The North American boreal forest, which stretches from the Great Lakes and Colorado Rockies north to the treeline of Arctic Canada, is the third-largest woodland on earth, behind only the Amazon rainforest and the Siberian taiga. It covers a quarter of North America's landmass, and comprises 24 percent of the world's intact forest. In a word, it's huge. So it's a supreme irony that it may fall to a foe no larger than a grain of rice: the mountain pine beetle.
Adult female beetles, which typically attack the lodgepole, ponderosa, or whitebark pine, bore into living sapwood to dig tunnels and lay eggs. The eggs hatch into legless larvae that feed outward, leaving a blue fungus that blocks water and nutrient flow, usually killing the tree within weeks.
These opportunistic pests have been around for eons, but sustained subzero winter lows or spring or fall cold snaps historically killed off most wintering beetles and larvae, protecting northern and high-altitude conifer forests. According to the Canadian Forest Service and numerous scientific studies, warmer winters and longer summers throughout subarctic North America and the Rockies have allowed massive infestations at altitudes and latitudes previously beyond the insects' reach. In some regions–Alaska, for example–warmer, longer summers have allowed the mountain pine beetle and the spruce bark beetle, a related species, to shift from a two-year to a one-year life cycle. The result has been catastrophic: The bugs have devastated nearly 3 million acres on the Kenai Peninsula alone.
In British Columbia, where average winter temperatures have risen 4°F in the last century, researchers are tracking the largest beetle infestation in recorded history. Nearly 23 million acres are already toast, and experts expect the province to lose 80 percent of its mature lodgepole stands by 2015.
Colorado's pine beetle epidemic exploded in 2006, killing 1,000 square miles of trees, most of them in the state's north-central sector, where annual winter lows have risen 2°F in the last 50 years. Dead, brown pines now blanket the slopes near Vail, Winter Park, and Keystone. Spruce bark beetles are also attacking the Flat Tops Wilderness, Colorado's second largest; much of the Routt National Forest surrounding Steamboat Springs; Utah's Dixie, Manti–La Sal, and Fishlake National Forests; Wyoming's Bridger-Teton National Forest; and huge swaths of Montana's Sawtooth Basin and Gallatin Range. "We're going to see a virtually complete loss of mature lodgepole pines in the state of Colorado by mid-century," says Jan Burke, silviculturist for the White River National Forest. "There's going to be a very different look to our forests, and this is not a hollow prediction. It's already happening right now."
It's not just the trees
While their coastal cousins are known for feeding on the salmon and trout they so expertly snatch from mountain streams, grizzly bears in places like Yellowstone and Glacier mainly rely on the tiny but nutrient-dense seeds of the whitebark pine. The seeds provide much-needed fat reserves for winter hibernation, but the supply dries up when whitebark pine groves die from beetle kill. The result: Seed-deprived grizzlies spend more time near humans, as they scavenge for replacement foods. During years when pine nuts are scarce–which is now occurring more often–rangers trap nearly twice as many problem bears.
One backcountry-loving scientist's wild theories have become a frightening reality. Jesse Logan may be a scientist, but he's no egghead. A lifelong Rockies resident, Logan is an avid fly-fisherman and expert backcountry skier, and he's spent an enviable number of days exploring off-trail. And like anyone who has logged serious wilderness time in the northern Rockies, he has sought shelter from the elements among the whitebark pines that grow just below treeline.
So it's fitting that Logan, who began his career as an agricultural entomologist, ultimately focused on the mountain pine beetles that led him straight back to those same trees.
Known in his field as the beetle guy, Logan spent most of the 1990s studying how these swarmers wreaked havoc on lower-elevation lodgepole pines. At the USDA Forest Service's Rocky Mountain Research Station in Logan, UT, where he dedicated 15 years to beetles, he noticed that the bugs were highly sensitive to temperature.
As more evidence of global warming emerged in the mid-'90s, Logan began to get a sinking feeling in the pit of his stomach. With difficulty, he wrangled funding from higher-ups in DC to plug some gently warming temperatures into a mathematical model. "It was really theoretical," he says. "Just an exercise." But what he found scared him. With just a slightly longer summer season, his model predicted, mountain pine beetles could complete their life cycle in one season instead of two, emerging in greater numbers to overwhelm trees and elevations that were once mostly beyond their reach.
