona to Alaska, their numbers bolstered by mild winters that are currently tilting the environmental playing field away from the trees and toward the pests.
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“The beetle outbreak that we’ve had in the last eight years or so has been at times very severe,” says University of Arizona Forest Health Specialist Tom DeGomez. He’s been studying the situation in that state for years and says, although the worst of it was in 2002 and 2003, some areas are still under attack. As a result, one Arizona forest has limited issuing permits to cut Christmas trees.
“Our outbreak (in Arizona),” says DeGomez, “has not been as sustained as in Colorado and other areas.” Among those other areas is British Columbia, which has been particularly hard hit.
One published scientific report has concluded that in Colorado, enough trees have died that there has been an impact on that area’s climate.
"The Forests Are Not Disappearing"
While some media reports have headlined the die-off as “Disappearing Forests,” DeGomez disagrees. He says, “The forests are not disappearing, however some stands are.” This is particularly devastating in cases where the stands are what foresters call “high-value,” meaning they’re located near homes or recreational areas.
The beetles are native to the forests they’re in. DeGomez notes that they “help break down the forest when it gets too dense,” and therefore have a natural role in the life cycle of the forest.
No single kind of beetle is responsible and no single species of tree is at risk. Each species of conifer tree has its own natural pests. In Arizona, the ponderosa pine and piñion trees have been the main victims. But De Gomez points out that elsewhere in the west — for example, where lodgepole pines are the dominant species — the situation is much worse because of the beetle native to those forests.
“The mountain pine beetle is a much more aggressive insect than are the beetles that typically kill ponderosa pine,” he says.
Climate Change Connection
What is a factor among the outbreaks are drought conditions that have plagued the west for several years, something that could be an early indicator of climate change. DeGomez says the warmer and dryer winters can stress the trees, adding, “If the tree is stressed, then the bark beetle can come in because the tree in its stressed condition cannot produce defense mechanisms.”
At a stand of damaged trees in Northern Arizona, he points out where a healthy tree has used pitch to cover a beetle hole, but a nearby dead Ponderosa Pine shows lots of holes.
In other parts of the west researchers are finding that warmer winters fail to kill off enough overwintering beetles, resulting in lots of hungry predators in the summer. In Arizona, DeGomez is researching the idea that winter precipitation is the key factor. He says wet winters give trees the chance to use stored up carbohydrates and that, “If the trees are healthy in April…then they’re going to be able to fend off these beetles pretty easily.” He says that’s about the time the beetles come out for the season, and fending off the first generation of beetles for that summer will slow things for the remainder of the season.
A single dry year is not enough to stress the trees, but several moderately dry seasons, followed by a very dry season, such as what happened in 2002, can result in an outbreak.
If current weather patterns persist, he doesn’t foresee another major outbreak for the next decade or so in the Southwest because the forest has already been thinned by the beetles to the point where there’s enough water to go around, even in moderately dry years. However, he agrees that a warmer west could change all that, saying, “What may happen if the temperatures rise even more is that some of the trees that were in a healthy situation after the last outbreak will become stressed because they will then be driven to a further need for moisture.”
Some of DeGomez’s published work in this field includes: US Forest Service annual aerial survey; U.S.D.A. Forest Service, 2003. Forest insect and disease conditions in the southwestern region, 2002. Southwestern Region Forestry and Forest Health Pub. No. R3-03-01.
The research was funded by the U.S. Forest Service.
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