Graduate student Marcus Schaub climbs into the canaopy to uncover how leaves respond to ozone pollution.

Forest trees hold thousands of leaves poised, like solar cells, collecting radiant energy from the sun. In the upper canopy—the layer of the forest most of us never see—leaves are tossed about in storms. They are chewed upon by insects. They lose essential moisture to the wind. But during the past century, trees have begun dealing with new hazards, such as ozone, acid rain, trace metals, and organic compounds. Scientists at Penn State and government agencies are conducting research to find out how the trees are being affected and what it could mean for Pennsylvania’s forests.

Plant pathologist John Skelly believes the most serious air pollutant facing our forests is ambient ozone. “When you fly over Pennsylvania, you can see that 60 percent of the state is forested, and most of those forests are healthy and productive,” Skelly says. “But when conditions are right, we see widespread leaf injury due to ozone, particularly on sensitive species like black cherry.”

High above the earth’s surface, the ozone layer acts as a sunscreen, protecting life below from harmful ultraviolet radiation. Without this shield, people would suffer more skin cancer, eye damage, and suppressed immune systems. “But at ground level, ozone is a very significant oxidant,” explains graduate student Marcus Schaub. “It ages things. It oxidizes everything from the rubber on your windshield wipers to the cell membranes of plants.”

Ozone forms when hydrocarbons and nitrogen oxides from burning fossil fuels cook in the hot summer sun. These pollutants travel for long distances with the prevailing wind, often affecting areas far from their source. High ozone levels send people with breathing problems to emergency rooms. People exercising or working outdoors may have chest pains and coughs. On hot, sunny days in Philadelphia, Pittsburgh, Harrisburg, and other urban areas, when ozone approaches unhealthy levels, people are advised to drive as little as possible and hold off on such activities as lawn mowing until evening. “In many parts of the state, we’re well above EPA standards for human health,” Skelly says.

“The first noticeable sign of ozone damage in vegetation is a purple or black stippling between the veins on the tops of the leaves,” explains research assistant Jon Ferdinand. “Each speck is a group of dead palisade mesophyll cells—the cells that perform most of the leaf’s photosynthesis. If the damage continues, the leaves begin to yellow, then drop prematurely.”

While ozone damages foliage, Skelly isn’t concerned that it will kill the forests. For one thing, he explains, not all trees are affected equally. Black cherry is very sensitive, for instance, while red maple is not. Also, seedlings suffer more damage than mature trees. But he is concerned about a reduction in growth over several decades. Also, when trees are weakened by ozone, other stresses, like drought and disease, can do more harm.