President Bush's proposal to thin the nation's forests to prevent forest fires won cheers from fellow Republicans in timber country.

But the high costs of thinning forests and the strong political opposition to both cutting old growth trees and suspending environmental laws could prove formidable obstacles.

The president's forestry plan, and other issues, drew hundreds of protesters when he spoke in Portland last week. And the chairman of the Senate subcommittee on forestry says the plan will face a fight in Congress if it goes beyond the goal of reducing fire danger and tries to overturn environmental laws that let the public challenge federal timber sales.

"Let's keep the focus on fire prevention,'' said Sen. Ron Wyden, D-Ore. "There is a history of seeing efforts tank quickly when someone overreaches.''

The cost of thinning forests can be staggering — $2.7 billion for the 1.6 million acres of forest just in the rugged Klamath Mountains region of southwestern Oregon, according to research by the U.S. Forest Service's Pacific Northwest Research Station.

"Economics is where the problems develop,'' said Jeremy Fried, team leader on the study, which has yet to be published.

Bigger trees bring in more revenue to help pay for the work.

But "if you just take big trees, you don't reduce fire danger. If you take small trees, it costs you an arm and a leg,'' Fried said.

In his computer model, Fried found when he limited thinning to trees less than 21 inches in diameter, the average cost was $1,685 per acre. In contrast, it cost $785 per acre for firefighters to control a 2,800-acre fire in southwestern Oregon that Bush toured on Thursday.

Proposals to cut bigger, more mature trees, which may shelter wildlife, also are more likely to attract protests.

Timber proponents have been frustrated for 12 years by a drastic cutback in national forest logging, caused by court orders that the U.S. Forest Service follow National Forest Management Act requirements to protect fish and wildlife habitat.

Meanwhile, scientists recognized that a century of indiscriminately putting out wildfires in forests that evolved with flames over millions of years has left a landscape loaded with fuel just waiting for hot, dry, windy weather to explode.

Even with widespread thinning, "We are never going to stop fires,'' said Mark Finney, research scientist at the Forest Service Fire Sciences Laboratory in Missoula, Mont. "That should not be the intent or the expectation.

"The question is what do we want to do to our fires. Do we want them to be less damaging, smaller, etc. That is what fuel management can do.''

Finney has found that random thinning does nothing to stop or slow fires. Fires just race around thinned areas.

But thinning strategically — in a herringbone pattern designed to make fire move perpendicularly to the prevailing wind — can have a huge effect, even when done on only 20 percent of the landscape, Finney found.

Finney's strategic thinning principals have been incorporated in the Sierra Nevada Framework, a management plan for 10 national forests in California, which targets 30 percent of the landscape for thinning to reduce fire danger.

Two years after it was adopted, the Sierra Nevada Framework has yet to be implemented. The Forest Service, which wants to offset thinning costs with timber harvest, is still reviewing objections raised by environmentalists, ranchers and timber companies.

Environmentalists accept the idea of thinning, said Wendell Wood of the Oregon Natural Resources Council. They just want to make sure it is done where it does the most good and does not become an excuse to cut old growth timber.

The timber industry accepts the idea that reducing fire danger will cost money and not always produce timber for mills, said Michael Klein, spokesman for the American Forest & Paper Association.

"This isn't about economics, it's about forest health,'' he said. "You have to value it against what would be lost if you do nothing.''