Creeping vines are increasingly invading Southern forests, choking out trees and altering forest makeups.

Scientists say increased levels of carbon dioxide might be to blame.

The invasion involves more than kudzu, the woody vine of Japanese origin that can cost hundreds of millions of dollars annually in the southeastern United States due to lost cropland and control measures.

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A survey of two forests in South Carolina over the past two decades has revealed that vines such as grapevines, trumpet vine, poison ivy and Virginia creeper have been infiltrating the areas at increasingly higher rates, especially in newer woodlands.

Researchers evaluated the difference in vine density in an old-growth forest in South Carolina's Congaree National Park and a newer forest along the Savannah River.

They found that the number of vines in the older forest had doubled within 12 years; there was a 10-fold increase in vines in the newer forest.

The patterns found in this study, detailed in an April issue of the journal Forest Ecology and Management, match those found by studies of other temperate and tropical forests, noted study leader Bruce Allen, of Ohio State University.

As the number of vines increases, the growth of trees may be jeopardized.

The leaves of the vines fill the canopy and block sunlight that would otherwise reach the forest floor, with the result that competing plants die because they can't get enough light.

"There are so many vines that they're starting to change the makeup of the forest," Allen said. "It appears that as the number of vines increases, the density of small trees decreases at a fairly uniform rate."

Just why the vines are taking over is uncertain, but Allen and his colleagues speculate that increasing carbon dioxide concentrations that are fueling global warming could be benefiting vine growth more than tree growth.

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

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