It’s 10:45 on a September morning. In Moshannon State Forest, on the Allegheny Plateau, there’s still a haze in the air. “It’s smog that often originates as far south as the Gulf Coast, or from the Midwest and Pittsburgh,” Skelly says. “Three hours ago, 8 million people drove to work in Chicago. I’ll bet we’re up to 75 or 80 parts per billion ozone.” Clean air averages between 20 and 40 ppb per hour, he explains. Once ambient ozone concentrations get above 100 ppb ozone, you begin to smell it. It’s the sweetish, acrid smell that hovers around electric arcs and copy machines.

Plant pathologist John Skelly tested an inexpensive ozone monitoring system that has helped researchers expand ozone monitoring to forests and other remote areas.

For three years, Skelly and his research assistants have traveled a 510-mile circuit to 11 remote sites to monitor ozone levels, including the area he calls “the ozone triangle”—a gradient of ozone concentrations surrounding the cleanest part of the state. “We’ve consistently found the highest readings at Penfield in the Moshannon State Forest,” Skelly says. “This makes sense, because it’s only a couple miles from the highest point on Interstate 80 east of the Mississippi. There’s no higher land from here to Chicago.” Traveling northeast, towards Mt. Pisgah State Park, he explains, the air becomes progressively cleaner, with the cleanest air in Pennsylvania found in Tiadaghton State Forest, above Williamsport.

At a windy site in Moshannon State Forest, where ozone levels are particularly high during late summer, the canopy of sensitive black cherry may be so thin and lacy you can see through it. Skelly points out a sensitive tree. Its leaves look as if they’ve been sprinkled with pepper. Yet, just behind it, an ozone-tolerant cherry stands free of symptoms and loaded with fruit. “Even within black cherry, genotypes vary,” Skelly says. “So you can’t simply say that at certain ozone levels black cherries are going to exhibit certain symptoms.”

Through genetic studies in a seed nursery, Skelly, forest geneticist Kim Steiner, and Jae Lee, visiting scientist from South Korea, learned that a quarter of the variation in the black cherry’s response observed in the forest is due to its genetics alone, not ambient ozone. The research team believes that site conditions, such as soil moisture, temperature, and sunlight, also play a big role in the plant’s response. In other words, you can’t predict what’s going to happen in the forest based on ozone levels alone.

To understand this, it helps to understand a little about trees.

In Skelly's lab, tree seedlings grow under controlled levels of ozone and other factors so scientists can observe their response.

Trees stand rooted in the earth like giant straws, sucking water and nutrients from the soil. When the sun comes out, the trees begin to photosynthesize, exchanging gases with the atmosphere through stomates—breathing pores in their leaves. In the morning and early afternoon, when sunlight is plentiful to power photosynthesis, the pores open wide, capturing as much carbon dioxide as they can. But as the day continues and the site dries out, water stress sets in. To protect itself from excess water loss, the plant closes its pores.

Because ozone can enter the tree through the pores, graduate student Marcus Schaub hypothesized that trees on wet sites would be more vulnerable to ozone damage because they wouldn’t close their pores as quickly.

To test this hypothesis, he had to climb into the canopy. Wearing a climbing harness and an instrument on his back that measures leaf gas exchange, Schaub mounted scaffolds up to 82 feet high. On platforms overlooking a fluttering sea of leaves, he measured gas exchange in three species: black cherry, which is very sensitive to ozone; white ash, which is moderately sensitive; and red maple, which is ozone-tolerant. He looked for a correlation between symptoms and factors such as gas exchange, soil moisture, and ambient ozone. He performed the same measurements on seedlings grown in outdoor open-top chambers, where he could partially control ozone concentrations and soil moisture. “Seedlings have a higher rate of gas exchange than mature trees,” he says, “so we expected them to show more injury.”

Three years of data collection on the seedlings confirmed Schaub’s hypotheses. Seedlings showed more injury than mature trees; seedlings on wet sites showed more injury than seedlings on dry sites; and the most sensitive species, black cherry, showed the most injury. However, what they observed in the mature trees came as a surprise. “Red maple and white ash didn’t differ consistently in their response at the wet and dry sites,” Schaub says. “And black cherry showed more injury at the dry sites. They also had better gas exchange and ozone uptake at the dry sites. I believe that over time these trees adjusted their rooting depth so they could reach deeper into the soil for water. Also, black cherry doesn’t tolerate ‘wet feet’ as well as the white ash, so the wet site may simply have been too wet.”

Schaub’s research proved the hypothesis that a tree’s sensitivity to ozone is related to species, age, and soil moisture. “Now, when people claim that ozone is seriously affecting forests, we can explain that it depends on the site and the species,’” he says. “We also can tell EPA the ozone levels at which plants begin to show injury and help them set up a better ozone standard for trees.”