Disrupted Forests

Western ecosystems already undergoing widespread disruption are our forests. Across western mountains, hotter temperatures have been promoting the spread of tree-killing bark beetles, which now are spreading to higher elevations than before and reproducing more quickly. In areas that used to be so cold that it took three years for one generation to produce another, it now takes just one year. As a result, the West is experiencing an uprecedented infestation of bark beetles. The most widespread beetle infestations are in Colorado, where the U.S. Forest Service says, bark beetles "will likely kill the majority of Colorado's large diameter lodgepole pine forests within the next 3 to 5 years."

Climate disruption is affecting western forests in other ways. Whitebark pines, which has unique ecosystem values in the West's highest forests, have recently been designated as a threatened species in large part because of the spread of mountain pine beetles into their high-elevation habitat, as well as other climate-change impacts. Something scientists have dubbed "sudden aspen decline" is reducing aspen stands across the West, triggered by the hotter and drier conditions that is the manifestation of climate change here. The areas of decline are the same areas where scientists project that hotter and drier conditions will lead to permanent decline. Other scientists have discovered increased death rates among trees of all types and ages across the West, not caused by insects or other disruptions, but instead apparently linked to higher temperatures.

Shown abover are dead and dying whitebark pines, indicated by reddish-brown color, in the Greater Yellowstone Ecosystem. Photograph copyright Wally Macfarlane, used by permission.

“[C]ool temperatures are believed to be the major restriction on
mountain pine beetle outbreaks at high elevations . . . Generally,
warmer temperatures promote bark beetle outbreaks both through
their favorable influence on the life cycle of the insect and through
drought-related declines in the trees’ abilities to withstand attack.”

U.S. Climate Change Science Program
Rocky Mountain/Great Basin Regional
Climate-Change Assessment (2003)