'Pristine' Amazonian rainforests are changing

18:00 10 March 04
NewScientist.com news service

Changes to apparently pristine parts of the Amazonian rainforests have been revealed by a survey of trees. Tall, fast-growing trees are doing better than those that grow slowly.

The changes - possibly the result of increasing levels of carbon dioxide in the atmosphere - threaten to reduce the Amazon basin's crucial ability to store excess CO2.

The discovery comes from a study of 18 undisturbed plots in the heart of the Brazilian Amazon that researchers have been monitoring 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 to see how their trees have changed over time.

Of 115 tree genera, they found that two became significantly more common and 14 became 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 their whole lives in the dim depths of the forest below the canopy.

Laurance and his colleagues could find no past anomalies in rainfall that might explain this pattern of change. Nor is it due to some peculiarity of their research techniques - a second, independent set of study plots nearby showed similar changes.

Dense wood
Instead, Laurance speculates that rising levels of CO2 in the atmosphere may be to blame. More CO2 means faster photosynthesis. Faster growth, in turn, favours those trees that already grow fastest.

"It's very difficult to predict what the long-term effects of these changes will be," says Laurance. However, because understorey trees grow slowly, they produce denser wood, which in turn means that the carbon content of each tree is greater.

If the forest composition shifts away from these trees toward faster-growing genera with lighter wood, the forest will be less able to take up CO2.

Many ecologists think the Amazon rainforest is one of the major "carbon sinks" that keep atmospheric CO2from rising more quickly than it already has.

If anything were to interfere with that, says Oliver Phillips, a tropical forest ecologist at the University of Leeds, UK, "that's bad news".

Journal reference: Nature (vol 428, p 171)

Bob Holmes