The effects of human-caused atmospheric changes on tropical forests

August 02, 2002

Panama City, Panama - For more than a century humans have been changing the composition of the world's atmosphere through the burning of fossil fuels, agriculture, and other activities. The resulting climate changes may already be having far-reaching impacts on tropical forests. A symposium at the 2002 meetings of the Association for Tropical Biology, hosted by the Smithsonian Tropical Research Institute (STRI), Panama, examined the evidence for these changes and their implications for the future.

Organized by Yadvinder Mahli of the University of Edinburgh and Oliver Philips of the University of Leeds, the symposium included 12 presentations and four general discussion sessions. A selection of some of the results follows.

Yadvinder Mahli provided an overview of ongoing climate changes as a result of increasing carbon dioxide and other greenhouse gases in the atmosphere. Since the mid-1970s all tropical forest regions have warmed, although with regional variation in intensity. There has been even more regional variation in precipitation, but there appears to have been an overall global decline. However, no global trend in dry season intensity has been detected.

Data analysis and models have suggested that increased temperature and atmospheric CO2 will increase the amount of carbon stored by tropical forests by stimulating tree growth. Deborah Clark of the La Selva Biological Station, Costa Rica, re-evaluated the evidence to suggest that tropical forests may not be carbon sinks after all, but instead end up contributing even more carbon dioxide to the atmosphere as temperature rises. Data from La Selva show a strong negative correlation between tree growth and higher temperatures. Temperatures experienced by canopy leaves may be close to the point at which respiration exceeds photosynthesis so that net production of CO2 results. Positive feedback between higher temperatures and CO2 production by tropical forests could be catastrophic by resulting in accelerated increase in global CO2 levels.

Tropical forests throughout the world appear to be changing significantly in structure, dynamics, and composition. Oliver Philips presented analyses (with collaborators T. Baker, S. Lewis, Y. Malhi, N. Higuchi, T. Killeen, W. Laurance, D. Neill, N. Silva, R. Vasquez, and B. Vincenti) of data from permanent plots in mature forests throughout the tropics. Tree turnover (the difference between mortality and the recruitment of new individuals into the population through growth) has doubled throughout the tropics in recent decades, from 1% annually in the 1950s to 2% in the 1990s. Basal area (the total area of the plot occupied by tree stems) has increased in Amazonia, but not in the rest of the tropics, and large lianas have increased in western Amazonia. Such widespread changes over such large areas suggest that a common mechanism is at work.

How resistant are tropical forests to declining precipitation? Daniel Nepsted (with collaborators P. Motinho, M. Dias-Filhod Ray, D. Ray, L. Solorzano, G. Gardinot, and I. Tohver) experimentally reduced the rainfall reaching the ground in forest plots in Amazonia by 40%. The growth of smaller trees declined within a few weeks, and their mortality increased three-fold over two years. However, litter fall increased only after two years of treatment. This forest apparently avoided drought-induced leaf-shedding and adult tree mortality for two years by tapping soil moisture to a depth of 20 meters.
-end-


Smithsonian Tropical Research Institute

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