During seven years of exposure to carbon dioxide concentrations 1½ 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.
Most scientists believe rising levels of carbon dioxide and other "greenhouse gases" are warming Earth's climate somewhat the way a greenhouse does when it traps the sun's heat. Policy makers who drafted the Kyoto Protocol to counteract global warming envision that planting more trees will help because that will remove some of the CO2. Under this scenario, trees may also grow faster in the higher CO2 atmospheres of the future.
To test that hypothesis, scientists at Duke's FACE site mounted pipes and valves on three separate rings of towers to release extra gas onto the forested test plots the towers surround. The computerized system maintains CO2 concentrations of 1½ times today's levels, regardless of wind or weather conditions. Three additional tower rings surround similar test plots but emit no gas, thus serving as experimental "controls." Scientists measure effects of the extra carbon dioxide by comparing results from the active and control sites.
The FACE tower rings are located in Duke Forest, a research reserve created out of former agricultural land that was replanted with trees. "Essentially there's no topsoil on that site," Schlesinger said. "The land was probably exhausted by cotton and tobacco farming in the 1800s and early 1900s."
Scientists at FACE had an unanticipated opportunity to assess how drought affects trees growing in a CO2 enriched atmosphere when 2002 proved to be one of the driest years on record in North Carolina, Schlesinger said. By comparison, 2003 was one of the area's wettest years.
"As the experiment has continued, we realized just how hard it is to see what ultimately controls tree growth -- whether CO2, water or soil nutrients," he added.
Apart from the impact of nitrogen deficiency and drought, the scientists have found some indication that pine tree growth declined over the years at the high CO2 levels, he said. The trees bathed in high CO2 also added more fine roots, which Schlesinger suggests is just another indicator of low nitrogen. "If trees don't have a lot of nutrients they grow a lot of roots looking for them," he said.
Meanwhile, some other species in Duke's CO2-bathed forest plots have grown at faster rates than the loblolly pines, scientists report. Still-unpublished data shows 70 percent growth increases for poison ivy, according to Schlesinger.
There is also evidence that the extra carbon dioxide has induced more underlying rock to weather into soil through dissolution by CO2-produced carbonic acid. While that action would also remove carbon dioxide from the atmosphere and "store" the remnants in the added soil, the impact would be "trivial" compared to expectations from boosted tree growth, Schlesinger said.
Based on available evidence from the Duke experiment, "I'd be surprised if the forests of the world will take up more than one-third of the carbon dioxide from fossil fuel emissions in the year 2050, which is what our experiment simulates," he predicted.