Studies Challenge Role of Trees in Curbing Greenhouse Gases
The New York Times, May 24, 2001
Two new studies are challenging the idea that planting forests could be a cheap way to absorb emissions of carbon dioxide, the main heat- trapping gas released by human activities.
In one, tracts of pine trees exposed to elevated levels of the gas initially absorbed large amounts and had a short growth spurt, but then reverted to typical growth rates.
A separate study of the soil around the exposed trees found that, although it accumulated carbon, much of the carbon was released back into the air as carbon dioxide when organic material in the soil decomposed.
The studies, described in today's issue of the journal Nature, were limited to loblolly pine forests in North Carolina, but the authors said their findings suggested a limit to the value of forest planting to counter carbon dioxide emissions from smokestacks and tailpipes that many scientists say are warming the climate.
"Such findings call into question the role of soils as long-term carbon sinks," wrote the authors of the soil study, Dr. John Lichter, a biologist at Bowdoin College, and Dr. William H. Schlesinger, a professor of biogeochemistry at Duke University, which owns the forest where the research was done.
Forest planting has figured in negotiations on a global agreement to reduce greenhouse gases, and the United States, Canada, Japan and some other large industrial countries have backed the idea.
But the new research suggests the approach is not as effective as advocates had hoped. The study of tree growth, led by Dr. Ram Oren, an ecologist at Duke, concluded that previous estimates of forests' carbon-absorbing abilities were "unduly optimistic."
Several scientists not involved in the studies said the research provided some of the first hard evidence showing the response of trees to carbon dioxide and, among other things, should help improve computer models used to predict how the rise in heat-trapping gases might affect the climate and ecosystems.
Others added that the work challenges a longstanding assertion of some coal and power companies that the main consequence of rising levels of carbon dioxide in the air will not be a damaging warming of the climate, but rather a flourishing of forests and other plant life.
Some scientists stressed that the Duke findings — despite the years of monitoring — still are preliminary because forests can take a long time to adjust to changes in the environment, and the conditions noted so far may only be a prelude to other shifts.
And some scientists involved in related experiments looking at the absorption of the gas by croplands and grassland said they thought that some of the researchers' conclusions were gloomier than their data.
Dr. Bruce A. Kimball, a soil scientist who has studied the response of wheat and cotton to elevated carbon dioxide at a Department of Agriculture laboratory in Phoenix, noted that the Duke soil findings, over all, still showed an increase in retained carbon. He said tree planting could have "some significant impact on offsetting some of our CO2 emissions."
He conceded, however, that the abrupt drop in the growth rate of the trees was "discouraging."
The study is described on a Department of Energy Web site at www.face.bnl.gov/.