Increase In Creeping Vines Signals Major Shift In Southern US Forests

Researchers charting the growth of vines in two forests in South Carolina found up to a 10-fold increase in the number of vines in just two decades. Vines commonly found in both forests include grapevines, trumpet vine, poison ivy and Virginia creeper. Most of the vines use adhesive roots or tendrils to climb trees.

The patterns observed in the south add to a growing number of studies that found similar patterns in temperate and tropical forests, said Bruce Allen, the study's lead author and a recent doctoral graduate of Ohio State University's School of Environment and Natural Resources.

“Collectively, we're talking about an increase of more than 500 vine stems in 27 acres of forest area that we studied,” he said. “And all of the growth is within the last 10 to 20 years. Old photographs from the sites indicate there may have been fewer vines historically.

“There are now so many vines that they're starting to change the makeup of the forest,” he continued. “It appears that as the number of vines increase, the density of small trees decreases at a fairly uniform rate.”

Although the specific reasons for this shift aren't fully understood, Allen and his colleagues say possible mechanisms include increases in carbon dioxide concentrations, which have been shown to increase vine growth more than tree growth.

“Many vines thrive on elevated levels of carbon dioxide,” he said. “Several studies suggest that vines like poison ivy benefit more than other plants from higher CO₂ levels.”

The findings appear in a recent issue of the journal Forest Ecology and Management. Allen conducted the study with P. Charles Goebel, an associate professor of environment and natural resources at Ohio State, and with Rebecca Sharitz, a senior research ecologist at the University of Georgia 's Savannah River Ecology Laboratory in Aiken, S.C.

The researchers collected 12 years' worth of data from six plots that each covered 2.5 acres – that's about the size of a football field – in an old-growth forest in South Carolina's Congaree National Park. They surveyed the plots every four years, beginning in late 1989. That was the same year that Hurricane Hugo struck the southeastern United States, killing an estimated 20 percent of the forest's trees.

The researchers also surveyed five 2.5-acre plots in a second-growth forest – a forest that was partially logged about 100 years ago and has grown back – along the Savannah River and a major tributary, Upper Three Runs Creek. The Savannah River creates a natural border between South Carolina and Georgia.

The researchers gathered data on vine growth in this protected forest every six to 10 years, beginning in 1979.

Allen said that he wanted to see if there was any difference in vine density between the old growth and newer forest.

During each survey session researchers would count the number of vine stems in each plot, identify new and dead stems, and look at vine growth from previous surveys.

Vine density in the old-growth, Congaree forest nearly doubled within 12 years. Right after Hurricane Hugo, there were about 100 vines in each plot. By 2002, that average had increased to slightly more than 200 vines per plot. While the second-growth, Savannah River forest had fewer vines by the end of the study, the researchers calculated a 10-fold increase in the number of vines in this forest -- they counted an average of 10 vines per plot by the end of the study in the Savannah River forest. In 1979, researchers had found just one vine in all of the research plots.

The five-fold difference in vine density between the two forests suggests that second-growth forests may be at more risk to threatening vine growth. Researchers wonder whether the steadily increasing growth in these forests will continue in the coming years. Unpublished data on vine growth in these forests gathered during the last four years suggests that this growth will continue. If it does, that could have an economic effect on the people who manage floodplain forests for timber.

“Although there was a substantial difference in the number of vines in each forest, the increase in density at the Savannah River site represents a linear increase in the number of vines,” Allen said. “We would expect vine density to increase after a disturbance, such as a hurricane.

“But to see such a clear, definitive increase at the undisturbed site along the Savannah River was unexpected, based on current models of floodplain forest development.”

A few years ago, Allen and his colleagues published a study suggesting that increased vine growth equates to a decrease in tree growth.

As the number of vines increase, their leaves fill a forest's canopy and essentially reduce the amount of sunlight that reaches the forest floor. Some of the competing plants die because they can't get enough light.

“The likely result is that more vines will grow on existing trees, and compete directly with tree seedlings,” Allen said. “A steady increase in vine numbers will likely influence the kind of trees that ultimately grow in these forests.”

Still, Allen says it would take many lifetimes to completely change the look of the forests.

“Both study sites lie in a floodplain and are often in the path of hurricane-force winds, so their landscape can change rapidly,” Allen said. “Data we've gathered from the Congaree forest shows a reduction in vine density after a hurricane. The winds wipe out many of the trees on which vines grow, thereby letting more light into the forest and also triggering a short-term increase in tree growth beneath the canopy.”

This work received support from several agencies, including the National Park Service, the U.S. Department of Energy, the Ohio Agricultural Research and Development Center, Ohio State and the Savannah River Ecology Laboratory.