NEW HAMPSHIRE—To millions of annual visitors, New Hampshire’s White Mountains represent the quintessential outdoors—fresh air, wild landscapes and abundant plants and animals. So, it may come as a surprise to learn that in the mid-20th century, the White Mountains were on the frontlines of an environmental crisis. In 1963, researchers at the Hubbard Brook Experimental Forest, nestled within the White Mountain National Forest, recorded highly acidic pH levels in on-site water samples—showing acidity levels nearly 100 times higher than would be expected in the region. 

The cause? Acid rain. Forest Service researchers at Hubbard Brook found themselves at the forefront of understanding this phenomenon and determining its potential impact on forest ecosystems. “After the initial discovery at Hubbard Brook, further research found that acid rain levels were even worse the further west one looked, in places like Ohio and Pennsylvania,” explains Amey Bailey, a forest technician at Hubbard Brook Experimental Forest. “Scientists were able to determine that the acidity in rainwater was being caused by emissions of sulfur dioxide and nitrogen oxide into the air, spurred prominently by midwestern industrial factories and power plants.” Carried by weather patterns, this polluted air moved eastward over the Mid-Atlantic and New England states.

This acid rain, caused by air pollutants, had a detrimental effect on the region’s ecosystems, with higher acidity killing sensitive aquatic plants and animals. In nearby soils, acid rain leached away vital nutrients such as calcium, essential for plant growth. “Calcium, an important nutrient, used to be a dominant element in streams and soil at Hubbard Brook,” observes Bailey. “Trees rely heavily upon calcium to grow strong and resilient cell structures. With acid rain, aluminum, an element toxic to trees’ roots, replaced calcium in the soil. Trees within Hubbard Brook Experimental Forest became less productive and more susceptible to environmental stressors such as drought and insect outbreaks. 

Amidst this backdrop, scientific evidence led to public advocacy, which in turn spurred action. In 1990, the United States Congress passed amendments to the 1970 Clean Air Act. “The story of researchers’ identification of acid rain and subsequent legislative action is one to celebrate,” says Bailey. “The legislation led to significantly cleaner air through cutting down on sulfur emissions from industrial plants.” In subsequent decades, researchers at Hubbard Brook and across the northeast have measured a rapid decline in acid rain.

This success story in curtailing the threat of acid rain is not the end of the story, however. Researchers at Hubbard Brook continue to study the legacy of acid rain. To take just one example, over the past two decades, scientists have looked at the real-world effects of adding calcium back to the soil. At study sites within Hubbard Brook, trees such as Sugar Maple have displayed significant improvements in health after calcium reapplication. These trees exhibit larger, healthier leaves, increased water use—known as evapotranspiration—and less winter injury. 

While the major problem of sulfur dioxide was addressed through public action in the 1990s, other threats remain. “Nitrogen oxide in the air—the big contributor being car and truck exhaust—also has impacts on ecosystems and watersheds,” says Bailey. “Scientists, including some of my colleagues, are actively seeking to understand the long-term implications of acid rain and how ecosystems are reacting or adapting to continued deposition of nitrogen from the atmosphere.” 

Bailey points with enthusiasm to a unique resource housed at Hubbard Brook: a massive archive of water samples comprised of nearly 30,000 weekly water samples dating back to the mid-1960s. “The Hubbard Brook archive holds immense potential for exploring emerging environmental challenges,” she says. “Recently, water samples have been used to study isotopes in precipitation, and I see new methods, which were previously unthought of, on the near horizon, all thanks to this unrivalled sample archive.

A teaching guide for grades 7 through 12 is available online that focuses upon acid rain and its interactions with air and ecosystems.

Forestcast, the USDA Forest Service’s podcast, explored the legacy of acid rain and the resurgence of the Red Spruce in the episode “Forestcast Special Episode: A Window of Resurgence for Red Spruce.”