President Clinton announced plans today to ban road-building and logging on at least 40 million acres of national forest wilderness, a move some environmental groups hailed as the biggest conservation initiative since Theodore Roosevelt's administration.

Standing before a spectacular vista of the Shenandoah Valley west of Harrisonburg, the president directed the Forest Service to begin a year-long process of soliciting public comments and devising plans to "permanently protect" at least 40 million acres of roadless national forest lands throughout the country. Timber companies have pressed the government to open such areas to logging, a practice Clinton seeks to sharply restrict or to eliminate.

"Today, we launch one of the largest land preservation efforts in America's history to protect these priceless, back-country lands," said the president, wearing a green, open-collar shirt, brown slacks and cowboy boots. "Within our national forests there are large parcels of land that don't contain roads of any kind and, in most cases, never have. . . . They offer unparalleled opportunities for hikers, hunters and anglers," and "they're absolutely critical to the survival of many endangered species."

The administration dealt gingerly with the most controversial national forest issue, singling out the vast Tongass National Forest in Alaska for special comment from the public. Covering 17 million acres, Tongass is the largest national forest and a prize both to conservationists and to loggers and others who believe that public land should be open to commercial use.

"We specifically solicit comments on whether or not the proposed rule [barring roads, logging and other activities] should apply to the Tongass National Forest," said a Forest Service memo.

Several conservation groups appeared pleased, for they had feared that the administration might omit Tongass from the protection plan. Taken as a whole, advocates said, Clinton's decision could protect vast regions of wilderness for decades.

"If this is done right, it will be the boldest conservation move of the century, sheerly because of the magnitude," said Ken Rait, director of the Heritage Forest Campaign. The group was created, largely at the instigation of the Pew Charitable Trusts, to push the wilderness protection plan for the roadless sections of the national forests. Rait predicted the logging industry will urge Congress to block Clinton's plan, scheduled to take effect late next year.

The national forest system contains 192 million acres in 46 states. There are 380,000 miles of roads in the forests, and logging is allowed on nearly one-fourth of the forest lands. But about 40 million acres--generally in parcels of 5,000 acres or more--have been identified as roadless, and at least 10 million more acres could be added to the list, officials said.

"It is very important to point out that we are not trying to turn the national forests into museums," the president said at this overlook in the George Washington and Jefferson National Forest. "This initiative should have almost no effect on timber supply. Only 5 percent of our country's timber comes from the national forests. Less than 5 percent of the national forests' timber is now being cut in roadless areas. We can easily adjust our federal timber program to replace 5 percent of 5 percent, but we can never replace what we might destroy if we don't protect these 40 million acres."

Henson Moore, president of the American Forest and Paper Association, said Clinton's statistics are misleading. The government, he said, has been steadily restricting logging on national lands, causing Americans to lose jobs and timber imports to rise. Vast roadless regions not only keep out loggers, he said, but also firefighters and those who could treat serious tree diseases.

"Recreationists may not realize what's happening here," said Moore, a former Louisiana congressman who wants a full-fledged debate on Clinton's proposal. "The American people don't understand you're telling Smokey the Bear, 'You stay out of the national forests along with everybody else.' "

CAPTION: Reddish Knob Overlook in George Washington and Jefferson National Forest provides scenic backdrop for forest protection announcement.