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.
The source of these disturbances to tropical forests, whose effects we are still seeing, remains a mystery. One hypothesis put forward sometimes is that man has been altering these forests for longer than is generally thought. However, climatic events or processes intrinsic to the ecosystem (e.g. attacks by pathogenic agents) could have exerted the same effects. A more detailed study of these natural disturbances would provide a clearer understanding of the consequences of human activities on tropical forests.
The observation of long-term variations at the sites studied suggests notably that these forests are particularly susceptible to episodes of drought. And some climate forecasting models have predicted a reduction in rainfall over tropical forests in the decades to come. Thus even if climate change was not shown by this study to have affected the functioning of these forests, it is probable that this factor will play an increasingly important role in the future…. It is therefore essential to pursue such large-scale studies and to ensure the conservation of tropical forests.
(1) Jérôme Chave is a research scientist in the Laboratoire Evolution et Diversité Biologique (CNRS/University of Toulouse 3/Ecole Nationale de Formation Agronomique).
(2) Forest biomass is the quantity of living material represented by trees in a forest, all components taken into account (roots, branches, leaves, etc.). Half of this biomass is made up of carbon.
(3) The results obtained at just one site contradicted this general trend, in the Sinharaja Forest of Sri Lanka. This forest displayed a decline in growing biomass and an increase in rapidly-growing species. But more detailed inspection of the results revealed that a single species of canopy trees, called locally the "Ceylon ironwood" (Mesua nagassarium), has seen a dramatic decline in its population, perhaps due to an attack by specific pathogens.
Journal reference: Assessing evidence for a pervasive alteration in tropical tree communities. Jérôme Chave, Richard Condit, Helene C. Muller-Landau, Sean C. Thomas, Peter S.Ashton, Sarayudh Bunyavejchewin, Leonardo L. Co, Handanakere S. Dattaraja, Stuart J. Davies, Shameema Esufali, Corneille E.N. Ewango, Kenneth J. Feeley, Robin, B. Foster, Nimal Gunatilleke, Savitri Gunatilleke, Pamela Hall, Terese B. Hart, Consuelo Hernández, Stephen P. Hubbell, Akira Itoh, Somboon Kiratiprayoon, James V. LaFrankie, Suzanne Loo de Lao, Jean-Rémy Makana, Md. Nur Supardi, Noor, Abdul Rahman Kassim, Cristián Samper, Raman Sukumar, Hebbalalu S.Suresh, Sylvester Tan, Jill Thompson, Ma. Dolores C. Tongco, Renato Valencia, Martha Vallejo, Gorky Villa, Takuo Yamakura, Jess K. Zimmerman, Elizabeth C.Losos. Plos Biology. 4 March 2008.
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