PORTLAND - Ten years after the Northwest Forest Plan sharply reduced logging on national forests in the region, the amount of old growth forest is up but northern spotted owl populations are down and with no clear reason why, scientists reported Tuesday.

Meanwhile, the plan has fallen far short of fulfilling its promise of a steady supply of timber or replacing lost timber jobs with new opportunities in the small towns near federal forest land, said Thomas Quigley, director of the U.S. Forest Service Pacific Northwest Reserarch Station in Portland.

"Many of the impacts were different than predicted," said Quigley.

The Northwest Forest Plan was adopted by the Forest Service and Bureau of Land Management under the Clinton administration in 1994 to lift federal court injunctions that had stopped logging in habitat for the northern spotted owl on federal lands.

It cut logging by more than 80 percent to increase the amount of old growth forest habitat for the northern spotted owl and the marbled murrelet, a threatened bird that lives and eats on the ocean but nests in old trees along the coast. It also called for improving the health of watersheds to help salmon and other fish.

The plan covers 24 million acres on 19 national forests and seven BLM districts in western Washington, Oregon and Northern California defined by the range of the northern spotted owl.

By embracing the new concept of ecosystem management, the plan marked a major change in federal management of natural resources. The government has spent $50 million to monitor how the plan has affected watersheds, wildlife, timber supply and other issues.

After declining significantly in the 1970s and 1980s, the amount of medium-aged to older forests increased 606,000 acres to a total of 7.9 million acres, 34 percent of the overall forest and right on target for the plan, said Melinda Moeur, Forest Service program leader for old growth monitoring.

Only 12 percent of that is considered classic old growth, dominated by huge trees forming a multistoried canopy. Clearcutting removed 17,300 acres, while 101,500 acres were consumed by fire.

Scientists estimate there are about 1,200 pairs in Washington, 2,000 to 3,000 pairs in Oregon and the same in Northern California, for a total of about 8,000 pairs.

The plan expected to see an average annual decline in owl numbers of 3.1 percent until enough habitat grew up to stabilize populations, but the actual decline has been sharper in some areas and less in others.

Four study areas in Washington, for example, saw an average 7.1 percent annual decline, while two study areas in southwestern Oregon saw a slight increase.

Populations have fared slightly better on lands covered by the Northwest Forest Plan than on state or private lands, said Joe Lint, a BLM wildlife biologist.

Scientists have no clear picture of what is causing the declines, but factors include invasion of spotted owl habitat by the barred owl, an aggressive cousin from Canada that often drives them off; habitat lost to past logging and wildfire; climate changes; and insect infestations, said Lint.

Eric Forsman, a Forest Service spotted owl biologist, said even killing off barred owls was unlikely to help the spotted owl, because the territory is so large and there is nothing to stop new barred owls from migrating in from Canada.

It is not yet clear whether owl habitat is being maintained by the plan, despite the increase in old growth forest acreage, but monitoring revealed no evidence to justify departing from the current strategy of maintaining large blocks of habitat across the region, Lint added.

Counts of marbled murrelets on the ocean show populations remained steady the past few years at about 22,000 off Washington, Oregon and Northern California, said Matt Huff, a biologist for the U.S. Fish and Wildlife Service.

It is not clear yet whether the plan is succeeding in maintaining or restoring nesting habitat for the bird, which lays a single egg on a mossy branch in a large tree along the coast, he added.

Susan Charnley, a Forest Service environmental anthropologist, said the 421 million board feet of timber the plan has been producing on average annually amounts to just 54 percent of the amount expected under the plan.

However, only 400 of the 11,000 timber jobs lost since 1994 can be blamed directly on less federal timber. The rest were caused by restructuring in the timber industry, she said.