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How a Computer Modeler Predicted the Mountain Pine Beetle Tree-Killing Rampage

Over the past decade North American forests have been decimated by outbreaks of mountain pine beetles
climate change, global warming, mountain pine beetle, deforestation, trees



Wikimedia Commons/Bchernicoff

Over the past decade, the forests of North America have been gripped by some of the worst mountain pine beetle epidemics in history. Driven by record-high temperatures and frequent drought, beetle kill has expanded more than twentyfold across the American West.

In central British Columbia, the insects have destroyed more than 14 million hectares of trees -- an area the size of Connecticut -- in the single largest outbreak the world has ever seen.

In the early years of the epidemic, public officials seemed almost at a loss. One Wyoming forest service researcher, interviewed by the Casper Star-Tribune in August of 2003, called it a "perfect storm" of events and trends unlike any his field had ever seen.

"What's surprising to me is how bad these infestations are at the same time," he told the Star-Tribune. The alliance of temperature and beetle, it seemed, had taken the world by surprise.

Except that it hadn't. A handful of scientists had known about the beetles' temperature threshold for some time -- had studied it extensively, in fact. After the Intergovernmental Panel on Climate Change (IPCC) released its first report in 1990, these scientists began to openly discuss the possibility that near-term changes in global temperatures could push the mountain pine beetle into new reaches of the continent.

Then, late in the decade, a Forest Service researcher named Jesse Logan pulled the pieces together. What he produced was a work of rare prescience -- a map of the catastrophe to come.

The ghost tree of Railroad Ridge
Somewhere in the Cloud Mountains of central Idaho, high above a sea of spruce and lodgepole pine, a jagged crest of peaks called Railroad Ridge juts up against the sky. Clinging to its rocky soil are some of the oldest trees in the western United States -- slow-growing whitebark pines, the oldest dating back more than a millennium.

At an elevation of 10,000 feet, these trees had long been spared the periodic beetle epidemics that ravaged lower-elevation forests.

When Logan and a handful of other entomologists visited the site in 1993, however, one tree stood out among the others. It had been dead for decades, preserved by the cold, dry air, and curiously, it showed all the signs of beetle kill.

"It was an eye-opener," Logan would later recall.

At first, the tree's presence seemed to fly in the face of much of what was understood of the mountain pine beetle -- called MPB by the experts -- at the time. Scientists were reasonably sure that pine beetles could only reproduce within certain climatic limitations -- and by all accounts, Railroad Ridge was beyond those limits. Yet sometime in the last century the beetle had flourished here, if only for a brief moment.

History offered a critical clue. The trees' dated mortality was sometime in the 1930s -- a decade that was, and remains, the hottest in recorded U.S. history. Could it be, Logan wondered, that a temperature threshold had been breached in that decade, allowing the beetles to erupt, albeit briefly, across Railroad Ridge?

More pressingly, could it be breached again? And when?

Death by gang attacks
The answer, it turns out, lay within the beetle itself -- in its cyclical pattern of birth, life, procreation and death. In particular, it rested on a delicate balance between temperature and the insect's development.

"There are certain temperature cues that prompt [the mountain pine beetle] to start developing at certain stages," said Barbara Bentz, a research entomologist with the U.S. Forest Service who, in the late 1980s, was a doctoral student working with Logan on modeling the beetle's life cycle.

It is critical to the beetle's survival that large numbers of mountain pine beetles emerge simultaneously, said Bentz. Pine trees are hardly passive victims -- as soon as beetles begin to burrow beneath the tree's bark, the unwilling host attacks with secretions of sap, attempting to drown the invader. To succeed, the beetles attack en masse, dehydrating the tree until it loses its ability to defend itself.

When temperatures pass a certain threshold, however, the beetles lose their ability to synchronize and begin emerging sporadically. Unable to coordinate mass attacks, they have greatly reduced chances of success.

These basic processes became the building blocks for Logan's climate models. Because the beetles developed at more or less constant rates relative to temperature, Logan was able to describe the relationship between reproductive success and temperature as a series of mathematical formulas.

Simplified, they might look something like this: At temperature X, an approximate number of beetles (Y) should emerge simultaneously, giving the insects a Z percent chance that a significant number will reproduce successfully.

Having established that equation, predicting the beetle's expansion into any particular region was just a matter of plugging in the IPCC's temperature projections and crunching the numbers.

After running his models, Logan arrived at a grim conclusion: The temperature thresholds for sites like Railroad Ridge, the greater Yellowstone region and much of the American West -- thresholds at which climatically impassable habitats would suddenly become benign -- were well within the short-term projections of the IPCC.

Forecasting a fast-moving plague
In 2001, Logan published his findings, along with co-author James Powell, in the journal American Entomologist. Already, the insect's reach had begun to spread, with the area of forest affected by beetle kill more than doubling between 1999 and 2001. However, few alarm bells had yet begun to sound.

Railroad Ridge remained Logan's most important litmus test. He had set up a number of weather stations on the ridge after his first visit in the 1990s, and these indicated that the area was steadily approaching the "tipping point" established by his models.

Then, on a visit in 2003, he found his predictions borne out. Here and there among the whitebark stands, patches of burnt orange, the color of rust, were appearing. The implications were obvious -- the beetles were back.

"It was the most magnificent whitebark ecosystem I'd seen," Logan would later recall in an interview with The New York Times. "It broke my heart."

It took less than four years for Railroad Ridge's ancient ecosystems to collapse. By 2007, virtually no living whitebarks remained.

Across the continent, the beetles were taking new ground. By 2008, much of the whitebark population of Yellowstone National Park would be similarly afflicted. And the beetles were spreading north, expanding into the boreal forests of Canada and exploding through central British Columbia. The maps and models Logan had created were suddenly being recreated -- this time, as the chronicle of a phenomenon well under way.

A story 'more bitter than sweet'
Logan retired from the Forest Service in 2006, although he has since collaborated on a number of articles with fellow scientists. His work, particularly the 2001 paper, is among the most widely cited in research explaining the momentous devastation wrought by the mountain pine beetle.

Yet the signature achievement of his career brings Logan little satisfaction. An avid skier and fly fisherman, he spends most of his time in the outdoors; during the coldest months of the year, he and his wife winter in a remote cabin in the Beartooth Mountains of southern Montana. Increasingly, the beetle's expansion has left dead and dying forests throughout the region.

"For me, this story is more bitter than sweet," he wrote in a recent exchange.

He is currently part of a group of scientists and citizens pushing the government to confer protected status on the whitebark pine.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500

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