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Amazonian rainforests are undergoing striking changes in in the way in which they grow and the species that they shelter, even in areas not directly affected by human activities such as logging or burning, according to a new study by Brazilian and US scientists.

Rising levels of carbon dioxide in the atmosphere are the most likely explanation, the researchers say. Plant growth requires carbon dioxide, so increased levels caused by industrial emissions, automobiles and forest burning may have allowed fast-growing species to gain the upper hand over their rivals.

"The changes in Amazonian forests really jump out at you," says William Laurance, a US scientist with the Smithsonian Tropical Research Institutes in Panama, who led the research, the results of which are published in this week's Nature. "It's a little scary to realise that seemingly pristine forests can change so quickly and dramatically."

The researchers studied nearly 14,000 trees scattered over 120 square miles over a period of two decades. They found that during the course of the study, most species of trees began to grow faster, and the species composition of the forest changed. The forests also became more 'dynamic', with existing trees dying faster and being replaced by young new trees.

"Sadly, this could be a signal that the forest's ecology is changing in fundamental ways," says Laurance. "Tropical rainforests are renowned for having lots of highly specialised species. If you change the tree communities then other species — especially the animals that feed on and pollinate the trees — will undoubtedly change as well."

These changes in the forest could exacerbate climate change, the researchers warn. Rainforests are thought to limit the greenhouse effect by storing carbon in their tissues, thus reducing the amount of carbon dioxide in the atmosphere. But fast-growing trees are less densely wooded than slow-growing ones, and may therefore take up less carbon.

"This appears to be yet another signal of effects on nature from increasing greenhouse gas concentrations and associated climate change," says Thomas Lovejoy of the Heinz Centre for Science, Economics and Environment in Washington, DC, who helped establish the tree study.

"We really need more research to see if these remarkable changes are also happening in other tropical forests around the world. If they are, then it's likely that even the world's remotest forests are now being altered by human activities.

Link to paper by Laurance et al in Nature 

Reference: Nature 428, 171 (2004)