GRANTS PASS, Ore. (AP) -- Big changes are in store for the nation's forests as global warming increases wildfires and insect infestations, and generates more frequent floods and droughts, the U.S. Department of Agriculture warns in a report released Tuesday.

The compilation of more than 1,000 scientific studies is part of the National Climate Assessment and will serve as a roadmap for managing national forests across the country in coming years.

It says the area burned by wildfires is expected to at least double over the next 25 years, and insect infestations often will affect more land per year than fires.

Dave Cleaves, climate adviser to the chief of the U.S. Forest Service, said climate change has become the primary driver for managing national forests, because it poses a major threat to their ability to store carbon and provide clean water and wildlife habitat.

"One of the big findings of this report is we are in the process of managing multiple risks to the forest," Cleaves said during a conference call on the report. "Climate revs up those stressors and couples them. We have to do a much better job of applying climate smartness ... to how we do forestry."

The federal government has spent about $1 billion a year in recent years combating wildfires. Last year was the warmest on record in the lower 48 states and saw 9.2 million acres burned, the third-highest on record, according to the National Oceanic and Atmospheric Administration's website.

Insect infestations widely blamed on warming temperatures have killed tens of millions of acres of trees.

Forest Service scientist James Vose, the report's lead author, said the research team found that past predictions about how forests will react to climate change largely have come true, increasing their confidence in the current report's predictions.

The report said the increasing temperatures will make trees grow faster in wetter areas of the East but slower in drier areas of the West. Trees will move to higher elevations and more northern latitudes, and disappear from areas on the margins of their range.

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Along with more fires and insect infestations, forests will see more flooding, erosion and sediment going into streams, where it chokes fish habitat. More rain than snow will fall in the mountains, shortening ski seasons but lengthening hiking seasons. More droughts will make wildfires, insect infestations, and the spread of invasive species even worse.

The nation's forests currently store 13 percent of the carbon generated by burning fossil fuels every year, and losing trees to fire and insects makes it likely in coming years that forests in the West will start giving off carbon as they decay, the report said. It suggested that burning the trees cut during thinning operations in bioenergy plants to generate electricity would help reduce the carbon emissions from burning fossil fuels.

Beverly Law, professor of global change forest science at Oregon State University, said in an email that her research in Oregon showed that despite more fire, the amount of carbon stored in forests continues to increase.

Tara Hudiburg, a postdoctoral researcher at the University of Illinois, said there is little conclusive evidence that burning trees for bioenergy helps reduce overall carbon emissions.

Andy Stahl of Forest Service Employees for Environmental Ethics, a watchdog group, said the agency traditionally has been guided by political pressures, and he has seen no evidence that concern over climate change is now playing a role.

Cleaves said climate coordinators are stationed at every national forest across the country, every regional headquarters, and at each research station. The threat of future flooding has prompted the Olympic National Forest in Washington state to start upgrading the culverts that carry storm water runoff on logging roads.

The report did not specifically address whether logging would decrease due to more thinning projects generated by global warming concerns. But it did say that privately owned timberlands would be much quicker to react to market pressures related to global warming than the national forests.

Cleaves said thinning projects designed to make forests more resilient to a changing climate were likely to produce less timber and revenue, because they tend to leave big trees standing.

The Forest Service has struggled to pay for thinning projects that don't generate revenue. Oregon Gov. John Kitzhaber has been exploring the idea of tapping state lottery funds to pay the Forest Service to plan timber sales in fire-prone areas.