Sadly, these theories have become hard fact. Logan caught his first glimpse of the evidence in 2003 at the rooftop of the Sawtooths outside Stanley, ID: The higher-elevation whitebark pines were under attack. By the next year, the needles would turn red and drop, leaving a wake of dry, weathered, birdless snags. Even as a scientist who had witnessed numerous beetle kills, Logan was devastated. "It's absolutely heartbreaking," he says. "I can't find other words."
His findings drove the Forest Service toward more long-term ecological research, but Logan left the agency last year–in part, he says, because of its lack of enthusiasm for such research. Now mostly retired at 63, he tries to enjoy the wilderness as much as he can. The beetle recently elbowed into one of his favorite ski slopes on the northeast aspect of Emigrant Peak in southern Montana, and Logan fears that whitebark pine forests could become functionally extinct in as soon as a generation. "It's now one of the most intact ecosystems in the world," he says. "But my grandchildren may never see it."
Forests and wildfires form an unlikely ecological yin and yang: In a healthy climate, the apparently destructive blazes actually keep an entire ecosystem in balance. Fire accelerates the decomposition of organic material, returning nutrients to the soil; it also helps evergreens' cones to open and lodgepole and jack pine seeds to germinate. But if flames come too often, or burn too intensely, they can sterilize the landscape, harming wildlife and opening the door to invasive species. Unfortunately, numerous studies have documented that global warming creates conditions that invite larger and more frequent conflagrations–which lead in turn to higher carbon emissions. Here's a look at the vicious cycle.
 Mercury rises
Average spring and summer temperatures throughout the West have increased 1.6°F in just the last two decades. As a result, mountain snowpacks, which keep the soil moist and serve as the main water source for Western forests and rivers, are melting one to four weeks earlier.
 Forests dry out
Within a month of snowmelt, trees weaken and vegetation dries, making forests more vulnerable to parasites and fires. Dry seasons now last 78 days longer, on average, than they did 20 years ago.
 Beetles thrive
The combination of warmer year-round temperatures and longer summers has spurred the proliferation of forest-slaying mountain pine beetles by speeding up the bugs' maturation and metabolism. "What's normally a two-year life cycle has shortened to one," says Don McKenzie, research ecologist with the Pacific Wildland Fire Sciences Lab in Seattle and lead author of a 2004 study examining the link between climate change and wildfires. The beetles lay their eggs in pines, eventually killing the trees and adding more fuel to the ecological tinderbox. Spruce beetles have killed more than 90 percent of spruce trees on Alaska's Kenai Peninsula.
 Fire becomes more devastating Parched vegetation, beetle-killed trees, and longer, warmer summers are provoking more fires, larger fires, and longer-lasting fires. A 2006 Scripps Institution of Oceanography study reports that four times as many fires burned 6 1/2 times more acreage between 1987 and 2003 than in the previous 16 years. The reason? Higher spring and summer temperatures.
 Nonnative plants invade
This increase in fire damage clears the way for invasive flora such as cheatgrass to enter areas like the Front Range and Great Basin. "Cheatgrass intensifies fires, because there used to be patches of nonvegetated areas on the plains that didn't burn," McKenzie says. "Now that the grass is there, the fire can spread more." Impact: the largest fire in Colorado history, the 2002 Hayman Fire, which burned 137,760 acres and cost $39 million in damages and manpower.
 Gases increase
Forests act as a carbon sink, extracting CO2 from the atmosphere during photosynthesis. Western forests alone account for 20 to 40 percent of carbon sequestration in the United States. But the combustion of trees and vegetation by fire releases enormous amounts of carbon dioxide–more than 3.8 billion tons last year (or nearly four times the 1980 level, according to one estimate). "Any time you release CO2, you're accelerating the cycle of global warming and forest fires," says McKenzie.
Since the 1970s, the average duration of wildfires has increased from 7.5 days to 37.1 days. Average summer temperatures in the western United States are expected to increase up to 9°F by 2069. The last two decades have been the warmest years in the West since recordkeeping began in 1895.