鈥淚n just two decades 鈥� a wink of time for a thousand-year-old tree 鈥� the ecosystem has been seriously degraded.鈥� So says William Laurance, an ecologist at the Smithsonian Tropical Research Institute in Balboa, Panama. It鈥檚 a familiar tale of ecological woe, but with a twist. After loggers and ranchers have cleared parts of the tropical rainforest, even those forest fragments that remain change far more rapidly than expected.
The finding comes from the longest-ever study of forest fragmentation, which has been running in central Brazil since the early 1980s. Researchers have been conducting censuses of tree species in 40 1-hectare rainforest plots, each within fragments ranging in size from 1 to 100 hectares.
Laurance and colleagues combed through these surveys to compare plots near forest edges against those in mature forest far from edges. The edge plots, they found, were more likely to lose their original tree species because of wind damage and drought, and were also more likely to gain fast-growing colonising species (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0609048103).
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These changes make the forest more liable to degradation than undisturbed forest, even though the total number of tree species on each plot remains about the same. Moreover, the new trees tend to be smaller and have less dense wood than those they replace, so the change represents a net loss of carbon storage 鈥� an ominous trend for forests鈥� ability to buffer global warming.
