Absorbing Fact: A Myth Is Dispelled -- Old-Growth Forests Beat Young Trees In Cutting Carbon Dioxide, Study By NW Colleges Shows

By doing what comes naturally - absorbing carbon dioxide, the gas chiefly responsible for the greenhouse effect - trees help ease the threat of global warming.

And conventional thought has been that, because young trees absorb carbon dioxide at a faster rate than older ones, logging off old-growth forests and replanting them with young, fast-growing trees would help reduce atmospheric levels of the gas.

Not so, according to a study by forestry scientists at Oregon State University and the University of Washington.

The research, reported in Friday's issue of Science magazine, found that old-growth forests are vastly more helpful in reducing atmospheric carbon dioxide than are modern plantations, where trees are harvested every 60 years or so.

The key was discovering the total amount of carbon stored in a forest rather than considering just the carbon uptake of growing trees.

``In some areas we studied. . . the supposedly dead parts of the (old-growth) forest actually stored more carbon than the trees, the living parts,'' said William Ferrell of Oregon State. ``And when the large trees are harvested, that decaying material (left behind) continues to release carbon into the atmosphere for years.''

Trees in an old-growth forest are so big, said Mark Harmon of OSU, that even though they grow slowly they absorb enough carbon dioxide to offset that released by decaying material on the ground. The result is a ``carbon equilibrium'' in the forest that does not affect atmospheric levels.

The study's computer projections indicated that when an old-growth forest is logged and replanted with trees, it takes at least 250 years before the original carbon storage and equilibrium are restored.

The scientists believe their study removes one reason for harvesting old-growth forests. Harmon said, ``We're concerned that decision-makers are going to be making important policies based on inaccurate, armchair ecology.'' But the researchers acknowledge that combustion of oil, coal, natural gas and wood is by far the largest source of atmospheric carbon dioxide.

Carbon dioxide has been accumulating in the atmosphere for the past century. The gas acts as a kind of blanket to hold the sun's heat close to the Earth's surface, known as the greenhouse effect. Most scientists agree that increasing atmospheric carbon dioxide will cause global warming. But whether it has begun or how much it will amount to are hotly debated.

Growing trees remove carbon dioxide from the air, store the carbon and release oxygen. When wood decays or is burned, the stored carbon recombines with oxygen and is released as carbon dioxide.

Old-growth forests on the western slope of the Cascade Range are among the most biologically productive environments on Earth. Harmon estimated that converting an acre of old-growth forest in Oregon to younger trees would have more impact on atmospheric carbon dioxide than converting an acre of Brazilian rain forest to pasture.

However, remaining old-growth forests in Oregon and Washington are small compared with the forests in tropical countries.

The researchers estimate that 12.4 billion acres of old-growth forests have been cut in Washington and Oregon in the past 100 years, more than 70 percent of the forests that greeted the pioneers. Even though that acreage comprises only 0.017 percent of the Earth's land surface, that logging ``appears to account for a noteworthy 2 percent of the total carbon released (to the atmosphere) because of (global) land-use changes in the last 100 years,'' they reported.

Jerry Franklin of the UW's College of Forest Resources and the Forest Service's experiment station at Corvallis took part in the study.