钙可以修复被酸雨侵害过的森林

Calcium can do much more than strengthen bones. The mineral is a critical nutrient for healthy tree growth, and new research shows that adding it to the soil helps reverse the decades-long decline of forests ailing（生病，苦恼） from the effects of acid rain. The paper, published Sept. 19 in the journal Environmental Science and Technology (EST) Letters, and led by John Battles, professor of forest ecology at the University of California, Berkeley, also presents strong evidence that acid rain impairs forest health.

The paper reports on 15 years of data from an ongoing field experiment in the Hubbard Brook Experimental Forest in New Hampshire led by study co-author Charles Driscoll Jr., professor of environmental systems engineering at Syracuse University.

"It is generally accepted that acid rain harms trees, but the value of our study is that it proves the causal link between the chronic loss of soil calcium caused by decades of acid rain and its impact on tree growth," said Battles. "The temporal and spatial scope of the study -- 15 years and entire watersheds -- is unique and makes the results convincing."

The researchers reported that trees in the calcium-treated watershed produced 21 percent more wood and 11 percent more leaves than their counterparts in an adjacent control site. The iconic sugar maple -- the source of maple syrup -- was the tree species that responded most strongly to the restoration of calcium in the soil.

The research site, managed by the U.S. Forest Service, was targeted because of the declining growth rates and unexpected death of trees in the area. Previous measurements of the forest soil showed a 50 percent depletion of calcium.

Acid rain forms when sulfur dioxide and nitrogen oxides -- gases produced from the burning of fossil fuels -- react with water molecules in the air. The mountainous regions in the Northeast have thin soils that are already acidic, so they have limited ability to withstand the assaults of nutrient-dissolving acid rain. Moreover, watersheds along the eastern corridor of the United States had been exposed to more acid rain because of the greater number of coal-burning power plants in the region.

The Clean Air Act of 1970 significantly reduced sulfur dioxide emissions, but decades of acid rain already had changed the soil chemistry of many sensitive regions, including the White Mountains of New Hampshire and the Adirondacks of New York.