The study addresses a variety of questions. How do forest trees respond to rising concentrations of atmospheric carbon dioxide from increased fossil-fuel combustion, forest burning, and other sources? Do they grow faster? Do their trunks and branches become larger than normal? Do they grow more efficiently even when deprived of nutrients? Will their response to elevated levels of atmospheric carbon dioxide be to absorb and store more carbon from the atmosphere, slowing the global warming? .
To help address these questions, ORNL has been conducting a series of long-term studies of forest tree species in a carbon dioxide-enriched atmosphere. The results of the study, which is sponsored by DOE's Global Change Research Program, were first reported in a letter to the May 28, 1992, issue of Nature by researchers in ORNL's Environmental Sciences Division. They are Richard J. Norby, Carla A. Gunderson, Stan D. Wullschleger, E. G. O'Neill, and Mary K. McCracken. The letter, entitled "Productivity and compensatory responses of yellow-poplar trees in elevated CO2," was the third most cited 1992 scientific paper in ecology and environmental sciences, according to the January 1995 issue of Science Watch.
ORNL studies of yellow-poplar tree responses to an atmosphere enriched in carbon dioxide have not found the dramatic growth seen in some studies with potted tree seedlings or irrigated and fertilized saplings. The ORNL researchers found an increase in growth efficiency - or the amount of stem wood mass produced per unit leaf area - in the yellow poplars.
They observed a doubling of photosynthesis per unit leaf area-the use of sunlight by leaves to produce carbohydrates that provide energy for tree growth. However, the relative amount of leaf area that was produced was less, and the overall stem mass-the girth of trunk and branches-remained about the same. Any additional carbon that was absorbed resulted in increased production of fine roots, rather than wood.
The increased fine root production, Norby says, suggests that some of the additional carbon in the atmosphere may be eventually absorbed and stored in the soil rather than in trees. Fine roots, he adds, die and decay rapidly, but their residue is an important source of the carbon in soil. The possibility of increased carbon storage in soil is an important focus of DOE's new Terrestrial Carbon Processes Program, which aims at identifying and quantifying natural mechanisms of the terrestrial ecosystems that may affect trends in atmospheric carbon dioxide concentration and to develop the scientific understanding needed to model, predict, and quantify the role of terrestrial ecosystems in regulating the balance of global carbon.
"Our results suggested strongly that, because less leaf area is needed to sustain the same growth rate and because fine root production increased, the yellow poplar trees grow more efficiently in a carbon dioxide-enriched atmosphere," Norby says. "The increased productivity showed up as additional fine roots rather than in wood.
"The increased efficiency implies that these trees may be better able to withstand environmental stresses, such as drought, and survive even when nutrients are limited. Thus, they may last longer as a mechanism for removing carbon from the atmosphere and storing it."
The interaction of elevated carbon dioxide with the stresses associated with predicted increases in air temperature is the current focus of the ORNL research group.
The results of the ORNL paper are sometimes incorrectly cited as evidence that trees will not grow bigger and faster in a carbon dioxide-enriched atmosphere unless they are provided with additional nutrients, including fertilizing pollutants such as sulfates and nitrates from acid rain.
"That this interpretation is wrong is shown by the large response of white oak trees to elevated carbon dioxide levels at the same experimental site at ORNL," Norby says. "Both species increased their growth efficiency despite a deficiency of nutrients at the site.
"We found that both white oak and yellow poplar trees showed about a 35 percent increase in growth efficiency in elevated carbon dioxide. These results suggest a potential increase in forest capture of carbon even when nutrients are limited."
ORNL, one of the Department of Energy's multiprogram research laboratories, is managed by Lockheed Martin Energy Systems, which also manages the Oak Ridge K-25 Site and the Oak Ridge Y-12 Plant.