SALEM, Ore. — The debate over immigration, which has filtered into almost every corner of American life in recent months, is now sweeping through the woods, and the implications could be immense for the coming fire season in the West.

As many as half of the roughly 5,000 private firefighters based in the Pacific Northwest and contracted by state and federal governments to fight forest fires are immigrants, mostly from Mexico. And an untold number of them are working here illegally.

A recent report by the inspector general for the United States Forest Service said illegal immigrants had been fighting fires for several years. The Forest Service said in response that it would work with immigration and customs enforcement officers and the Social Security Administration to improve the process of identifying violators.

At the same time, the State of Oregon, which administers private fire contracts for the Forest Service, imposed tougher rules on companies that employ firefighters, including a requirement that firefighting crew leaders have a working command of English and a formal business location where crew members can assemble.

Some Hispanic contractors say the state and federal changes could cause many immigrants, even those here legally, to stay away from the jobs. Other forestry workers say firefighting jobs may simply be too important — and too hard to fill — to allow for a crackdown on illegal workers.