Researchers have found soils in an old-growth forest in southern China are
storing carbon at a rapid rate.
If common to the soils of other old-growth forests, the finding could add
combating global warming to the reasons for preserving them from logging, some
scientists say.
The finding from soils in southern China goes against the generally accepted
idea that old-growth forests are in balance, giving up as much carbon through
decomposition as they take in from falling leaves and dying roots.
Why the soils act as a carbon sink remains unknown, but the study in the Friday
edition of the journal Science gives a new way to think about how carbon works
in old-growth forests, said Xuli Tang, a scientist with the South China
Botanical Garden in Guangzhou, China, and co-author of the study.
"No country in the world has been trying to (preserve) old growth forest from
carbon sink perspective because of the existence of the traditional paradigm,"
although China is doing some large-scale forest restoration projects that could
serve as significant carbon sinks, he said by e-mail.
Many scientists believe that an increase in carbon dioxide and other greenhouse
gases from burning oil and coal is causing rising temperatures around the globe
that are changing the climate, melting polar ice caps, raising sea levels and
contributing to a growing number of plant and animal species going extinct.
One strategy for combating global warming is finding ways to store carbon so it
is not released into the atmosphere.
By virtue of their age, old-growth forest soils are generally believed to be in
balance, with the amount of carbon added to the soil from falling leaves,
branches and dying roots offset by an equal amount of carbon released by the
microbes that eat the leaves, branches and roots.
The research was conducted in the Dinghushan Biosphere Reserve in the southern
China province of Guangdong, which is covered with 400-year-old subtropical
evergreen broadleaf forest.
Taking a total of 230 samples over 24 years from the top eight inches of soil,
the researchers found organic carbon levels went from about 1.4 percent in 1979
to 2.35 percent in 2003, an increase of 68 percent.
John Aber, professor of environmental studies at the University of New
Hampshire, who did not take part in the study, said he was skeptical of the
results, because they go against the prevailing research and are based on a
relatively small number of samples.
However, given the high rate of change found in the study, the increased carbon
may be coming from some recent change in the environment, such as industrial
pollution, he said.
That could happen two different ways, he said. Increased nitrogen in the air
could fertilize the forest, leading to more leaves falling to the ground and
increasing carbon in the soil. An increase in toxins falling to the forest floor
or increased acidity would slow down decomposition, leaving more carbon in the
soil.
"This isn't business as usual for an old-growth forest," said Aber. "If this can
be found to be generally the case in old-growth forests everywhere, it could
have huge implications.
"I think at this point it is an unusual result, and would have to be repeated
and documented in other old-growth forests."
Tang said that over the course of the study, the researchers observed a decrease
in the amount of leaves and branches falling to the forest floor, a decrease in
soil moisture, and an increase in soil acidity.
Christopher Field, director of the Carnegie Institution Department of Global
Ecology at Stanford University, did not take part in the study, but has worked
in the Dinghushan Biosphere Reserve. He said the study may have caught the
forest soils during an upswing in carbon storage, which is unlikely to be
sustained.
"I think that this doesn't really change anything about the way we think of
old-growth forests," he said. "I guess it's solid evidence for the emerging
consensus that old-growth forests are quite dynamic, that they have high rates
of growth, high rates of decomposition, and at any period the balance between
those could result in a net carbon storage or a net carbon loss."