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THE pristine rainforests of the Amazon are changing. And this time the loggers aren鈥檛 to blame. Taller, fast-growing canopy trees are becoming more common, and the slow-growing trees of the understorey are becoming rarer.

The subtle shift may be caused by increasing carbon dioxide in the atmosphere, and it threatens to reduce the Amazon basin鈥檚 role as a sink for excess CO₂.

This discovery comes from a study of 18 undisturbed plots in the heart of the Brazilian Amazon that have been monitored since the 1980s. Tropical ecologist Bill Laurance of the Smithsonian Tropical Research Institute in Balboa, Panama, and his colleagues combed through census records of the plots and found that, of the 115 tree genera that were common enough to provide good samples, two had become significantly more common and 14 significantly rarer over the 15 years of the study. The winners tended to be tall, relatively fast-growing canopy trees, while the losers tended to be slower-growing trees that live in the dim depths of the forest below the canopy (Nature, vol 428, p 171).

Laurance and his team could find no past disturbances or anomalies in rainfall that might explain this pattern of change. Nor is it due to some peculiarity of their research techniques as a second, independent study on plots nearby showed similar changes. Instead, Laurance conjectures that rising levels of CO₂ in the atmosphere may be to blame. More CO₂ means faster photosynthesis 鈥� and the trees on the plots grew faster in the second half of the study period than in the first. Faster photosynthesis, in turn, favours those trees that are already the fastest growers.

鈥淚t鈥檚 very difficult to predict what the long-term effects will be,鈥� says Laurance. However, because understorey trees grow slowly, they produce denser wood, which means the carbon content of each tree is greater. If the composition of the forest shifts away from these trees toward faster-growing trees with lighter wood, the forest will not absorb so much CO₂.