The Heat Is Online

Tropical Forests May Soon Turn from Sinks to Sources

Global Warming is Changing Tropical Forests
Environmental News Service, Aug. 7, 2002

Human activities are changing the global climate, and these changes are having far reaching effects on tropical forests, according to scientists from around the world gathered here last week for the Association for Tropical Biology annual meeting.

The scientists were hosted in Panama City by the Smithsonian Tropical Research Institute. They explored the Smithsonian's tropical biology research station at Barro Colorado, located on the hilltop that became an island when central Panama was flooded during the construction of the Panama Canal in 1911.

The Association for Tropical Biology says that tropical forests are undergoing unprecedented changes as 1.2 percent of the remaining forest is removed each year, as atmospheric carbon dioxide which fuels plant growth increases by 0.4 percent each year, and as global climate change begins in earnest.

Yadvinder Mahli from the University of Edinburgh's Institute of Ecology and Resource Management 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, Mahli said, 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. No global trend in dry season intensity has been detected.

Higher global temperatures and increases in atmospheric carbon dioxide, the major greenhouse gas, will increase the amount of carbon stored by tropical forests by stimulating tree growth, data analysis and models have suggested.

University of Missouri scientist Deborah Clark, who works at the La Selva Biological Station in Costa Rica, re-evaluated the evidence and told the symposium that tropical forests may not be carbon sinks that can be used to absorb carbon dioxide generated by the burning of fossil fuels.

Instead, tropical forest may end up contributing even more carbon dioxide (CO2) to the atmosphere as temperature rises, she said.

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 carbon dioxide results, Clark suggests.

Positive feedback between higher temperatures and CO2 production by tropical forests could be catastrophic by resulting in accelerated increase in global CO2 levels, she said.

Dr. Oliver Philips of the University of Leeds School of Geography presented analyses, conducted with Malhi and others, 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, he said, from one percent annually in the 1950s to two percent in the 1990s.

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, said Dr. Philips.

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