FLAGSTAFF, Ariz. — Long on opposing sides when it comes to forest use, timber interests and environmental groups have agreed on how thinning and prescribed burns should be done on nearly 1 million acres of Arizona's ponderosa pine forest.

The upfront agreement could take tough disputes out of the courtroom and lead to fewer delays in implementing projects, U.S. Forest Service officials say. Some 170 lawsuits over the past 51/2 years have been filed challenging timber harvest and fuels reduction on national forests, according to the agency.

"We are looking for more opportunities to replicate this," said Faye Krueger, deputy forester for the Southwest region that includes Arizona and New Mexico.

A handful of similar collaborations have been recently used in forest restoration, range management and forest road access plans across the West.

One of the largest western forest restoration projects cleared a major hurdle in April, when two environmental groups and a Flagstaff logging company agreed to limit the size of trees that could be cut along Arizona's Mogollon Rim. The trees would be removed as part of efforts to restore a natural fire regime.

The Mogollon Rim, a prominent line of cliffs that cuts across north-central Arizona and divides the state's high country from the desert, has been the site of some of the state's biggest wildfires.

But what the agreement doesn't include is a promise from any of the parties to not sue the Forest Service over the Four Forest Restoration Initiative, a 20-year project still in the planning stages.

Ethan Aumack, director of restoration programs for the Grand Canyon Trust, said no one expects that lawsuits — often used by environmentalists to change Forest Service policies — will completely stop. But as long as the level of agreement on a specific project is maintained, he said there should be no need to file suit.

"I think for any of the parties to backtrack on support they've provided without due cause would be very difficult and would carry a significant cost," he said.

Logging on national forests has been a contentious issue for decades. A 2008 report to the Forest Service by the State University of New York found that logging, harvesting and timber sales were among the top three reasons the agency was sued from 1989 to 2005. Most lawsuits sought less resource use.

Logging of old-growth forest, as well as fire suppression, have left Arizona forests with dense populations of small trees that can fuel fire growth. The Forest Service has been developing plans to thin the small trees, but cost and an agreement on how to restore the forest to a more healthy condition have kept work from being done on a large scale.

The state's largest-ever wildfire, which burned 468,000 acres in June 2002, focused added attention on the overgrown forests and the major fire hazards they posed. That prompted talks of large-scale thinning projects that would be partially funded by industry groups.

The agreement reached in April was signed by the Center for Biological Diversity, the Grand Canyon Trust and Arizona Forest Products Restoration. It covers restoration efforts on parts of four forests — the Coconino, Kaibab, Tonto and Apache-Sitgreaves. The cut trees would provide logs for plywood-like sheets called oriented strandboard.

Pascal Berlioux, president and CEO of Arizona Forest Products Restoration, said the groups agree on about 97 percent of issues, but the point of contention has been limiting trees cut to those no more than 16 inches in diameter.