The mountain pine beetle infestation that has ravaged swaths of Western forests is slowing down for the second year in a row, the Forest Service said yesterday.

The massive area of public forestland -- about 750 million acres -- with trees killed by the mountain pine beetle decreased by 3 million acres between 2010 and 2011, according to a report from the Forest Service. Northern Idaho saw a decrease of nearly 80 percent in beetle-damaged lands from last year.

Dwindling reserves of the beetle's choice food, the lodgepole pine, have limited its ability to proliferate, said Robert Mangold, associate deputy chief for research and development for the Forest Service.

"They are running out of susceptible hosts," Mangold said. "We expect the beetle population numbers to drop dramatically over time."

In 2009, acres destroyed by the mountain pine beetle -- mostly in the forests of Wyoming, Montana, Colorado and Idaho -- reached a historic peak of 9 million acres. Since, the numbers have been tumbling. About 6.8 million acres of forests showed pine beetle damage in 2010, and 3.8 million acres was affected in 2011.

A series of interacting factors that include drought, wildfires and tree health play into the cycle of pests in forests, but the interactions can be elusive, Mangold said. Drought can make trees weaker and more susceptible to insect attacks. In turn, trees killed by pests can provide fuel to wildfires, aiding the spread of flames.

Not out of the (fire-prone) woods, yet

Warmer winters encouraged beetle populations in the years before the 2009 peak, said Mangold, who sees the decline as part of a cyclical pattern in forest ecology.

The downward trend in pine beetle infestations is a positive one, said William Anderegg, a researcher at the Carnegie Institution for Science in Stanford, Calif., who studies how and why forests die. However, to assess the effect of climate change, observations must be made over decades, not years.

"Year-to-year variation is often the noise, but it's the long-term trend (which has been increasing substantially in the bark beetle case) that is a sign of climate change," he wrote in an email.

More research has been conducted on lodgepole pine and mountain pine beetles than any scientific investigations on forest die-off, said Anderegg, who recently co-wrote a paper documenting the patterns of dying forests around the world for Nature Climate Change.

Nevertheless, research into the roots of forest mortality has been minimal to date, he said. Anderegg's recent paper called for a coordinated international effort to better understand the interactions that drive dying forests.

"Sadly, our monitoring efforts are so incomplete that it's tough to make these cross-nation and cross-continent comparisons," he said.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500