Study Finds Climate Change Increases Risk of Plant Invasion in Eastern United States

AMHERST, Mass.—If present climate trends continue, three invasive plant species—kudzu, privet and cogongrass—are likely to continue their destructive march north, according to new computer modeling results predicting that climate change will greatly expand the range of all three species.

The study, co-authored by Bethany Bradley, a visiting professor at AmherstCollege, and D.S. Wilcove and M. Oppenheimer of PrincetonUniversity is forthcoming in the journal Biological Invasions and was recently posted online.

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It finds that the risk from privet and kudzu expands north into Ohio, Pennsylvania, New York and New England states by 2100 if not earlier, and that the risk of cogongrass expands as far north as Kentucky and Virginia.

“There have been quite a number of studies that show kudzu, for example, is marching northwards, but this is the first real mapped study that provides hard evidence that this indeed would be the case, for all three of these species and not just kudzu,” Bradley said. “The extent of increased invasion risk uncovered by our study is actually pretty staggering.”

This rogues’ gallery of invasive plant species is currently kept from spreading outside the South by factors such as low winter temperatures and varying precipitation levels. But climate change models suggest that these species have the potential to disrupt ecosystems they enter, including the forests of New England that are celebrated for their spectacular fall foliage and tasty maple syrup.

“Kudzu grows up the side of trees and takes over the top of them,” Bradley said. “That one especially would have the biggest impact on the aesthetics of the forest.”

Kudzu, which may be the best known of the three plants in the study, was introduced in the 1800s for erosion control.

“It climbs anything,” said Bradley, who is one of five so-called Copeland Fellows brought to AmherstCollege this year to teach and research about environmental issues. The other Copeland Fellows are Chris Cuomo, C. Josh Donlan, Seth Shulman and Diana Pei Wu.

“It smothers trees, it climbs up utility poles, it climbs over houses and garages,” Bradley added. “It creates havoc for everyone, from the homeowner to the forester.”

Privet, a shrub which is still sold in some garden centers in New England, has been spreading its range in southern forests, Bradley said.

“Up here it hasn’t been as much of a problem – it hasn’t escaped cultivation,” she said. “In the southeast it really has. That’s why we’re really concerned about climate change. If it gets warmer does that mean our forests up here are in for it as well?”

Cogongrass is less well-known than the other two species, but potentially a bigger problem, Bradley said. Currently confined to the GulfCoast in the Florida Panhandle, it’s a grass that fills in spaces between trees and forests, burning very easily and at higher temperatures than native plants.

“Trees that normally would survive a burn are not acclimated to the super hot temperatures, and they die,” Bradley said.

To conduct the study, Bradley and her co-authors collected data on invasive species distribution, temperature changes and other variables.

“At the core of this methodology, we’re using maps of where these species are distributed currently as well as maps of temperatures, not just annual temperatures but monthly temperature, and not just mean temperatures but minimum and maximum temperatures and precipitation as well,” she said. “You’re trying to put in a model all of the variables that are best going to map the species distribution. That’s at the core of this – biogeographical modeling.”

What the model found, according to the paper, is that “climate change is likely to enable all three species to greatly expand their ranges. Risk from privet and kudzu expands north into Ohio, Pennsylvania, New York and New EnglandStates by 2100. Risk from cogongrass expands as far north as Kentucky and Virginia.”

Although 2100 seems like a long way off, Bradley notes that climate change doesn’t typically occur gradually over time, but in fits and starts that are often quite dramatic.

“We’re assuming that the future is a gradual thing but a lot of scientific evidence suggests that is not the case,” she says. “What these findings really suggest is that we should be putting some management practices in place now in these threatened areas with the assumption that they’re going to get there eventually.”

Eradicating invasive plant species is much easier when they are first spotted in a region and are confined to small pockets of growth.

“The trouble with eradicating invasive plants is not so much that we don’t know how to do it, it’s that we don’t have the resources to do it,” Bradley said. “If a population is small enough, you can have pretty good success eradicating it. If an entire state gets taken over then you’re not going to make much headway.”

For more information, contact Bethany Bradley, visiting professor and Copeland Fellow, Amherst College, visiting assistant professor, University of Massachusetts – Amherst; email:  bbradley@amherst.edu; or through Peter Rooney, director of public affairs, Amherst College, 413-542-84542; prooney@amherst.edu.