Got calcium? Mineral key to restoring acid rain-damaged forests

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 from the effects of acid rain.

Helicopter distributes calcium pellets throughout research watershed at Hubbard Brook Experimental Forest. (Photo courtesy Hubbard Brook Research Foundation)

The paper, published today (Thursday, Sept. 19), in the journalEnvironmental 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.

Acid deposition has altered the calcium cycle in watersheds in the Northeastern United States in ways that are similar to changes observed at Hubbard Brook Experimental Forest. The bar graphs in the photos show the differences in calcium (Ca) levels between 1950, bars on the left, and 1995. (Graphic courtesy of Hubbard Brook Research Foundation)

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.

For the Hubbard Brook study, a helicopter spread 40 tons of dry calcium pellets over a 29-acre watershed over several days in October 1999. The calcium was designed to slowly work its way into the watershed over many years.

“This was restoration, not fertilization,” said Battles. “We were only replacing what was lost.”

Researchers monitored the forest over the next 15 years, comparing the treatment area with an adjacent watershed that had the same characteristics, but did not get the added calcium.

Hubbard Brook Experimental Forest with a view of the south-facing experimental watersheds. (Photo courtesy of the Hubbard Brook Ecosystem Study)

“The treatment increased the forest’s resilience to major disturbances,” said Battles. “The trees in the calcium-treated watershed were able to recover faster from a severe ice storm that hit the region in 1998.”

“This study has important implications that go well beyond the forests of the northeastern United States,” said Dave Schindler, a professor of ecology at the University of Alberta in Canada who was not part of this research. “Similar depletion of soil nutrients by acid precipitation has occurred in much of eastern Canada and Europe. This long-term study indicates that the calcium problem can be reversed, and that is heartening.”

Both Schindler and Battles noted that the high cost of adding calcium to the soil would likely limit its use to targeted watersheds rather than as a treatment for vast areas of affected forests.

“Prevention is always preferable, and with our study’s clear evidence that acid rain is hurting forests, other countries will hopefully be motivated to intervene sooner by implementing air pollution standards to reduce emissions,” said Battles.

Funding from the National Science Foundation helped support this research.