Ecologist Gene Likens and his colleagues considered the effects of acid deposition on the experimental forest. They noticed that after SO2 and particulate levels in the atmosphere declined as a result of pollution controls mandated by the Clean Air Acts, damaged trees in the forest did not recover as expected. One hypothesis they considered to explain this was that nutrients required for tree health and growth—particularly calcium and magnesium—had been stripped from the soil by acid precipitation. It takes decades for soil to rebuild these nutrients, which would explain the lag in recovery of the trees.