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.