Invasive plant may impact air quality

LOS ANGELES – Kudzu, a fast-growing and invasive Asian vine introduced in the American South several decades ago, has now blanketed more than 7 million acres of the region, making it sometimes seem more common than the hallmark azaleas, dogwoods and peach trees.

Now there’s evidence that the plant also increases air pollution.

A paper published last week in the Proceedings of the National Academy of Sciences reported a link between kudzu and the production of ozone, the colorless and odorless gas that is the main component of smog. Ozone can damage lung tissue, increasing inflammation and the risk of asthma attacks.

Some crops and plants are known to contribute to ozone. But this study is the first to establish a connection between an invasive plant and poor air quality, said lead researcher Jonathan Hickman, a fellow at the Earth Institute at Columbia University.

“Tying the two together is an important aspect of this research,” said University of California Davis plant sciences professor Arnold Bloom, who was not involved in the work. The findings suggest yet another way to assess the effects of invasive species.

Kudzu was planted widely in the Southeast in the early 20th century to help control erosion. But it grew almost too well, not just controlling soil erosion, but taking over abandoned farmland and climbing over stands of native plants, including trees.

A legume, it captures nitrogen from the air and transfers it into surrounding soil.

Normally this is a good thing, Bloom said. Plants need nitrogen to grow. Until nitrogen from the air is broken down into an easier-to-digest form by “nitrogen-fixing” plants and bacteria that work with them, other plants can’t access it.

“But there’s a downside,” Bloom said. The nitrogen-rich soil around kudzu can also emit gases that react with chemicals known as volatile organic compounds, creating ozone.

To measure kudzu’s effect on ozone emissions, Hickman and his colleagues identified three locations in Madison County, Ga., where they could study paired plots, or sites where patches of soil covered by kudzu sat next to nearly identical patches of soil without the plant.

They found that the presence of kudzu more than doubled the concentration of nitric oxide coming from the soil.

Plugging their data into a computer scenario in which kudzu covered all nonagricultural, nonurban soils in the region by 2050, they then calculated that, in some areas – specifically parts of Mississippi, Arkansas and Tennessee – the number of “high-ozone events” would increase by as many as seven days per year, up more than 35 percent compared with another hypothetical scenario without kudzu.

A high-ozone event was defined as occurring when ozone levels reach 70 parts per billion. The Environmental Protection Agency air-quality standard for ozone is 75 parts per billion. The agency has proposed changing the standard to 60-70 parts per billion.

Aside from altering the visual landscape, invasive species can crowd out native plants, upending long-established, but fragile, ecosystems.

In the Southeast, kudzu is “a poster child for invasive species,” Hickman said. Only a few other plants can coexist with it because it grows so dense in summer months that it allows less light through to the ground than a mature forest does.

In addition to facilitating the release of nitrogen oxides from soils into the air, kudzu emits isoprene, a volatile organic compound. “Kudzu is kind of like nature’s tailpipe – a tiny ozone machine,” Hickman said.