Scientists have sounded the alarm after spotting changes in the environment in Brazil's tropical rain forests.
Some trees are beginning to dominate the forests
They say they have found worrying signs that the forests may become less able to absorb the carbon dioxide emissions blamed for global warming.
Their long-term study in supposedly pristine areas reveals trees have been growing and dying faster than before.
The trends could have consequences for other plant and animal species, the authors warn in the journal Nature.
The Amazon accounts for more than half of the world's remaining rainforests.
They are thought to limit the greenhouse effect of global warming by storing carbon and hence reducing the amount of carbon dioxide in the atmosphere.
Scientists from Brazil and the US detected the changes while engaged on a 20-year study of the effects of rain forest clearance by humans.
They marked out plots in the central Amazon region, tagged almost 14,000 trees with a girth of more than 10 centimetres (four inches) and monitored their growth.
But it was in "control" areas, where there was no human activity such as logging or burning, that they discovered that bigger, quicker-growing species were flourishing at the expense of the smaller ones living below the forest canopy.
"It is clear that this is not random variation. Rainforest dynamics are changing," said William Laurance, of the Smithsonian Tropical Institute, who led the study.
Plant growth requires carbon dioxide, and the researchers speculate that the Amazon trees are getting an extra boost from the rising levels of the gas caused by vehicle exhausts, factory emissions and other industrial processes.
Call for more research
"Undisturbed Amazonian forests appear to be functioning as an important carbon sink, helping to slow global warming, but pervasive changes in tree communities could modify this effect," the study says.
"In particular, increases in forest carbon storage may be slowed by the tendency of canopy and emergent trees to produce wood of reduced density as their size and growth rate increases, and by the decline of densely wooded sub-canopy trees."
The changing composition of supposedly "untouched" rain forest would also have an effect on other inhabitants.
"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," Mr Laurance was quoted as saying.
The scientists say their findings could have global consequences and that more research is urgently needed in other tropical forests.