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 ne of the most contentious debates during the recent
climate talks in the Hague centered on the possible use of forests
as credit towards reducing atmospheric carbon dioxide. A team
of researchers supported by the National Science Foundation (NSF)
working on eucalyptus plantations in Hawaii has discovered an
important aspect of how carbon processes work in tropical tree
plantations. The researchers, who have published their findings
in the December edition of Ecology, discovered that carbon
cycling, or sequestration, was significantly boosted when the
composition of tree stands included nitrogen-fixing trees.
 "The
results of this research illustrate the complex relationships
between the carbon and nitrogen cycles of ecosystems and can
be applied to the future management of both natural and planted
forests," said Henry Gholz, director of NSF's long-term
ecological research program.
The ramifications
of the findings could have an impact on the way in which the
carbon sequestering potential of tropical tree plantations is
measured. The global coverage of tropical tree plantations has
increased dramatically in the past two decades. In 1980, it was
estimated that some 21 million hectares of tropical land were
being used for tree plantations globally; by 1999, estimates
stood at 60 million.
In addition,
nitrogen deposition has become an increasing concern worldwide,
and so information indicating that nitrogen deposition may aid
in carbon sequestration may prove to be especially relevant to
policymakers such as those who were in attendance at the recent
Hague meetings.
Scientist
Jason Kaye and his colleagues at Colorado State University researched
carbon storage on a former sugar cane farm that had been turned
into a plantation for eucalyptus trees (eucalyptus saligna) in
Hawaii. The team discovered that the acres that were interplanted
with Albizia trees (albizia falcataria) were able to sequester
more carbon than areas where eucalyptus trees were planted alone.
The researchers believe that this is due to the nitrogen-fixing
qualities of the albizia trees.
Kaye and his
colleagues studied carbon storage in forest stands planted 17
years ago with differing species composition. Some stands were
planted as pure eucalyptus, some as pure Albizia, and some with
the two trees planted together. The researchers found that, in
stands where the two species were interplanted, the forest contained
twice as much carbon in trees as one-crop areas. In addition,
areas of pure albizia sequestered about 20 percent more carbon
in soil than did the pure eucalyptus stands.
"Carbon
sequestration is the balance of inputs and outputs from a system,"
explains Kaye. "What we've shown here is that carbon outputs
from soil are lower in stands that have more nitrogen-fixing
trees. If decomposition is inhibited because of nitrogen inputs,
then increased biological nitrogen fixation, nitrogen fertilization
and nitrogen deposition may promote carbon sequestration."
The mechanisms at work in those processes, however, are still
very poorly understood.
"Changes
in the composition of tree species which result from land use
or climate changes may have important feedbacks to terrestrial
carbon sequestration," Kaye says. "We still need to
learn more about how species composition may be affecting the
soil of these and other forests in order to fully understand
their ability to act as carbon sinks." 
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