Forest ThreatNet

Long-term study reveals acid rain impacts on high elevation forests

By Stephanie Worley Firley, EFETAC

When great numbers of dying spruce trees were observed in New England and the southern Appalachian region during the early 1980s, scientists scrambled to find the cause. Pollution was suspected, but the presence of interacting pollutants in the areas of concern complicated the search for answers. In 1988, Steve McNulty, ecologist and team leader of EFETAC's Southern Global Change Program (SGCP), established a research site on southeastern Vermont’s Mount Ascutney to test a hypothesis that nitrogen deposition, a major component of acid rain, was to blame for the high rates of spruce decline and mortality.

Nitrogen is an essential element used by plants for growth and survival, but can cause serious problems in excess amounts. When nitrogen compounds in air pollution from automobile exhaust and coal-fired power plants react with moisture in the atmosphere, they form acids that reach the earth’s surface in nitrogen rich, acidic precipitation.

In this study, a series of ten research plots across a high elevation red spruce forest on Mt. Ascutney have been fertilized annually with low amounts of nitrogen to simulate long-term nitrogen deposition increases resulting from acid rain. Every four years, researchers have taken measurements and analyzed changes in tree growth, foliage, soils, and the forest floor to determine the ecosystem’s response to the elevated nitrogen levels. Two decades after it began, the study continues in the hands of SGCP biological scientist Johnny Boggs.

Above: EFETAC biological scientist Johnny Boggs collects data on Mt. Ascutney to assess potential for species regeneration in a red spruce forest impacted by acid rain.

“We now know that acid rain is not killing the red spruce trees directly,” says Boggs. “Instead, nutrient imbalances caused by acid rain are changing conditions in this forest ecosystem.” He further explains, “Essential nutrients are leached from spruce needles when trees are exposed to acid rain. Specifically, when calcium is lost, needles become more susceptible to freezing injury and death. Soils are affected by acid rain, too. Aluminum concentrations in soils increase, causing displacement of essential nutrients as well as toxicity in tree roots.” When healthy growth is altered by acid rain, trees also become more vulnerable to damage from other stressors, like drought, insects, and disease.

Acid rain is not a new phenomenon; the air pollution that causes acid rain has been present in parts of the U.S. since the Industrial Revolution. Unfortunately, the damage is not always immediately visible. “The study on Mt. Ascutney will continue indefinitely to monitor acid rain’s effects on forest health and species regeneration through time,” says Boggs. Researchers will visit the site to take measurements again in 2010.

Right: Display panels on Mt. Ascutney highlight Boggs' acid rain research.

“Though impacts of nitrogen deposition have generally only been associated with decline in high elevation spruce-fir forests, high rates of added nitrogen could potentially have negative effects on lower elevation deciduous forests as well. When forests receiving high amounts of acid rain continue to decline, new trees will no longer be able to grow. This could result in major shifts in forest species compositions,” says Boggs.

He adds, “We still have a lot to learn, but there are things we can do now to deal with acid rain and its consequences. By reducing our energy consumption and pollution from automobiles, we can increase the chances of keeping our forests healthy for future generations.”

The Mount Ascutney Research Project is a collaborative venture. The study site has been provided by the Vermont Department of Forests, Parks, and Recreation. Other partners from the University of Vermont, University of New Hampshire, Duke University, and the U.S. Forest Service Northern Research Station have contributed peer-reviewed publications on a variety of topics related to ecosystem responses to nitrogen deposition.

Interpretive display panels based on this study were recently installed on Mt. Ascuntey. Click on the images to enlarge.