Calcium helps strengthen bones in people but it is also 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. 

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.
 

A new paper reports on 15 years of data from an ongoing field experiment in the Hubbard Brook Experimental Forest in New Hampshire which finds 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. 

Aerial view of helicopter distributing calcium pellets throughout a research watershed at Hubbard Brook Experimental Forest. Credit: : 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. For the 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.  

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.

"The treatment increased the forest's resilience to major disturbances," said lead author Professor John Battles from the University of California, Berkeley. "The trees in the calcium-treated watershed were able to recover faster from a severe ice storm that hit the region in 1998."

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.