Carbon Sequestration: Seeing The Forest For Its Trees

Jason Kaye and his colleagues from 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. Albizias, which are sometimes also referred to as mimosa trees, are not a cash crop like eucalyptus. Although many tree plantations employ monoculture planting schemes, planting one tree species over hundreds of acres, the albizia trees' effect on soil quality has prompted some farmers to test the potential benefits of interplanting the two species. This kind of practice is increasingly common in tropical areas, where the nutrient level of soil is often limited.

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 monocrop areas. In addition, areas of pure albizia sequestered about 20 percent more carbon in soil than did the pure eucalyptus stands.

The researchers suggest that nitrogen which is added to the soil by the albizias may be inhibiting the decomposition of old, sugarcane derived, soil carbon relative to eucalyptus. Many previous studies have demonstrated that nitrogen additions tend to inhibit humus decomposition in laboratory experiments. This study, however, is the first to examine the possibility that nitrogen additions may decrease carbon turnover in the field.

The ramifications of the findings could have a large 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 any 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.

"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."

This study is part of an ongoing project funded by the National Science Foundation through the University of Hawaii at Hilo. The research team will follow the legacy of these species on the fertility of soils in the next generation of trees.