PORTLAND — People against logging old growth forests won an Oregon Court of Appeals ruling Wednesday that struck down a state law on grounds it treats an environmental dispute differently.

The case grew out of arrests in March 2005 in the Siskiyou National Forest when protesters claimed old growth trees were being illegally logged as part of the timber salvage from the Biscuit fire three years earlier.

Josephine County sheriff’s deputies arrested people who refused to leave when loggers arrived for work.

The appeals court said Wednesday the law didn’t violate the state constitution. But, the court said, it did violate the equal protection clause of the U.S. Constitution because it allows a protest over labor conditions but prohibits other protests.

An attorney for the protesters said it was like allowing the police to arrest somebody for carrying a sign that said “Save Our Trees” without arresting somebody carrying a sign that said “Save Our Jobs.”

Lawyers for the state had argued that the law targeted conduct rather than expression. The appeals court said, however, that allowing picketers in a labor dispute but not those in an environmental dispute is a form of discrimination based on the kind of expression involved.

“The problem is people are treated differently under the law,” said Lauren Regan, executive director of the Civil Liberties Defense Center in Eugene.

“In this case, because there was an exception for labor protesters versus all other kinds of protesters, the court ruled that violated the federal equal protection clause,” Regan said.

She noted the center made public records requests that showed the agricultural operations law had been used almost exclusively against forest protesters.

“The overwhelming percentage of people ever prosecuted under the statute were environmental activists,” said Dan Kruse, attorney for the Cascadia Wildlands Project.

“They tried to frame it broadly so it wouldn’t be seen to be targeting a specific group, but in practice it was absolutely an anti-environmental activism statute,” Kruse said.

The appeals court also noted the statute could lead to other problems because it could reach beyond protesters.

As an example, the court said, an attorney for an environmental group who went to a company headquarters to represent an employee objecting to an allegedly unsafe practice “could likewise be guilty of attempting to obstruct an agricultural practice — even if the attorney is invited to the headquarters.”

The 2002 Biscuit Fire burned 500,000 acres of national forest in southwestern Oregon — the nation’s biggest fire of the year — and became a battleground between the Bush administration and environmental groups over roadless areas to logging.