Image: Pine tree destroyed by beetles
David Zalubowski  /  AP
The bough of a pine tree destroyed by pine beetles is shown amid the fall colors of trees near Keystone, Colo., Oct. 1, 2008. There are about 80 mills in North America that produce at least 1 million tons of wood pellets per year. Pellets are made of recycled wood waste such as sawdust or beetle-killed trees.
updated 7/6/2011 7:30:27 PM ET 2011-07-06T23:30:27

Marauding insects have become a leading threat to the nation's forests over the past decade, a problem made worse by drought and a warming climate, a federal report says.

Bark beetles, engraver beetles and gypsy moths are the primary culprits behind a threefold increase in forestland mortality caused by insect attacks between 2003 and 2007, according to a U.S. Forest Service report obtained Wednesday by The Associated Press.

The volume of forests in the lower 48 states killed by bugs totaled 37 million acres during the period, up from 12 million during the previous five years. Millions of additional acres have perished since.

When defoliated trees are added to those killed outright, the acreage significantly damaged by insects since 2003 totals about 50 million — 8 percent of forest area in the lower 48 states, the report says. The victims range from Rocky Mountain pine forests hammered by bark beetles to ash stands in Northeastern and Upper Midwestern states, where authorities have struggled to contain an emerald ash borer invasion.

By comparison, about 13 million acres were scorched by fires during the same period, less than 2 percent of all forest acreage.

Other dangers include overdevelopment, pollution and storm damage. Still, the nation's 751 million acres of forestland have remained "remarkably stable" over the past half-century, says the federal agency's first comprehensive report card on their health in eight years.

"Forests are incredibly resilient, and from an ecological point of view this is how they regenerate," said Rob Mangold, the agency's director of forest health protection. "They go through this cycle of death to produce a new forest. You have to look at things in the long term, although locally there are some really big impacts."

Drought early in the reporting period weakened many Western trees, making them more vulnerable to beetle attack, said Richard Guldin of the Forest Service's research and development arm. Milder winters have boosted the beetles' survival rates and numbers.

"We get enormous numbers of beetles that overwhelm even healthy trees," Mangold said.

Aerial surveys indicate a drop-off in beetle depredation in some parts of the West over the past couple of years, he said. While it's too early to say whether that represents a trend, previous infestations have risen and fallen over time.

The Southern pine beetle killed nearly 28 million loblolly pines in the mid-1980s, but the total dropped to a few million two decades later as the number of host trees declined. Spruce budworm epidemics have broken out in 30- to 50-year cycles, as sections of forest died and regrew.

While little can be done to prevent periodic attacks by native insects, officials say people can help prevent the spread of foreign invaders such as the emerald ash borer and Asian longhorned beetle by refraining from moving firewood. The ash borer has killed tens of millions of trees in the Northeast and Upper Midwest.

"It's a terribly difficult insect and there's still a big effort to figure out the best way to contain it, but we know that firewood is the primary means of spreading that pest," said Mike Philip, pest survey program manager with the Michigan Department of Agriculture.

Fires increased in number and intensity during the period covered by the report. They burned nearly 40 million acres of all land types between 2003 and 2007, up from 25 million during the previous five years. Of the acreage burned from 2003-07, about 13 million were forests.

About 1.7 million acres died from drought and other weather-related events, up from 788,000 acres during the preceding five years.

Although overall acreage has changed little over the past 50 years, the nation's forests also are being weakened by fragmentation from urban and recreational development, which is increasing by about 1.6 percent annually, the report said. Many paper and wood products companies have sold large tracts previously managed for logging and conservation.

"Our nation's trees and forests are the very lifeblood for the clean air and water we all take for granted in this country," Forest Service chief Tom Tidwell said.

Copyright 2011 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

Photos: Beetles kill in Canada’s warming forests

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  1. Canada's pine beetles

    Ahead of the global climate talks in December 2009, nine photographers from the photo agency NOOR photographed climate stories from around the world. Their goal: to document some of the causes and consequences, from deforestation to changing sea levels, as well as the people whose lives and jobs are part of the carbon culture.

