Results of a new study show that episodes of reduced precipitation in the Southern Rocky Mountains, especially during the 2001-2002 drought, greatly accelerated a rise in numbers of mountain pine beetles. The overabundance is a threat to regional forests. The research is the first to chart the evolution of the current pine beetle epidemic in the southern Rocky Mountains.

It compared patterns of beetle outbreaks in the two primary host species, the ponderosa pine and lodgepole pine, said University of Colorado at Boulder (CU-Boulder) researcher Teresa Chapman.

A paper on the subject is published in the current issue of the journal Ecology. Chapman is lead author of the paper; co-authors include CU-Boulder scientists Thomas Veblen and Tania Schoennagel.

The National Science Foundation (NSF) funded the research.

"This study confirms that warming temperatures and drought are likely triggers of the widespread bark beetle outbreaks that have devastated forests over vast areas of the West," said Richard Inouye, program director in NSF's Division of Environmental Biology.

"It also suggests why bark beetle outbreaks may vary for two different tree species," he said, "and how different forests may be more or less susceptible to these insects that are transforming mountain landscapes."

The current mountain pine beetle outbreak in the Southern Rockies--which ranges from southern Wyoming through Colorado and into northern New Mexico--is estimated to have affected almost 3,000 square miles of forests.

While the 2001-2002 drought in the West played a key role in pushing the pine beetle outbreak into a true regional epidemic, the outbreak continued to gain ground even after temperature and precipitation levels returned to levels nearer the long-term averages.

The beetles decimated lodgepole pine forests by moving into wetter and higher elevations and into less susceptible tree stands--those with smaller-diameter lodgepoles that share space with other tree species.

"In recent years some researchers have thought the pine beetle outbreak in the Southern Rocky Mountains might have started in one place and spread from there," said Chapman.

"What we found was that the mountain pine beetle outbreak originated in many locations. The idea that the outbreak spread from multiple places, then coalesced and continued spreading, really highlights the importance of the broad-scale drivers of the pine beetle epidemic, like climate and drought."

Mountain pine beetles range from Canada to Mexico and are found at elevations from sea level to 11,000 feet. These native insects have shaped the forests of North America for thousands of years by attacking old or weakened trees, resulting in younger forests.

The effects of pine beetle overpopulation are especially evident in recent years on Colorado's Western Slope, including Rocky Mountain National Park, with a particularly severe epidemic occurring in Grand and Routt counties.

The most recent mountain pine beetle outbreak began in the 1990s, primarily in scattered groups of lodgepole pines at low elevations in areas of lower annual precipitation.

Following the 2001-2002 drought, the outbreak was "uncoupled" from the initial weather and landscape conditions, triggering a rise in beetle populations on the Western Slope and propelling the insects over the Continental Divide into the Northern Front Range to infect ponderosa pine, Chapman said.

The current pine beetle epidemic in the Southern Rocky Mountains also was influenced by extensive forest fires that ravaged Colorado's Western Slope from roughly 1850 to 1890.

Lodgepole pine stands completely burned off by the fires were followed by huge swaths of seedling lodgepoles that eventually grew side-by-side into dense mature stands, making them easier targets for the pine beetles.

"The widespread burning associated with dry years in the 19th century set the stage by creating vast areas of trees in the size classes most susceptible to beetle attack," said Chapman.

Veblen said a 1980s outbreak of the pine beetle in Colorado's Grand County ended when extremely low minimum temperatures were reached in the winters of 1983 and 1984, killing the beetle larvae.

But during the current outbreak, minimum temperatures during all seasons have been persistently high since 1996, well above the levels of extreme cold shown to kill beetle larvae in laboratory experiments.

"This implies that under continued warming trends, future outbreaks will not be terminated until they exhaust their food supply--the pine tree hosts," said Veblen.

Chapman said there has been a massive and unprecedented beetle epidemic in British Columbia, which also began in the early 1990s and now has affected nearly 70,000 square miles.

"It is hard to tell if this current beetle epidemic in the Southern Rockies is unprecedented," she said. "While warm periods in the 16th century may have triggered a large beetle epidemic, any evidence would have been wiped out by the massive fires in the latter 19th century."

The rate of spread of the mountain pine beetle in lodgepole pine forests has declined in the southern Rocky Mountains during the past two years because of a depletion of host pine population.

But surveys indicate that the rate of beetle spread in ponderosa pine forests on the Front Range has increased sharply over the past three years.

The current study suggests that under a continued warmer climate, the spread of the beetle in ponderosa pines is likely to grow until that food source also is depleted.

"Our results emphasize the importance of considering different patterns in the population dynamics of mountain pine beetles for different host species, even under similar regional-scale weather variations," said Chapman.

"Given the current outbreak of mountain pine beetles on the Front Range, the effect on ponderosa pines is certainly something that needs further study."