The Future

For those concerned about future biodiversity losses and global climate change, our analyses suggest both worrying trends and some apparently "good news". The Amazon, the world's largest remaining tract of tropical forest, has shown concerted changes in forest dynamics over the past two decades. Such unexpected and rapid alterations—apparently in response to anthropogenic atmospheric change—raise concerns about other possible surprises that might arise as global changes accelerate in coming decades. Tropical forests are evidently very sensitive to changes in incoming resource levels and may show large structural and dynamic changes in the future, as resource levels alter further and temperatures continue to rise (Lewis et al., 2004a). The implication of such rapid changes for the world's most biodiverse region is unknown, but could be substantial.

Moreover, old-growth Amazonian forests are evidently helping to slow the rate at which CO2 is accumulating in the atmosphere, thereby acting as a buffer to global climate change—certainly "good news'' for the moment. The concentration of atmospheric CO2 is rising at a rate equivalent to 3.2 gigatonnes of carbon per year; this would be significantly greater without the tropical South American carbon sink of 0.5 to 0.8 gigatonnes of carbon per year. However, this subsidy from nature could be a relatively short-lived phenomenon. Mature Amazonian forests may either (i) continue to be a carbon sink for decades (Chambers etal., 2001, Cramer etal., 2001), or (ii) soon become a small carbon source because of changes in functional and species composition (Cramer etal., 2001; Phillips etal., 2002b; Korner, 2004; Laurance etal., 2004), or (iii) become a mega-carbon source, possibly in response to climate change (Cox et al., 2000; Cramer et al., 2001). Given that a 0.3% annual increase in Amazon forest biomass is roughly equivalent to the entire fossil-fuel emissions of the European Union (in January 2004), a switch of tropical forests from a moderate carbon sink to even a moderate carbon source would have profound implications for global climate, biodiversity, and human welfare.

Finally, it is important to emphasize that climate-based models that project the future carbon balance in Amazonia (and future climate-change scenarios) have made no allowance for changing forest composition. This omission is likely to lead to erroneous conclusions. For example, lianas contribute little to forest biomass but kill trees and suppress tree growth (Schnitzer and Bongers, 2002), and their rapid increase suggests that the tropical carbon sink might shut down sooner than current models suggest. Large changes in tree communities could undoubtedly lead to net losses of carbon from tropical forests (Phillips and Gentry, 1994; Korner, 2004). The potential scope for such impacts of biodiversity changes on carbon storage is highlighted by Bunker et al. (2005) who explored various biodiversity scenarios based on the tree species at Barro Colorado Island. When slower-growing tree taxa are lost from an accelerated, liana-dominated forest, as much as one-third of the carbon storage capacity of the forest could be lost. Clearly, projections of future carbon fluxes will need to account for the changing composition and dynamics of tropical forests.

Acknowledgements

The results summarized here depended on contributions from numerous field assistants, rural communities, and field-station managers in Brazil, Bolivia, Ecuador, French Guiana, Peru, and Venezuela, and more than 50 grants from funding agencies in Europe and the U.S.A. This support is acknowledged in our earlier publications. Simon Lewis and Yadvinder Malhi are supported by Royal Society University Research Fellowships. Timothy Baker is supported by NERC grant NE/B503384/1 and a Roberts Fellowship at the University of Leeds. We thank M. Alexiades, S. Almeida, L. Arroyo, S. Brown, J. Chave, J. A. Comiskey, C. I. Czimczik, A. Di Fiore, T. Erwin, N. Higuchi, T. Killeen, C. Kuebler, S. G. Laurance, W. F. Laurance, J. Lloyd, A. Monteagudo, H. E. M. Nascimento, D. A. Neill, P. Nunez Vargas, J. Olivier, W. Palacios, S. Patino, N. C. A. Pitman, C. A. Quesada, M. Saldias, J. N. M. Silva, J. Terborgh, A. Torres Lezama, R. Vasquez Martinez, B. Vinceti for contributing data and discussions to earlier papers on which this overview is based.

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