Scientists who used a roof to shelter a wooded area from acid rain say they found the environment can stage a remarkable recovery once pollution ceases, at least in some places.

In an effort to gauge the ability of the environment to rebound, researchers put a clear, plastic roof over about 1,000 square yards of a sparsely wooded region of southern Norway subject to high levels of acid rain from pollution.During the four-year course of their ingenious experiment, the roofed area was "watered" by rain and snow from which acidic chemicals were removed.

Reporting Wednesday in the British journal Nature, Richard Wright of the Norwegian Institute for Water Research said the experiment found "chemical changes caused by acid deposition are largely reversible."

Just two weeks after the protective roof was installed near Risdalsheia, nitrate concentrations in the water runoff from the soil dropped by 60 percent, the researchers found, while sulfate concentrations showed a general decline starting at four months. After 3 1/2 years, sulfate levels were about 50 percent of those in a control area subject to acid rain.

Courtenay Riordan, an acid rain expert with the Environmental Protection Agency, said the Scandinavian study "confirms what a lot of people have been saying - if you don't have thick soil and you do eliminate acid deposit, you would expect the water and soil to recover fairly rapidly."

A major problem facing the United States, however, is that its topsoil tends to be deeper than Norway's and could mean U.S. soil would leach sulfate for a longer period of time, making habitats inhospitable longer, Riordan said.

Still, although the Norwegian study does not offer practical solutions to ending the problem of acid rainfall, Riordan said, it helps to quash arguments that reducing such pollutants would be futile in areas already damaged.