A federal appeals court has determined an 11,700-acre thinning project near Oregon’s Mt. Hood requires more extensive environmental review, effectively halting timber harvest.

The 9th U.S. Circuit Court of Appeals has ruled the impacts of the Crystal Clear project, which aims to reduce wildfire risk, are “highly controversial and uncertain” due to scientific disputes about the effectiveness of thinning.

Because the appellate court has ordered the U.S. Forest Service to conduct an “environmental impact statement” that could take years, the decision affects several mills in Oregon and Washington that were relying on that timber, said Lawson Fite, an attorney for the American Forest Resource Council, a timber industry group.

“Just for the wood basket around Mt. Hood, it could be significant,” Fite said. “There could be some pretty negative effects from this if we’re not able to find some path forward.”

High Cascade Inc., a timber company that’s won contracts to conduct thinning within the project, is still deciding whether to ask the three-judge panel or the full 9th Circuit to reconsider the decision, said Fite. AFRC is representing the company in the lawsuit.

Bark and several other environmental groups that filed a complaint against the Forest Service said the ruling vindicated their arguments that the project’s necessity is “unsupported by science and inconsistent with conditions on the ground.”

The environmental plaintiffs claimed the agency didn’t take a “hard look” at the project as required under the National Environmental Policy Act because the benefits from logging in the area are “highly uncertain.”

Last year, Chief U.S. District Judge Michael Mosman rejected their arguments that the Forest Service violated NEPA and other federal laws, ruling that the agency thoroughly evaluated the forest’s health and arrived at rational conclusions about its management.

“Although Bark disagrees with some of the USFS’s data, assessing the condition of the Project area is an area within the USFS’s expertise — a determination that is entitled to the highest level of deference,” Mosman said.

However, the environmental groups appealed that ruling, which the 9th Circuit has now overturned.

The appellate court has agreed the Forest Service’s environmental assessment, or EA, “drew general conclusions” but “did not engage with the considerable contrary scientific and expert opinion.”

“This dispute is of substantial consequence because variable density thinning is planned in the entire project area, and fire management is a crucial issue that has wide-ranging ecological impacts and affects human life,” the 9th Circuit said.

The appellate court also determined that a more comprehensive environmental impact statement, or EIS, is necessary because the agency didn’t sufficiently consider the “cumulative impacts” on threatened spotted owls from surrounding timber projects.

“The USFS’s failure to engage with the other projects identified by appellants leaves open the possibility that several small forest management actions will together result in a loss of suitable owl habitat,” the ruling said.

Fite, the attorney for AFRC, said the 9th Circuit’s ruling reflects several errors and misunderstandings.

The issues with the Forest Service’s environmental assessment appear “fixable,” but instead of remanding that review to the agency, the 9th Circuit ordered a more time-consuming environmental impact statement of the project, he said.

The appellate court also didn’t give enough deference to the Forest Service in evaluating and treating fire risks, Fite said. “They didn’t understand what kind of forestry this was. They thought it was clear-cutting old growth and that’s not what this is.”