Forests keep active in old age
Established forests may continue to increase soil carbon stores.
Old-growth forests can keep on squirreling away carbon from the atmosphere long after they have reached maturity, a study suggests. The discovery runs counter to the theory that established forests, although valuable stores of carbon, will not help to alleviate the greenhouse effect because they are already 'full' of carbon.
Researchers sampled forest soils in the Dinghushan Biosphere Reserve in Guangdong Province, China, from 1979 to 2003. As they report in Science1, the amount of carbon compounds stored in the soil increased by almost 68%, rather than remaining constant.
It is not yet clear how the increase occurred, says Xuli Tang of the Chinese Academy Sciences, Guangzhou, who led the study. The researchers speculate that below-ground processes, such as altered geochemistry or microbe behaviour, may be responsible.
Pristine forests such as the Amazon are often referred to as the planet's 'lungs', because their leaves produce oxygen and cycle carbon at a high turnover rate. But the overall carbon content of such mature forests is thought to be steady. Environmentalists are placing their hope in newly planted forests to help suck up the excess carbon pumped into the atmosphere by fossil-fuel burning, because young trees are expected to store more and more carbon as they grow.
If old-growth forests really are increasing their soil carbon content as a result of increasing atmospheric carbon dioxide levels, it would be "a surprise", admits Gnter Hoch, who studies soil carbon at the University of Basel, Switzerland.
"This could be just a special case," he points out. If the soil conditions are unusual when the forest is first established, then it could continue to behave in surprising ways for centuries after, he says. "And young-growth forests will probably grow faster in a high-CO2 world, and reach maturity faster," he adds.
That's not to say that established forests are not valuable, Hoch notes both as reservoirs of carbon and as habitat for wildlife. "It would make no sense to cut down an old forest to grow a new one."
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- Zou G., et al. Science, 314 . 1417 (2006).