Releaf for greenhouse? Don't cut old forests.

Releaf for greenhouse? Don't cut old forests

Can chopping down old forests help curb global warming? In recent years, some logging proponents have maintained the United States could help slow the atmospheric accumulation of carbon dioxide by replacing old-growth forests with faster-growing young trees. But a new study of young and old forests in the Pacific Northwest refutes that idea.

"The argument that we're somehow reducing the greenhouse effect is just totally fallacious. It is not an argument for cutting down old growth," says Mark E. Harmon of Oregon State University in Corvallis, who conducted the study with William K. Ferrell of Oregon State and Jerry F. Franklin of the University of Washington in Seattle.

The loggers' logic may seem reasonable at first glance, says Harmon. Younger trees grow much faster than older ones, so a hectare of young forest does pull more carbon dioxide out of the atmosphere each year than a hectare of old-growth forest. Meanwhile, timber gets harvested and incorporated into buildings, keeping dead wood from decomposing and releasing carbon dioxide into the atmosphere.

But this quick analysis disregards the facts that older forests store much more carbon than younger ones and that much of the harvested wood does not get stored in long-lasting structures.

To compare a 60-year-old hemlock forest with a 450-year-old Douglas fir/hemlock forest, Harmon and his colleagues reviewed the literature and added up all the carbon stored in living trees as well as in dead wood and organic debris on the forest floor. The young forest held less than half the carbon of the old-growth forest, the report in the Feb. 9 SCIENCE. Using a computer model, the researchers determined that it takes about 200 years for the storage capacity of a replanted forest to approach that of an old-growth forest.

In their analysis, the researchers also examined the fate of harvested wood from old-growth forests. They found that large portions are burned or turned into paper and wood chips that rapidly decompose and release carbon dioxide. Almost half the carbon in the cut trees may be lost into the atmosphere during a few years, says Harmon.

Paul Alaback of the U.S. Forest Service in Juneau, Alaska, has reached similar conclusions in studying old and young forests in Alaska and Chile. In both cases, he says, replacing old forests actually adds carbon dioxide to the atmosphere.

The new findings have not fallen on deaf ears. Last February, while testifying before a Senate subcommittee, Forest Service Associate Chief George M. Leonard agreed with a statement by Sen. Frank H. Murkowski (R-Alaska) that replacing old growth with younger trees "is part of an answer rather than part of the problem" with regard to the atmospheric buildup of carbon dioxide. Leonard now says Harmon's study has helped change the Forest Service's thinking on that issue.