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Invasive pests kill as many trees every year as equivalent to 5 million cars

Great smokyIn the Great Smoky National Park in North Carolina, the woolly adelgid decimates the trees. (Photo courtesy of Songlin Fei)

WEST LAFAYETTE, Ind. – Invasive insects and pathogens wreaked havoc on ashes, elms, chestnuts and others, wiping some of them almost entirely from American forests. In addition to environmental impact, a Purdue University study found that carbon storage lost each year by these pests is the same as the amount of carbon emitted by 5 million vehicles.

Songlin Fei, a professor at the Department of Forestry and Natural Resources, said that trees killed each year by the 15 most invasive pests contain 5.53 teragrams of carbon (TgC), which corresponds to about 6 million tons of the US. His findings were published on Monday (August 12) at the Proceedings of National Academy of Sciences.

"Not all dead trees immediately become sources of carbon, but they are taken from living biomass that functions as a carbon sink. Some dead biomass will eventually get into the atmosphere, "Fei said.

Losses are particularly worrying because some suggest that carbon sequestration in forests can counteract climate change by capturing and retaining more carbon from the atmosphere. Fei said losses due to invasive species could destroy these hopes.

Evans CountyIn Evans County, Georgia, laurel wilt disease destroyed Redbay's laurel stand. (Photo courtesy of Kevin Potter)

"If we think of forests as a tool to mitigate climate change, the tool itself is being questioned by these invasive pests," Fei said. "The tool not only gets damaged, but becomes an obstacle."

Fei and colleagues at the US Forest Service have analyzed 10 years of forest research covering 93,000 plots in neighboring United States. They measured the loss of trees due to invasive pests – beyond the natural death of trees – for the most harmful pathogens and insects tracked by the Forest Service.

Of the 15 invasive pests covered, nine are pathogens, four are juice media, one is auger and the other is leaf medium. The most harmful pests when it comes to the loss of biomass above those that would be expected from natural losses were emerald borer, Dutch elm disease, beech bark disease, and common woolly. The most harmful species in terms of mortality rates, measured as a percentage of biomass loss, were laurel wilt diseases (11.4 percent), chestnut blight (6.3 percent) and musk cancer (5 percent).

The study authors said that while the current annual loss due to invasion accounts for only 0.04 percent of total living biomass in the neighboring United States, the problem may increase. Of the 15 pests, three attacked only about half their potential range, and seven attacked less than 35 percent.

"Although the total biomass losses reported here represent only a relatively small percentage of the total biomass, it should be emphasized that the trajectory of future effects of these pests can be expected to increase because most of the pests analyzed here did not invade the entire range of their hosts," they wrote. "Given the further increase in the extent of existing pests and the anticipated establishment of new unnatural pests in the future, proactive policies to reduce future invasions are likely to bring additional benefits in terms of reducing greenhouse gas emissions."

The researchers also warn that their estimates are certainly low because they do not take into account losses from urban areas. Also included are hundreds of other pests that inhibit the growth of trees and the development of root systems that can retain significant amounts of carbon.

Scientists plan to work on determining the amount of carbon from dying trees that returns to the atmosphere and the amount of soil trapped. The National Science Foundation and USDA financed this work.

/ Public Release. View in full here.

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