Undisturbed Amazonian forests are changing, say scientists

A research team of U.S. and Brazilian scientists has shown that rainforests in central Amazonia are experiencing striking changes in dynamics and species composition. Although the cause of these changes-in what are believed to be completely undisturbed, old-growth forests-is uncertain, a leading explanation is that they are being driven by rising levels of carbon dioxide in the atmosphere.

Carbon dioxide levels have risen by 30% in the last 200 years as a result of industrial emissions, automobiles, and rapid forest burning, especially in the tropics. Much of this increase has occurred since 1960. Plants use carbon dioxide from the air for photosynthesis.

"The changes in Amazonian forests really jump out at you," said William Laurance, a U.S. scientist with the Smithsonian Tropical Research Institute in Panama. Laurance was the lead author of the paper, which appeared this week in the scientific journal Nature (Mar 11). "It's a little scary to realize that seemingly pristine forests can change so quickly and dramatically."

For the past two decades, the research team studied the fate of nearly 14,000 trees in the central Amazon, scattered across a landscape of 120 square miles in area. During the course of the study, most species of trees began growing faster. The forests also became more dynamic, with existing trees dying faster and being replaced by young new trees.

Even more important is that the species composition of the forest is changing. "There clearly are winners and losers," said Alexandre Oliveira of the University of So Paulo, Brazil, another team member. "In general, large, fast-growing trees are winning at the expense of smaller trees that live in the forest understory."

"The decline of many small trees is intriguing because they tend to be so specialized," said Henrique Nascimento, a Brazilian researcher at the Smithsonian Tropical Research Institute. "They live in the dark interior of the forest, and are the o

Contact: Dr. William F. Laurance
Smithsonian Tropical Research Institute

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