Carbon cycling and species composition: Seeing the forest for its trees

provided by National Science Foundation

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."