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Since 1997, a host of native beetle species has chewed through more than 40 million acres of Western forests, according to aerial surveys by the U.S. Forest Service. That's as much space as all of America devotes to its lawns. And the beetles show no signs of crashing.
They kill by burrowing into the fleshy layer of nutrient-conducting phloem - just under the bark - which their larvae eat and effectively girdle.
Various bugs have swallowed 2 million acres of Utah forests, a patchwork about as large as Yellowstone National Park.
In Montana, with more than 6 million acres of beetle carnage, second only to Colorado, the frost-free season has extended by two weeks in the past half-century. That's pleasantly milder for people who once routinely bundled themselves against 40-below zero winters, but it's torturous for pine trees.
"In the arid West, you only have a growing season if it has enough water, just like your garden," said Steve Running, ecology professor and University of Montana Climate Change Studies Program director.
But the region hasn't gained water, and most climate models predict it will lose moisture to evaporation even when more falls as snow or rain.
"It means a longer stress period for the trees," said Running, whose research on global forests helped the U.N. Intergovernmental Panel on Climate Change share the 2007 Nobel Peace Prize with former Vice President Al Gore. "Stress on the tree host is to the advantage of the invading insect."
So trees around the Rockies will have to grow with more spacing, Running said, as a response to changes in precipitation and evaporation. That could mean the forests look more like they did a century ago, before fire suppression allowed them to close in.
But some species likely will struggle to hold on. It could be, Running said, that the forests will need help adapting, and foresters will plant drought-tolerant pines from Arizona and New Mexico into Utah, Idaho or Montana.
Killer beetles • Scientists say warming temperatures have pressed the accelerator on the natural cycle of tree deaths by cranking up the beetle birthrate.
Each year for centuries, mountain pine beetles - the most prolific species of the latest outbreak - have emerged to strike new trees, though in colder places a generation might take two years. Now they are popping up more often; some will live to breed into a second season.
Barbara Bentz, an entomologist with the Logan branch of the U.S. Forest Service's Rocky Mountain Research Station, monitors temperatures around the West. She has found that, at high elevations, where mountain pine beetles previously needed two years to complete a generation, now they work faster in warmer trees and can reproduce in a year.
Another development is adult beetles that breed one summer and then survive a mild winter appear to have eggs left over for a second go. They'll emerge earlier than July, when they usually fly from dead trees in search for their next victims.
Some foresters and entomologists believe two generations can now emerge in a single summer, perhaps indicating there's enough frost-free time for new larvae to mature and target new trees before their winter slumber.
But Bentz is not convinced. She has watched for that, especially at warmer research sites such as around California's Lassen Volcanic National Park.
"I have been going to the warmest places I can find, looking for it," she said, "but I can't find it."
Others say they have. Nancy Bockino, ecologist for Grand Teton National Park, said mountain pine beetles attacking the western Wyoming trees she's trying to protect spawned two generations in 2007. Her mom, along on a working field trip, pointed out that a green tree had both entry and exit beetle holes. Believing that impossible - any tree attacked the previous year would be showing yellow or red needles - the scientist told her mom it couldn't be so.
Then she took a look for herself - and was a believer.
"It was a warm spring," Bockino said. "They were killing [trees] in May." Follow-up visits later in the season found a second wave of afflicted trees.
University of Montana entomologist Diana Six said she, too, has documented a double batch of beetles. It was in the St. Regis Valley, near where Montana meets the Idaho Panhandle, and there were two "perfect" generations. Sometimes an extended warm season will fool new adult beetles into thinking they have time to fly and attack, she said, but usually they don't make it.Next Page >
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