News & Observer (Charlotte, N.C.), August 9, 2007

TREES OF LIMITED VALUE VS. WARMING

[Rachel's introduction: A decade-long study finds that planting trees is not a real solution to the problem of global warming. (Still, there are many good reasons to plant trees.)]

By Margaret Lillard, The Associated Press

RALEIGH -- A decade-long experiment led by Duke University scientists indicates that trees provide little help in offsetting increased levels of the greenhouse gas carbon dioxide.

That's because the trees grew more, but only those that got the most water and nutrients could store significant levels of carbon.

"The responses are very variable according to how available other resources are -- nutrients and water -- that are necessary for tree growth," said Heather McCarthy, a former graduate student at the private university in Durham who spent 6 1/2 years on the project. "It's really not anywhere near the magnitude that we would really need to offset emissions."

McCarthy, now a postdoctoral fellow at the University of California at Irvine, presented the findings this week at a national meeting of the Ecological Society of America in San Jose, Calif. Researchers from the U.S. Forest Service, Boston University and the University of Charleston also contributed to the report.

All helped in the Free Air Carbon Enrichment experiment, in which pine trees in Duke Forest were exposed to higher-than-normal levels of carbon dioxide.

The scientists also gathered data on whether the forest could grow fast enough to help control predicted increases in the level of carbon dioxide.

The loblolly pines grew more tissue, but only those that got the most water and nutrients were able to store enough carbon to have any impact on global warming, the scientists discovered.

"These trees are storing carbon," McCarthy said Wednesday. "It's just not such a dramatic quantity more."

That means proposals to use trees to bank increasing amounts of carbon dioxide emitted by humans may depend too heavily on the weather and large-scale fertilization to be feasible.

"It would be an attractive solution, for sure," McCarthy said. "I don't know how realistic people thought it was, but I think there were certainly high hopes."

Scientists blame the worldwide buildup of carbon dioxide -- due largely to the burning of fossil fuel -- for global warming. The United States is second only to China in the level of greenhouse gas it emits as a nation.

The experiment site, funded by the U.S. Department of Energy, holds four forest plots dominated by loblolly pines in Duke Forest, in north-central North Carolina.

The trees are exposed to extra levels of carbon dioxide from computer- controlled valves that are mounted on rings of towers above the treetops. The valves can be adjusted to account for wind speed and direction, and sensors throughout the plot monitor carbon dioxide levels, McCarthy said. Four more plots received no extra gas.

Trees exposed to the gas produced about 20 percent more biomass -- wood and vegetation -- than untreated trees. But the researchers said the amounts of available water and nitrogen nutrients varied substantially between plots.

Ram Oren, the project director and a professor of ecology at Duke's Nicholas School of the Environment and Earth Sciences, said in a written statement that replicating the tree growth in the real world would be virtually impossible.

"In order to actually have an effect on the atmospheric concentration of CO2, the results suggest a future need to fertilize vast areas," Oren said. "And the impact on water quality of fertilizing large areas will be intolerable to society."

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