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.