The delicate balance of tree species growing in some of the most remote regions of the Amazon has been altered significantly and the scientists believe it is a direct result of increases in carbon dioxide emissions caused by humans. Small, slower-growing trees that thrived beneath the forest canopy are losing out to faster-growing varieties because the Amazon is in effect being artificially "fertilised" with increased levels of carbon dioxide in the atmosphere.

The result is a change in the composition of the major groups of trees, which could have a long-term impact in the vital role played by the Amazon in soaking up excess carbon dioxide, the scientists said. Amazonian rainforests are renowned for their rich diversity of trees. But some areas appear to be doing better at the expense of others, as atmospheric levels of carbon dioxide continue to rise, said William Laurance of the Smithsonian Tropical Research Institute. For the past two decades, the scientists, whose study is published in the journal Nature, tracked the growth of nearly 14,000 trees in 18 plots of land which were scattered throughout 120 square miles.

Dr Laurance said the researchers deliberately chose the plots because they were remote and well away from any human activity, such as logging and deforestation, but the changes they saw were nevertheless dramatic. He added: "The changes in Amazonian forests really jump out at you. It's a little scary to realise that seemingly pristine forests can change so quickly and dramatically.

"Sadly, this could be a signal that the forest's ecology is changing in fundamental ways. Tropical rainforests are renowned for having lots of high 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."

Alexandre Oliveira, of the University of Sao Paulo in Brazil, said that during the period of the study most trees grew faster, but at different rates, leading to a scenario of winners and losers.

Dr Oliveira said: "In general, large, fast-growing trees are winning at the expense of smaller trees that live in the forest understorey."

The scientists found that out of a total of 115 tree genera in the Amazon, 27 had changed significantly either by increasing or decreasing in growing density, a rate of change that is 14 times greater than expected if it was due to chance.

Henrique Nascimento, a Brazilian scientist with the Smithsonian Tropical Research Institute, said that the decline of smaller, shade-tolerant trees could have important implications for the overall health of the Amazon.

Dr Nascimento said: "The decline of many small trees is intriguing because they tend to be so specialised. They live in the dark interior of the forest and are the only trees that can flower and reproduce in the shade." One possible reason for the change is that rising levels of carbon dioxide - which have increased by 30 per cent in the past 200 years - could be fertilising the trees and causing them to grow faster, especially the larger species with wood that is not as dense as the shade-loving varieties.

If this proves to be the case then the changes could have an important effect on the ability of the Amazon to act as a "sink" for atmospheric carbon, an impact that could exacerbate global warming.

Thomas Lovejoy of the Heinz Centre for Science, Economics and Environment in Washington, which helped to fund the study, said: "This appears to be another signal of effects on nature from increasing greenhouse gas concentrations and climate change. We need more research to see if these remarkable changes are also happening in other tropical forests."

http://news.independent.co.uk/world/environment/story.jsp?story=499968