The study, performed by researchers at the Department of Energy's Oak Ridge National Laboratory and 10 other institutions in the United States and Europe, revealed a strong relationship between productivity of forest plots in the current atmosphere and productivity in plots experimentally enriched with carbon dioxide.
"The median response indicated a 23 percent increase in productivity in the future atmosphere," said ORNL's Rich Norby, lead author of the paper to be published Dec. 13 in the Proceedings of the National Academy of Sciences. "What was especially surprising to the research team was the consistency of the response across a wide range of productivity."
Researchers analyzed data from four experiments in which young forest stands were exposed for multiple years to an atmosphere with a carbon dioxide concentration predicted to occur in the middle of this century. The experiments were conducted in a deciduous forest in Tennessee, a pine forest in North Carolina, a young hardwood stand in Wisconsin and a high-productivity poplar plantation in Italy.
The team calculated net primary productivity - the annual fixation of carbon by green plants into organic matter - for each of the sites from data on wood, leaf and fine-root production. The results proved surprising.
"When we got together to analyze these data, we expected to spend our time explaining the differences between sites," said Norby, a member of ORNL's Environmental Sciences Division. "We were really surprised and excited when all of the data fell neatly onto a single line."
More detailed analysis of the data revealed the mechanisms of the forest productivity response. In forest stands with a relatively low amount of leaf area, the response to elevated carbon dioxide levels was explained by increased absorption of light. With greater leaf area, however, the response was an increased efficiency of conversion of light energy to organic matter. In separating the overall response into leaf area and light-use efficiency, the analysis meshes well with broader scale analyses based on satellite imagery, Norby said.
Norby notes that this analysis will be especially valuable as a benchmark to evaluate predictions of ecosystem and global models.
"Climate change predictions are dependent on assumptions about the interaction between the biosphere and atmosphere," Norby said. "However, the contribution of carbon dioxide fertilization to the future carbon global carbon cycle has been uncertain and the models are poorly constrained by experimental data. The close agreement of the productivity predictions of models with the new experimental data should add confidence to overall model results."
Norby cautioned against viewing these results as a reason to ignore the steadily increasing amount of carbon dioxide in the atmosphere.
"Although carbon dioxide fertilization of forests might slow the rate of increase of atmospheric carbon dioxide, a 23 percent increase in productivity is insufficient to stabilize the concentration in the atmosphere," he said. "The increase in productivity demonstrated in these experiments will most likely be tempered by the stresses of climate warming, ozone pollution or insufficient nitrogen supply. In addition, some of the increased organic matter entering the forest is not sequestered in wood but is rapidly returned to the atmosphere. Understanding the controls on carbon processing by ecosystems remains a priority research challenge."
This study, funded primarily the DOE's Office of Science, Biological and Environmental Research and the National Science Foundation, reinforces earlier findings and challenges reports that question the importance of carbon dioxide fertilization based on observations of a few trees.
UT-Battelle manages Oak Ridge National Laboratory for the Department of Energy.
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