SEATTLE (AP) _ A plan to reduce logging by 46 percent in Washington's largest national forest was announced Wednesday, and more drastic reductions may be ordered to protect the northern spotted owl, officials said.

Mount Baker-Snoqualmie National Forest supervisor Doug MacWilliams briefed a few dozen interest-group representatives on the 10-to-15-year plan for land and resource management.

He said it represents a major shift in favor of recreation and a ''very severe'' blow to the timber industry.

The proposed timber harvest reductions would cost about 1,800 jobs, MacWilliams said. ''Every one of us in this room will feel it in some way or another,'' he said.

''When you go to buy a 2-by-4, you're going to be paying through the nose for it,'' he said.

The plan, replacing one dating from 1963, takes effect 90 days after publication Friday in the Federal Register and will remain in effect pending the outcome of appeals, MacWilliams said.

Representatives of industry, landowner and environmental groups all say they will appeal various aspects of the plan. It is the product of 10 years of research and debate, but is subject to amendment by the Forest Service.

Mount Baker-Snoqualmie, covering 1.7 million acres from the Canadian border to the area around Mount Rainier National Park, is the second largest timber- producing national forest in Washington.

At the same time, it probably has the heaviest recreation demands of any national forest in the state, said forest spokesman Warren Olney.

The plan would set the average annual timber harvest at 108 million board feet - 46 percent less than the average for the past decade and 36 percent less than proposed in a draft plan two years ago.

The forest would produce $526 million in income a year, MacWilliams said.

The harvest would drop to 45 million board feet, however, if a scientific report on protection of the owl is accepted by the U.S. Fish and Wildlife Service, he said.

Fish and Wildlife officials are scheduled to decide by June 23 whether to list the owl as threatened, a decision that would trigger further logging restrictions.

However, MacWilliams said, the harvest would rise by a total of roughly 40 percent over the two decades following the planning period and then level off at a sustained-yield level.

About 95 percent of the roadless acreage would remain intact and 97 percent of the 643,000 acres of old growth would remain in the year 2000.

Old growth was defined in the plan as trees at least 21 inches in diameter measured 4 1/2 feet from the ground.

Using the Wilderness Society's definition of at least 32 inches, however, there is only about half as much virgin forest left, said Jean Durning, regional director of the environmental group.

Other features of the plan include a recommendation that 30 rivers stretching along 462 miles be designated as wild and scenic, a protection now accorded in Washington only to the Skagit River system. The draft plan had proposed wild-and-scenic designation for five rivers.

The plan also calls for construction of 220 miles of trails, rebuilding of 493 miles and maintenance of 891. The draft plan had called for 80 new miles, 240 for rebuilding and 1,144 for maintenance.