Hostile Takeovers

To do her master's thesis research in plant biology, Tammie Beyerl had to fend off blackberry brambles, poison ivy, and legions of ticks. "I used lots of Deep-Woods Off and put duct tape around my pants legs," she says. 

Her perseverance paid off with the first detailed information about the ecology of an exotic plant that threatens forests and watersheds in 23 states. The interloper is the Chinese yam (Dioscorea oppositifolia or D. batatas), an ornamental vine native to Asia that has escaped from U.S. gardens and backyards. It's been reported from Georgia north to Vermont and west to Kansas and Oklahoma. 

A perennial that's sometimes used medicinally, the plant has a large tuberous root system, grows in dense colonies, and can produce vines some 12 to 15 feet tall, which overrun other vegetation. In this country, it spreads by means of bulbils—little tubers, resembling tiny potatoes, that grow at the junction of leaf and stem and drop off in early fall.

Beyerl worked in the lab of David Gibson, a professor of plant biology whose students are studying plant communities and invasive exotic plant species in the nearby Shawnee National Forest.  She first learned about the Chinese yam problem from Mark Basinger, a Ph.D. student in the department who has since graduated. When she found that no one had studied this plant in the wild in the United States, she decided to make it her thesis subject, research that would have "an immediate effect." 

Indeed, her work has laid the foundation for understanding where and how the plant is growing and what ecological effects it may be having. The Illinois Department of Natural Resources (IDNR) underwrote her research with a $1,000 grant from the state's Wildlife Preservation Fund, which has supported many SIUC studies since its inception in 1985. IDNR is working to publicize Beyerl's findings via the Illinois Nature Preserves Commission web site, which includes information on exotic species.

"This plant was first reported [growing wild] in southern Illinois in 1986," says Jody Shimp, a regional biologist with IDNR. "It wasn't until the early 1990s that we found it in the Shawnee, and now we're finding it everywhere. It's probably one of the most invasive exotic plants in the area, and we're finding it in natural areas that are usually resilient to invasion. So we're really concerned about this one."

In spring and summer 2000, Beyerl studied Chinese yam colonies at six sites in four southern Illinois counties. "All the populations I found seemed to be traceable back to an old home site," she says. More-remote populations were clustered along streams, "and if you follow the streams back up, you find an old or existing home site where there's a colony." That's the case at Lusk Creek Natural Area in Pope County, for example.

"This plant seems to be spread by streams over pretty large distances," says Beyerl, "but we don't really understand the mechanism of distribution through the water, because the bulbils don't float."

To see whether and when bulbils are being moved this way, Jennifer Thomas, a new master's student working with Gibson, has strung traps across streams at Giant City State Park, southeast of Carbondale. She's also experimenting by attaching bulbils to fishing line and seeing how far they can be carried by stream water before they settle out, especially during high stream flow. And, to answer a related but separate question, she's testing to see how long bulbils can remain viable. Her research is supported by a $10,500 IDNR grant to Gibson and Beth Middleton, an associate professor of plant biology, to map populations of Chinese yam and study its spread.

Human disturbance seems to help the plant get a roothold in the wild by removing competitors. Once established, however, it readily invades pristine areas. It prefers forest edges and silty, nitrogen-rich soil, and it grows well in sunlight or shade. 

"We're finding it in the richest kind of habitat—the type we're trying to preserve because of its biodiversity," says Beyerl. A 2001 survey by Mark Basinger turned up colonies growing in every state nature preserve in southern Illinois.

That's worrisome. Beyerl discovered that significantly fewer native species, and fewer individuals of each species, grow in areas hosting Chinese yam colonies. This blow to species diversity and abundance is typical of invasive exotics and accounts for much of the havoc they can wreak on ecosystems. 

There may be a silver lining, but land managers need to act fast. The more-remote populations in Beyerl's study, such as those at Lusk Creek, consisted mostly of short plants with few bulbils, rather than the smothering, bulbil-laden vines found closer to home sites. This may have something to do with the age of the colony—the more-remote populations are younger—yet bulbils planted in the greenhouse can produce long vines after just one growing season.

"If we could figure out what's making the difference between these two types of colonies, it could help us manage this plant," says Beyerl. But if colonies in pristine areas are allowed to progress to the vine form—as has happened already with one colony at Lusk Creek—the plant will be "nearly impossible" to control, she warns. 

Jody Shimp adds, "Awareness can go a long way. If we can get people to stop planting this, that'll really help the state and our natural areas, which are so vulnerable."
 

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