WASHINGTON - In the name of science, the United States Forest Service has proposed the experimental logging of half a million acres in two forests in the Sierra Nevada to see how it will affect the habitat of the California spotted owl and the ferocity of forest fires. But skeptical environmentalists are saying the real purpose is simply to give timber companies a chance to cut more big trees on some of the nation's 190 million acres of public land.

The study is to be conducted in the Plumas and Lassen National Forests, two of the 11 national forests that run along the mountainous spine of California.

The Bush administration's experiment is designed on such a grand scale that it will vastly increase the amount of timber being taken from the two northern California forests, which have been heavily logged in the past. Some trees to be cut are much larger than current forest regulations would allow: in some cases, up to 34 inches in diameter, or almost nine feet in girth.

After a year in which forest fires raged through the West, affecting seven million acres, the administration has been pushing plans to thin the trees in places where years of mismanagement - including the practice of putting out every single fire - have left dense thickets of undergrowth. Often these projects are aimed at protecting small communities at the forest edge. But rarely do they involve cutting so many trees, or such big ones, especially in sensitive wildlife habitat deeper in the woods.

In its announcement of the project, the Forest Service referred to the logging euphemistically as "management-caused changes in vegetation," and said the study would test whether the benefits of the cleared areas, which would create firebreaks, exceed the ecological damage, especially to the spotted owl habitat. Like the more famous northern spotted owl of the Pacific Northwest, the California species is struggling for survival.

Environmental advocates who have long fought logging in the region, and some scientists, see this proposal as science on the model of Japanese whalers, who take their harpoons to sea in what they call a research project - one that happens to put whale meat on the menus of pricey restaurants in Tokyo.

"This comes to almost 30,000 acres per year of suitable owl habitat that would be logged," said Chad Hanson, an anti-logging advocate at the John Muir Project and a Sierra Club board member.

The conservation groups say the plan is an attempt to reverse existing rules, including those adopted during the Clinton administration, that put much of the forest off limits.

As evidence they pointed to the administration's announcement last week of changes in rules governing logging - changes that the government said were aimed at limiting forest fires. The administration's goal was to cut through environmental reviews, court appeals and litigation that slow approval of the projects.

Mark Rey, the assistant secretary of agriculture who oversees the Forest Service, said adversaries in the debates should learn to trust each other and the government. "I certainly trust the environmental groups," said Mr. Rey, who was formerly a lobbyist for a forest industry group. "They've spent millions of dollars on political ads to demonize the administration, but that doesn't mean I don't trust them." He spoke with tongue firmly in cheek, knowing that environmental groups are certain to challenge the administration's proposals in court.

In fact, there was a big ruling last week on a related issue, when a federal appeals court in San Francisco decided to reinstate a ban on building roads in 60 million acres of national forest. The policy, put in place under President Clinton and challenged by the industry and some local governments, is one that the Bush administration wants to change.

Road construction is one problem that environmentalists see in the California experiment. Another is the reduction in canopy cover in some California forests to 40 to 50 percent, compared with 60 to even 90 percent before logging - a result that is prohibited under the current forest plan because of the likely harm to owls. Limits on cutting large trees, on building roads and on thinning the canopy were put into the regulations for the region after intensive scientific study. The Forest Service, however, said it would amend those rules, calling the changes insignificant.

ENVIRONMENTALISTS are sure to object during the 45 days of public comment that began last week. "I think this is quickly going to spiral into a device for getting around other restrictions on forest practices, under the guise of scientific analysis," said Don Erman, emeritus professor of forestry at the University of California at Davis. While the scientific question of how different methods of logging affect the survival of animals and the health of the forest is perfectly valid, he said, it remains to be seen whether the experiment needs to be so huge, or indeed whether its design is appropriate. These are matters that scientists, not timber lobbyists or environmentalists, should decide, he said, adding, "I don't think science works very well from ideology."

Copyright 2002 The New York Times Company