A team of university scientists has found that federal clean-air laws have helped forests recover from pollution.

Kansas State University associate biology professor Jesse Nippert collaborated on the project with West Virginia University researchers.

Kansas State says in a news release the researchers spent four years studying more than 100 years’ worth of rings in Eastern red cedar trees in the central Appalachian Mountains of West Virginia. The region is downwind of the Ohio River Valley coal power plants and experienced high amounts of acidic pollution in the 20th century.

The scientists found that the trees fared better in the decades since the Clean Air Act was passed in 1970.

Their findings appear in the scientific journal Proceedings of the National Academy of Sciences of the United States of America.