    Two beneficiaries of a warmer climate, beetles and wildfires, are consuming northern forests in Canada and the U.S. This forest near Kelly Lake in the Cariboo District of British Columbia, Canada, shows the twin plagues: Beetle-infested trees surround healthy green trees, while the patch itself is flanked by trees killed by a recent wildfire. (Nina Berman / Consequences by NOOR) Back to slideshow navigation
  2. Ponderosa pine trees, which can live to be hundreds of years old, are among the victims of beetles and wildfire. Moreover, as the land grows drier, trees grow weaker, and pests, abetted by milder winters, grow stronger. (Nina Berman / Consequences by NOOR) Back to slideshow navigation
  3. The owners of a ranch near Kamloops, British Columbia, had to cut down this ponderosa killed by pine beetles.

    By 2020, beetles will have done so much damage that British Columbia's forest is expected to release more carbon dioxide than it absorbs, according to the Canadian Forest Service. In two decades, that could mean nearly 1 billion tons more carbon dioxide in the atmosphere — about the same as five years of emissions from Canada’s transportation sector.

    While gaps in climate science exist, leading some to question the degree of mankind’s impact as well as whether anything should be done, most governments as well as the science academies of the U.S. and other industrial nations agree that mankind is a significant factor and that greenhouse gas emissions should be reduced. (Nina Berman/noor / Consequences by NOOR) Back to slideshow navigation
  4. Pine beetles drill holes into trees, eating away and making nesting areas. Eventually that cuts off water and nutrients, essentially starving trees to death.

    For more information on this project go to Consequences by NOOR (Nina Berman / Consequences by NOOR) Back to slideshow navigation
  5. About the size of a grain of rice, pine beetles are small in stature but many in number — and thus able to devastate miles of forest.

    Freezing weather can kill off larvae, but low temperatures are happening less often. Moreover, warmer summers enable some beetles to complete their reproductive cycle in one year instead of two, speeding up population growth.

    In the United States, Forest Service officials believe most lodgepole pines — the predominant pine tree at higher elevations — will be killed by 2013, and it could take decades before those pines return. (Nina Berman / Consequences by NOOR) Back to slideshow navigation
  6. This popular lookout over Kamloops Lake shows miles of trees dead from beetle kill and drought.

    The Western U.S. holds similar views. In Colorado, a dozen towns are surrounded by dead forests and another dozen are on the outskirts. The state's affected region is also home to ski resorts such as Aspen, Vail, Breckenridge and Winter Park. (Nina Berman / Consequences by NOOR) Back to slideshow navigation
  7. In British Columbia, beetles have laid waste to 35 million acres — an area the size of New York state — and are expected to kill 80 percent of its lodgepole pines.

    "To the untrained eye, the (beetle) attack appears beautiful at first," says photographer Nina Berman. "Swaths of green trees turn red, like autumn leaves changing. But these pines are evergreens, and a color shift is a sign of inevitable mortality. From red, the leaves turn purple, brown and finally gray. At this point, they can no longer stand and wither to the ground, their pine cones dried out and scattered across the forest floor, their branches ready fuel for fires." (Nina Berman / Consequences by NOOR) Back to slideshow navigation
  8. Dead pine stands like these in British Columbia are fuel for wildfires. In 2009, British Columbia saw more wildfires over more acres than in any year in its recorded history.

    In the United States, an unprecedented years-long epidemic of mountain pine beetles has killed 6.5 million acres of forest from Colorado to Washington state. That's an area about the size of Massachusetts.

    Share your thoughts about these slideshows and climate change. (Nina Berman / Consequences by NOOR) Back to slideshow navigation
  9. A wildfire near Sorrento, British Columbia, becomes evening entertainment.

    The U.N. panel of experts convened to assess climate impacts has stated with "very high confidence" that "disturbances such as wildfire and insect outbreaks are increasing and are likely to intensify in a warmer future with drier soils and longer growing seasons."

    For more information on this project go to Consequences by NOOR (Nina Berman / Consequences by NOOR) Back to slideshow navigation
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