Scientific Method / Science & Exploration

Clean Air Act was boon to Appalachian forests

Sulfur pollution shown to have slowed tree growth.

Eastern Red Cedar
Scott K. Johnson

In the US, the early 1970s saw a major shift in the management of the environment around us. As public recognition of pollution grew, there was an outcry to clean things up, one that eventually led to the passage of the Clean Air Act and Clean Water Act and the establishment of the Environmental Protection Agency. These regulations set limits on the discharge of pollutants from things like power plants and industrial facilities.

Emissions of sulfur dioxide were one of the things dialed back by the Clean Air Act. A subsequent focus on sulfur as a driver of “acid rain” led to a 1990 amendment setting up a cap and trade system. Together, these measures helped bring about a drastic reduction in sulfur coming out of US smokestacks.

But this wasn't a case where potential ecosystem damage was avoided before it took place. In reality, sulfur emissions had already had an effect that lasted even after emissions began to drop. A new study describes how this happened in the forests of West Virginia.

Researchers from West Virginia University and Kansas State University collected tree ring cores from a stand of old eastern red cedars near the south branch of the Potomac River. The rings showed that although the trees were from one to five centuries old, they are now in the midst of a youthful growth spurt that began around 1980. By measuring the isotopes of carbon in the wood, the researchers were able to track changes in the trees’ physiology over the last 100 years.

Trees, of course, combine carbon dioxide and water (using sunlight) to produce carbohydrates for energy and growth. They take in CO2 through stomata—open pores in the leaves—through which they also lose water. An increase in the amount of CO2 will typically allow most plants to constrict their stomata, losing less water while obtaining the same amount of CO2.

But carbon dioxide isn't the only thing that influences stomata. Exposure to the sulfuric acid in acid rain constricts stomata without the corresponding boost in the availability of CO2. Through this and other mechanisms, acid rain limits the growth of plants.

The carbon isotopes show that the trees’ stomata increasingly constricted through the early to mid-1900s before reversing course around 1980. The 1930s bucked the trend, however, with stomata opening back up a bit. If the only thing affecting the stomata was rising atmospheric CO2, you wouldn’t expect this sort of pattern.

One factor can explain this behavior nicely: sulfur pollution. The region is downwind of the coal-fired power plants in the Ohio River Valley and has received its share of sulfuric acid over the years. Emissions rose in the early 1900s, fell temporarily during the Great Depression, and declined from their peak starting around 1980—and all this is reflected in the carbon isotopes of those trees. The researchers even measured isotopes of sulfur in the wood and found clear differences before, during, and after the period of high sulfur emissions.

While rising CO2 has had an effect on these trees, sulfur pollution did as well, with the increased sulfuric acid in rainwater altering the behavior of the trees’ stomata and hampering their growth. This adds a layer of complexity to research focusing on how trees are responding to increasing CO2—failing to account for sulfur can confuse estimates of CO2’s influence.

More generally, the study shows a concrete benefit of reducing sulfur pollution that would not have been apparent without careful research. These ecosystems really were being impacted and have been recovering since emissions were reduced. Sulfur dioxide emissions haven’t been eliminated—even in the US they can still be problematic—but these trees help highlight the real progress that has been made.

PNAS, 2013. DOI: 10.1073/pnas.1308115110  (About DOIs).

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