In 1970, the main threat to sugar maples and many other trees in the region was acid rain, caused by air pollution from factories. The effects of acid rain on forests, mostly weakening trees and stressing the soil are well documented. But in the intervening decades, pollution controls have limited the amount of acid rain in the region, so the trees should be rebounding. For some reason, they aren't, and scientists aren't sure why. It could have something to do with the lingering effects from acid rain, or climate change, but further studies are needed to pinpoint a cause.