Agriculture Secretary Bob Bergland today is expected to propose that 15 million acres of national forests from Alaska to Virginia be designated as federally protected wilderness.

The recommendation, which would almost double the size of the nation's wilderness system, follows eight years of controversy over the vast roadless areas within the national forests. At the same time, the secretary is to recommend opening 36 million acres of forests to timbering and commercial development. Eleven million acres would be set aside for further study.

If approved by President Carter and Congress, the proposal would resolve the status of 2,686 areas in 37 states and Puerto Rico that have been off limits to development since 1970.

Both industry and environmental groups predict that the proposal -- known in bureaucratic circles as RARE II, or the second Roadless Area Review and Evaluation -- will be the hottest environmental issue since the controversy over parks in Alaska.

"This affects a lot of people more directly than Alaska," said Tim Mahoney of the Wilderness Society. "It's a lot of acreage in a lot of people's back yards."

The review, covering a third of the Forest Service's domain, has prompted the largest response ever recorded to an environmental impact statement, partly as a result of a highly organized campaign by the timber industry.

A team of more than 100 specialists was assembled in Salt Lake City to computerize the 265,000 letters, petitions, and resolutions that flooded Forest Service headquarters. Of those 178,000 were form letters and petitions, showing overwhelming opposition to wilderness.

Mahoney said timber companies handed out perprinted postcards in paychecks and at least one Idaho millworker was fired for refusing to write an anti-wilderness letter. However, timber companies say environmentalists were also organizing, handing out leaflets at Mount Rushmore.

Designating the land as wilderness means it would be closed to timbering. Hardrock mining, oil and gas exploration and cattle grazing would be sharply restricted. No roads could be built, and no motorized vehicles allowed.

For conservationists, the need is clear: man must have some places where he can be completely alone, away from the noise of motorcycles and power saws. Ecosystems, those intricate webs of animals and plants, must have a chance to survive undisturbed, in this view.

But "many people think wilderness is Rock Creek Park or Disneyland," said Hugh Mullins of the National Forest Products Association. "They don't realize they won't be able to drive into the wilderness. They'll be kept out if they can't walk or canoe in."

Mullins said he is worried that setting aside more wilderness will make it harder for the timber industry to meet a projected doubling of demand by the year 2020. However, administration sources said most of the commercial timberland is excludecd from the wilderness recommendations.

Oil companies, such as Atlantic-Richfield and Amoco, have been pressuring the Forest Service to exclude the overthrust belt, a vast stretch of the Rocky Mountains, from the new wilderness. Administration sources said most of the area will be open to development.

Of the 15 million acres to be recommended for wilderness, more than 9 million are in Alaska, where President Carter recently designated 56 million acres for parks and wildlife refuges. The wilderness classification would be more restrictive, however.

Today's recommendation will also contain the first wilderness proposals for grasslands in the Great Plains. "People have traditionally thought of wilderness as spectacular mountain peaks and rushing streams, but all kinds of ecosystems should be included," said one Agriculture Department official.

Proposed wildernesses in eastern states of New Hampshire, West Virginia, Virginia, Georgia, North Carolina, Florda and Tennessee will be especially controversial. Eastern forests, unlike pristine western wildernesses, were denuded but have grown wild again.

Bergland's recommendations are expected to be forwarded to the White House after 45 days -- the usual period is 30 days -- to allow for what department officials predict will be considerable political reaction from governors and members of Congress who have yet to see the proposal. However, major changes are not expected, administration sources said.

Congress must approve any additions to the 18-million-acre wilderness system -- and this proposal would be the largest increase since the Wilderness Act was passed in 1964. HOWEVER, THE 36 MILLION ACRES TO BE FREED FOR DEVELOPMENT CAN BE OPENED ADMINISTRATIVELY.