The growth rate of Loblolly pine trees in a North Carolina forest increased by 12 percent when the trees were exposed for one growing season to carbon dioxide levels projected for 50 years in the future, according to initial data gathered by scientists from four institutions.
While appearing to be good news about the ability of trees to absorb increasing amounts of carbon dioxide from fossil-fuel combustion and deforestation, researchers caution that the growth rate likely cannot be sustained. University of Illinois scientists suggest that the findings achieved during a drought and in soil low in nitrogen and phosphorus may have implications in soil management.
"Depending on trees may not be a usable mitigation policy," said Evan DeLucia, a plant biologist at the U. of I. "Trees have large reserves of nutrients, but it is likely that the growth stimulation we saw will drop in time," he said. "It may abate to zero in 10 years as the trees adapt to the higher carbon dioxide concentrations and growth exceeds the capacity of the soil to provide limiting nutrients.
"We may need to put more focus on the issue of soil management so carbon dioxide can be stored in the ground," DeLucia said. "The key is long-term, locked-up storage below ground. Everything we see above ground will end up back in the atmosphere in one or a few human lifetimes. All of it will die and decompose. Trees are short-term carbon storage. Carbon must go into soil to remove it from the atmosphere."
The findings - presented as part of eight reports in August at the Ecological Society of America annual meeting in Baltimore - were from the first year of work at the Forest-Atmosphere Carbon Transfer and Storage Experiment in a forest near Durham, N.C. The report on overall tree growth was made by Shawna Naidu, a postdoctoral researcher at the U. of I., who also reported that early results from this year appear to indicate the growth increase already may be stabilizing.
Naidu's research is in collaboration with DeLucia and project co-directors William Schlesinger of Duke University and George Hendrey of Brookhaven National Laboratory. The overall experiment includes principal investigators from the U. of I., Duke, Brookhaven and West Virginia University.
For the project, researchers have surrounded trees with vertical PVC pipes arranged in a circle 30 meters in diameter. Carbon dioxide is pumped out of the pipes to 120 trees within each ring. Three circles are exposed to carbon dioxide concentrations of 560 parts per million, an amount projected for the year 2050; another three rings are maintained at the existing worldwide average of 360 parts per million.
DeLucia, who is heading the U. of I. effort, is studying the forest's carbon budget. Like managing a checkbook, he is measuring the income of carbon into the plants, how much is saved, how much is spent on new leaves, roots and stems, and how much is lost back into the atmosphere through respiration.
So far, DeLucia said, trees can use higher levels of carbon in photosynthesis, but the question is how much more can they use and ultimately store. "This is the first experiment that lets us address this question in the real world. This is really a huge advance."
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