As an investigation by scientists at Duke University and the Cary Institute of Ecosystem Studies shows, the value of long-term studies can't be understated. In it, investigators looking at the impacts of acid rain on the soil acidity of a New Hampshire forest found that everything behaved as expected for about a decade. But after that, the system went haywire.
A legacy of acid rain has acidified forest soils throughout the northeastern US, lowering the growth rate of trees. In an attempt to mitigate this trend, in 1999 scientists added calcium to an experimental forest in New Hampshire. Tree growth recovered, but a decade later there was a major increase in the nitrogen content of stream water draining the site.
Beavers are one of nature's most industrious engineers. Using branches and mud, the intrepid animals create dams that slow moving water. In New York's Hudson Valley, their constructions are a common sight on streams and in wetlands.