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Fragmentation Quickly Destabilizes Amazon Rain Forest

Amazon rainforest clearing



© COLLART HERVE/CORBIS SYGMA
The towering--and tiny--trees of the Amazon can live for hundreds of years. But a 22-year study of what happens when the rain forest is sliced up by timber cutting, cattle ranching and soy farming has revealed that survivors in various fragments do not last for long. "In just two decades," notes William Laurance of the Smithsonian Tropical Research Institute, who led the study, "a wink of time for a thousand-year-old tree, the ecosystem has been seriously degraded."

Since 1980, researchers have been studying 40 different one-hectare plots in nine rain forest fragments in central Amazonia near Manaus. Covering roughly 32,000 individual trees composed of 1,162 species, 24 of the hectare plots rest near the edges of the remnant fragments while 16 lie deep within intact interiors. Comparing the two reveals that trees located on the edges of such fragments quickly perish, dying nearly three times faster than their interior peers. "When you fragment the rain forest, hot winds from the surrounding pastures blow into the forest and kill many trees, which just can't handle the stress," explains team member Henrique Nascimento of Brazil's National Institute for Amazonian Research. "Also, winds build up around the fragment and knock down a lot of trees."

Although overall tree species richness did not change over the two decades of the study, the type of species that predominated at the edges changed radically: from specialized trees capable of persisting in the dark understory to so-called generalist species. "These species are fast-growth, short-lived species with low wood density," Nascimento explains, such as Cecropia sciadophylla, which has increased by more than 3,000 percent after fragmentation. Such edge fragments are also highly unstable, with one species replacing another in rapid succession, and the trees themselves remain generally smaller than their undisturbed, towering brethren in the interior.

The biggest losers appear to be those trees specifically adapted to the shady undergrowth of virgin rain forest whose seeds are spread by some of the Amazon's many species of animals. This, of course, means that those animals are potentially similarly affected. "In the long term, such wide-ranging disruptions could pose an important threat to tropical biodiversity, given the myriad ecological linkages among rainforest trees and their many dependent animal, plant and fungal species," the researchers conclude in their paper presenting the findings, published online November 27 in the Proceedings of the National Academy of Sciences USA. "These changes occur remarkably fast," Laurance adds. "And when you completely alter something as basic as the trees, the other species that live in the rain forest will surely be affected too."

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