Tropical Forests Not Likely To Limit Expected Rapid Rise In Carbon Dioxide, Major Study Suggests

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http://www.sciencedaily.com/releases/2008/03/080330214448.htm

More than two million trees belonging to nearly 5000 species, growing in tropical forests spread over 12 sites and three continents, have been monitored since the 1980s. The aims of this major study were to analyze the carbon storage capacity of tropical forests and measure the effects of climate change on how they function. This work was carried out by an international team, coordinated by Jérôme Chave(1), a CNRS researcher. Their results suggest that the tropical forests studied did indeed act as carbon sinks, but appeared to react principally to intrinsic phenomena rather than climate change. They also demonstrated the complex functioning of forest ecosystems, their vulnerability and the importance of efforts to ensure their conservation.

Tropical forests account for nearly two-thirds of terrestrial biodiversity and store more than half of the carbon in the biosphere. Recent studies have predicted that in a carbon dioxide-enriched environment, physiological changes will affect tropical plants; their functioning will be modified, their biomass will increase and they will sequester more carbon(2). Under these conditions, rapidly-growing tree species should be favored over slow-growing species, and globally, the carbon sinks represented by tropical forests should contribute to limiting atmospheric emissions from fossil fuels.
The international research group led by the Smithsonian Tropical Research Institute, and coordinated by Chave, tested these hypotheses for the first time using forestry inventories initiated in the early 1980s. This project involved 38 scientists from 15 countries. More than two million trees (with a diameter of more than 1 cm) belonging to nearly 5000 species were inventoried. The scientists developed novel statistical methods which, for a given species, made it possible to estimate the biomass of trees as a function of wood density and trunk diameter. They were also able to define two groups of species: rapid-growing and slow-growing. For each studied plot, carbon assessments were performed at the scale of the ecosystem and for both of these groups of species.

Chave and his colleagues confirmed that carbon storage capacity had increased significantly during the last two decades. Ancient tropical forests are thus indeed major carbon sinks. What is the mechanism underlying this carbon sequestration? At all sites but one, the biomass of slow-growing species had increased, but not that of rapid-growing species(3). There was thus no clear evidence that tropical forests have modified their functioning in response to climate change over the past twenty years. Indeed, these results tend to suggest that the forests are now rebuilding themselves after disturbances in the past. Consequently, tropical forests will not be able to limit the rapid rise in atmospheric carbon dioxide levels for a long time to come.

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