JUNEAU -- Top federal officials overseeing the Tongass National Forest are disputing environmental group claims that an announced shift to logging second-growth timber instead of controversial old growth has "stalled" but are acknowledging the transition will take time.

Robert Bonnie, who oversees the U.S. Forest Service, said the change will take 10 to 15 years, and that old-growth harvests will have to continue in the meantime to protect local communities reliant on the industry.

"There's going to be a need for old-growth timber supply for several years," said Bonnie, U.S. Department of Agriculture undersecretary for Natural Resources and Environment.

The Tongass is the nation's largest national forest and has for decades been the site of bitter battles between the timber industry and environmentalists.

Now, there's broad agreement that the Forest Service should switch to harvests of less sensitive and less controversial second-growth. The debate now is about the rate of that transition and how much additional old-growth is logged while it's happening.

Bonnie, speaking to reporters Wednesday in Juneau, said there simply aren't enough young trees reaching marketable age in the next few years to keep the industry going without continuing some old-growth harvest.

"Those trees have to be old enough or large enough to be economically viable, but there hasn't been timber harvest on the forest long enough" for sufficient second-growth volume to grow, Bonnie said.

Bonnie also warned there are costs to speeding the transition by harvesting new growth earlier than planned.

Alaska Forest Association executive director Owen Graham echoed that concern.

"This is what drives me as a forester crazy," he said. "Jeez, don't cut them now -- they're just about to enter their period of fastest growth."

By waiting the planned 10 to 15 years, there will be much more timber to harvest from the same acreage, Graham said.

Bonnie also said the industry needs time to retool to mill smaller-diameter trees. It can't do so profitably now on the limited volume that's available, he said.

The discussion of the Tongass transition Wednesday was spurred by a report issued by Trout Unlimited making the claim that the transition had "stalled." The group based its claim on an analysis of four years of Forest Service budgets and said the agency was spending to much money preparing timber sales and should instead be spending it on more viable industries such as tourism and fishing.

The Trout Unlimited report was prepared by Headwaters Economics of Montana, but the report's authors later told reporters the budget data they reviewed did not include sufficient detail to determine whether the Forest Service timber sale program was focused on old or young growth.

The Forest Service is continuing to develop its plans for the Tongass transition to second-growth, and Bonnie will be meeting in Sitka this week with the Tongass Advisory Council to further develop those plans.