Rocky Mountains, Colorado, USA
Dead red trees cover the mountainsides in Rocky Mountain National Park. This is just one location in the Rockies where mountain pine beetles have killed millions of trees. The destructive beetles have multiplied quickly throughout the mountain range, largely because of warmer temperatures.1
The mountain pine beetle's ability to survive and multiply rapidly is highly sensitive to temperature2,3 and precipitation.4 Warmer average temperatures allow pine beetles to complete their life cycle in just one year instead of two.3,5 Rising minimum temperatures in the Colorado Rockies have allowed more beetles to survive the winter.2,6
When conditions are right, mountain pine beetles can multiply rapidly, devastating large areas of forest. The beetles bore through the bark of pine trees, killing them. Their preferred host is mature lodgepole pines, but they can also attack younger lodgepole pines, ponderosa pines, jack pines, and western white pines.10
The extensive mountain pine beetle epidemic in Colorado has two basic causes. First, the climate in the Rocky Mountains has become warmer, on average, with temperatures more conducive to pine beetle survival.11 Second, a large number of lodgepole pines across the region were susceptible to the beetle, because many were of a similar mature age that the pest prefers—largely because of past fire suppression practices.5
The ability of the mountain pine beetle to survive and thrive is highly sensitive to temperature and precipitation. Outbreaks have been correlated with warmer winter temperatures, which allow more beetles to survive.2 In one study of higher elevations, biologists found that warmer temperatures allow the pine beetles to complete a full generation in just one year instead of two, allowing them to multiply faster.3 And recent periods of drought have stressed trees, making them more susceptible to attack.4
Stations set up throughout the Rockies to manage water resources measure temperature and snow depth. From 1986 to 2007, temperatures at these stations rose by an average of 2.5° F (1.4° C).12 The strongest trend toward higher temperatures occurred from November to January, when the average increase ranged from 3.6 to 5.4° F (2.0 to 3.0° C).12 Warmer temperatures like these could allow more mountain pine beetles to survive the winter.
From March to May, the average increase in temperature ranged from 2.2 to 3.2° F (1.2 to 1.8° C) from 1986 to 2007.12 The amount of warm temperatures in spring and summer determines whether mountain pine beetles complete their life cycle in one year or two.5 And if they have to survive only one winter instead of two, they are less likely to experience a killing cold snap.13
Rocky Mountain National Park—one of the 10 most-visited national parks in the United States8—is among the areas in Colorado most ravaged by the mountain pine beetle.7 The park, about 70 miles northwest of Denver, is home to the headwaters of the Colorado River.8 Since it was established in 1915, the park has never faced an outbreak of mountain pine beetle as large as the one starting around 2002.14,7
There is no effective means of controlling a large beetle outbreak in such a vast area as a national park. Annual spraying can save a few trees, but campgrounds and trails must be closed to allow workers to deal with dead trees.14 At Rocky Mountain National Park, workers had to remove all the trees at some campgrounds because they had died from mountain pine beetle attacks, leaving barren campsites.15
One of Colorado's largest concerns is dealing with the aftermath of the devastation. Dead trees present a serious threat as they begin to fall. Park and other employees are particularly concerned with clearing dead trees near communities and developed recreation areas.7 The state also needs to protect critical infrastructure such as roads, watersheds, and power lines.7
Some of the most widely visited ski resorts in Colorado have also suffered extensive beetle damage, including Vail, Breckenridge, Steamboat Springs, and Winter Park.16,17,7,18 Beetle infestations are also expanding in the Aspen and Snowmass resort areas.7
Colorado's major infestations of mountain pine beetles in 2010 occurred along the Front Range, east of the Continental Divide. These beetles are attacking lodgepole, limber, and ponderosa pine from the Wyoming border down to Clear Creek County, just west of Denver.7 An aerial survey showed that pine beetle activity in the lower–elevation stands of ponderosa pine on the Front Range had expanded more than 10–fold to over 200,000 acres (80,000 hectares), in 2010 compared to 2009.7 This area, at the foot of the Rockies, is closer to towns and cities than Rocky Mountain National Park.
In areas in Colorado west of the Continental Divide, the mountain pine beetle population was in decline in 2010, because the beetle had killed off most of its preferred host—the lodgepole pine.7
Part of a Larger Pattern
The mountain pine beetle has wreaked devastation on a scale beyond that of most other forest pests. Beetles attacked some 11.8 million acres (4.8 million hectares) of forests in the United States in 2009, and some 75 percent of those losses stemmed from the mountain pine beetle.19
Along with Colorado infestations, epidemic outbreaks of the mountain pine beetle have occurred in Wyoming, Montana, and Idaho. And outbreaks are growing in California, Oregon, Washington, and South Dakota, and just recently appeared in Nebraska.19
A warming trend in the mountainous interior and northern regions of British Columbia—much steeper than the global trend, particularly in winter20,21—has set the stage for rapid mountain pine beetle expansion there as well. The beetle has destroyed the lodgepole pine forests of central British Columbia, and the insect is continuing to expand into northern British Columbia and across the Continental Divide into Alberta.10,22,23
In fact, mountain pine beetles have the potential to spread across the continent, through jack pine forests to the East Coast of the United States and Canada.23,24 The destruction in North America occurs in the wake of a typical pattern of climate change: faster warming over continents, at higher latitudes, and in winter.
What the Future Holds
A healthy forest normally stores significant amounts of carbon. By killing off millions of acres of trees, which then decay or provide fuel for wildfires, the mountain pine beetle has instead turned lodgepole pine forests into a source of carbon emitted into the already overloaded atmosphere.9
The pine beetle epidemic shows how warming temperatures can lead to a problem that, in turn, causes temperatures to rise even further. Such a feedback loop serves to amplify global warming. The added carbon in the atmosphere warms mountain temperatures even more, allowing the beetles to continue to multiply and destroy more trees, which give off more carbon. The feedback loop continues until the preferred host trees are gone.
The choices we make today can help determine what our climate will be like tomorrow. Significantly reducing our heat–trapping emissions—by developing cleaner energy technologies such as solar and wind power—would help minimize the rise in temperatures and the resulting severe impact on our forests and parks.