Experiments conducted in a North Carolina pine forest show that after an initial growth spurt, trees grow more slowly and do not absorb as much excess carbon from the atmosphere as expected.
The results suggest that planting trees may not thwart global warming or serve as an adequate substitute for reducing heat-trapping greenhouse gas emissions.
Forests typically grow in poor soils. Researchers said that once forests have initially capitalized on the extra carbon in the atmosphere, they quickly deplete nutrients in the soil. Their growth slows dramatically and their ability to store excess carbon is curtailed.
``The impact of existing forests on carbon in the atmosphere may not last very long and it might not be very significant,'' said Duke University ecologist Ram Oren, the lead researcher in one of the studies.
``The forests' impact on carbon dioxide may not materialize in any important way,'' he said.
Scientists who did not participate in the studies said results are ``potentially very important'' in determining the role of forests play in regulating the changing climate.
``If this is true for forests in general, we may not be able to count on existing forests to maintain a high capacity to absorb carbon,'' said University of Illinois plant biologist Evan DeLucia, who conducted earlier experiments in the same plots.
``We cannot look to forests to eliminate the threat of global warming,'' DeLucia said.
As plants grow, they absorb carbon dioxide in their tissues and emit oxygen.
Carbon dioxide levels in the atmosphere have been increasing for decades in conjunction with fossil fuel consumption, traffic and industrialization.
Many scientists believe the rising levels of CO2 and other emissions in the atmosphere are trapping more of the sun's heat, driving up temperatures and causing global warming. Some computer projections predict levels of these so-called greenhouse gases will rise 35 to 50 percent in the next 50 years, with temperatures increasing along with them.
Conservationists have advocated protecting existing forests and planting new ones so they can serve as ``carbon sinks'' that would help to regulate the atmosphere and moderate global warming.
However, the pair of studies published in the latest issue of the journal Nature suggests those hopes are too optimistic.
The experiments were conducted on experimental rings of Loblolly pines near Duke University.
In one test plot, pipes steadily pumped an air mixture containing 560 parts per million of carbon dioxide into the tree canopy. It simulated what computer models predict atmospheric conditions will be in 2050.
In a control plot, pipes pumped conventional air with a carbon dioxide level of about 365 ppm.
For two years, the trees exposed to more carbon dioxide grew at a rate up to 25 percent faster. Those results were reported by DeLucia in 1999.
Since then, Oren said, growth in the tree rings fumigated with extra carbon dioxide slowed to about the same rate as the control trees. Expectations that the trees would continue to sequester large amounts of additional carbon were ``unduly optimistic,'' he said.
Oren blames the trees' return to slow growth on a shortage of other nutrients, notably nitrogen. When the researchers added nitrogen to the test plots, those trees grew quickly again.
Farming, air pollution and other activities are increasing the levels of some forms of nitrogen in the environment. But those sources probably would not directly fertilize forests to a meaningful degree, the researchers said.
``Forests may slow down the rate of increase of carbon dioxide in the atmosphere,'' Oren said. ``But most forests exhaust soil nutrients very quickly. From that point on, the trees show no response.''
In the second study, researchers from Duke and Bowdoin College in Brunswick, Maine, examined decomposing leaves and roots on the floor of the experimental forest plots.
They found the total amount of litter increases in a carbon-dioxide-enriched atmosphere, but so does the rate at which it is broken down.
The carbon returns to the atmosphere, rather than being incorporated into the soil, reported William Schlesinger and John Lichter. Once a leaf falls from the tree, its carbon is recycled back in the atmosphere in about three years, they said.
The new studies were conducted in forest plots that are comprised primarily of pine trees, although sweet gum and other native species are moving in.
Researchers disagreed whether the results can be applied to other regions, such as the tropics, where conditions and tree species are different.
DeLucia said tropical forests may show even less response to elevated carbon levels.
In a Nature commentary, Eric A. Davidson and Adam I. Hirsch of the Woods Hole Research Center in Massachusetts said the results ``will be essential'' in improving computer models that predict global climate change.