BEIJING, July 20 (Xinhuant) -- Scientists say increased levels of carbon dioxide might be the reason creeping vines are invading Southern forests in the United States, choking out trees and altering forest makeups.

    A survey of two forests in South Carolina during the past 20 years 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.

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

    Researchers evaluated the difference in vine density in a South Carolina old-growth forest in Congaree National Park and a newer forest along the Savannah River and discovered the number of vines in the older forest had doubled within 12 years, while 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 Allen.

    As the number of vines increases, the growth of trees may be endangered. The leaves of the vines fill the canopy and block sunlight that would otherwise reach the forest floor, so competing plants die because they can't get enough light.

    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."

    (Agencies)