LEWISBURG, Pa. — Bucknell University biology professor Chris Martine calls poison ivy “perhaps the least popular plant in America.” But in a new study published this week in the Biodiversity Data Journal, Martine and two recent Bucknell alumni found that poison ivy may be an unlikely hero by combatting Japanese knotweed and potentially saving new trees in Susquehanna River forests.
Martine reports that dozens of studies have looked at the effects of Japanese knotweed on natural communities in Europe and North America. He noticed that its presence along the Susquehanna River was reason for concern.
“In addition to the prevalence of this single invasive species, it looked like the very existence of these forests was under threat,” said Martine, who holds the David Burpee Professorship in Plant Genetics & Research at Bucknell.
Martine noticed what local nature lovers and biologists with the Pennsylvania Natural Heritage Program were also starting to see: these forests, specifically those classified as Silver Maple Floodplain Forests, were not regenerating themselves where knotweed had taken a foothold.
In the new study, the Bucknell researchers conclude that Japanese knotweed has not only excluded nearly all of the native understory plant species in these forests, but it has prevented the trees already established in the canopy from leaving behind more of themselves.
“If you were to fly over these forests, or even look at a Google Earth image, you’d see a nice green canopy along the river consisting of mature silver maples, river birches, and sycamores,” said Martine, “but below that canopy there is almost nothing for tens of feet before you reach an eight-to-twelve-foot-tall thicket of knotweed. Few new trees have been able to grow through that in the last 50-60 years and our surveys found that seedlings of these species are quite rare.”
The authors suggest that as mature trees die of natural causes over the next several decades and are not replaced, these systems will shift from tree-dominated riverbank habitats to “knotweed-dominated herbaceous shrublands” incapable of supporting a rich diversity of insects, birds, and other wildlife. Loss of trees in these habitats could likely also lead to riverbank erosion and increase the severity of flood events.
The few places where knotweed has not taken over offer a bit of hope, and they may exist because of poison ivy.
“What we see in the data is that poison ivy often trades understory dominance with knotweed. That is, when knotweed isn’t the big boss, poison ivy usually is,” Martine said. “The difference is that whereas knotweed knocks everyone else out of the system, poison ivy is more of a team player. Many other native plants can co-occur with it and it even seems to create microhabitats that help tree seedlings get established.”
The prevalence of poison ivy in these sites didn’t go unnoticed by undergraduate Anna Freundlich (see photo), who collected most of the plant community data — more than 1,000 data points — in a single summer as a research fellow.
“Anna developed a pretty serious methodology for avoiding a poison ivy rash that included long sleeves, long pants, gloves, duct tape, and an intense wash-down protocol,” said Martine, her research advisor, “and even after crawling through the plant for weeks she managed to never once get a rash.”
Martine cautions against too much optimism regarding the chances of poison ivy saving the day, however.
“Righting this ship is going to require eradicating knotweed from some of these sites, and that won’t be easy work. It will take some hard manual labor,” he said. “But it’s worth doing if we want to avoid the imminent ecological catastrophe. These forests really can’t afford another half-century of us letting knotweed run wild.”
Freundlich is a now pursuing a master’s degree in plant ecology at the University of Northern Colorado. Lead author Matt Wilson, a Bucknell master’s student at the time of the study who analyzed the dataset, now works for the Friends of the Verde River in Cottonwood, Ariz.
CONTACTS: Mike Ferlazzo, 570-577-3212, 570-238-6266, firstname.lastname@example.org; Chris Martine, 570-577-1135, email@example.com