During seven years of exposure to carbon dioxide concentrations 1 times higher than today's, test plots of loblolly pines have indeed boosted their annual growth rates by between 10 and 25 percent, found the researchers. But "the highest responses have been in the driest years, and the effect of CO2 has been much less in normal and wet years," said William Schlesinger, a professor of biogeochemistry and dean of Duke's Nicholas School of the Environment and Earth Sciences.
These counterintuitive findings suggest that nitrogen deficiencies common to forest soils in the Southeastern United States may limit the abilities of loblolly pine forests to use the extra CO2 to produce more tissues as they take in more of the gas, he said.
"In a dry year trees naturally grow less so the amount of nitrogen doesn't make any difference," he said. "In a wet year, when there's plenty of water, the amount of nitrogen does make a difference." Tree growth depends on the availability of nitrogen, which foresters routinely add to Southeastern soils in the form of fertilizer when they plant trees, he added.
Schlesinger will report on the first seven years of results from Duke's Free Air Carbon Dioxide Enrichment (FACE) experiment at a symposium on "CO2 Fertilization: Boon or Bust?" beginning at 8 a.m. PT on Monday February 16, 2004 during the American Association for the Advancement of Science's 2004 Annual Meeting in Seattle.
Funded by the U.S. Department of Energy, Duke's FACE experiment is set up as an open-air test of how higher CO2 outputs produced by fossil fuel emissions and other human activities could change a Southeastern forest ecosystem about 50 years from now.