NSF PR 00-97 - December 19, 2000
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Carbon Cycling and Species Composition: Seeing the
Forest for its Trees
One of the most contentious debates during the recent
climate talks in 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 which had been turned into a plantation
for eucalyptus trees (Eucalyptus saligna) in
Hawaii. The team discovered that the acres which 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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