Ground-Level Ozone Is a Creeping Threat to Biodiversity

It’s well established that chronic exposure to high ozone levels is a serious threat to human health, exacerbating heart and lung problems such as asthma and emphysema, and causing decreased birth weights. One study found that more than 1 million premature deaths are caused globally each year by high levels of ozone.

Research also shows that crops and forests are damaged or killed by ozone, either directly or indirectly, as ozone makes them more susceptible to insects, disease, and drought. Ozone does more damage to plants than all other air pollutants combined, according to the US Department of Agriculture. The gas is predicted to cause a substantial decline in global food production. One recent study predicted that by 2050, wheat yields would decline by 13 percent, soybeans by 28 percent, and corn by 43 percent because of rising temperatures and ozone.

While it’s clear that ozone can take a toll on all living organisms, research has not, until recently, looked at its effects on biodiversity. Scientists believe, however, that the impacts are substantial. This month the International Union of Forest Research Organizations, a global network of scientists, is holding a conference titled Air Pollution Threats to Plant Ecosystems. Ozone is at the top of the list.

In a paper published last year, 20 researchers in Europe and Asia, including Agathakleous, modeled what could happen to ecosystems in coming decades as a result of ozone pollution. They concluded that ozone will affect “the composition and diversity of plant communities by affecting key physiological traits” and can cause a cascade of changes that diminish biodiversity. In their paper, the researchers urged officials to take ozone into account in efforts to protect and restore biodiversity and said its effects should be included in assessments of atmospheric pollution and climate change.

Research is showing that ozone affects plants in a wide variety of ways.

“It paralyzes the plants’ stomata,” said Howard Neufeld, a plant ecologist at Appalachian State University, “and so they release more water than they take in.” Stomata are the microscopic openings on the surface of leaves where trees exchange gases with the atmosphere. Ozone damages them and interferes with a variety of processes, including photosynthesis.

Ozone also damages leaves and accelerates their aging. “As leaves are injured, photosynthesis goes down; a plant makes less sugars, and it has fewer resources,” says Neufeld. “It also affects the movement of sugars to roots, which reduces root growth, making them more susceptible to drought and nutrient deficiencies and disease.”

Ozone damage can also alter the timing of leaf fall and shrink leaf size, reducing the amount of litter and affecting the microbial communities that thrive in decomposing leaves. Microbes in the litter and soil are critical to taking up nutrients, helping trees resist disease and use water efficiently.

Ozone’s impacts on soil also affect the rhizosphere—the root system and its associated microbes, fungi, and other organisms. “When the plants respond to ozone, they consume energy,” said Agathokleous. “When they use so much energy, there is less to provide to organisms in the soil, and the chemical composition can be affected.” Less nutritious leaves can also affect the life cycle of animals that feed on them.