Even in pristine rainforests unaffected by human activities such as logging or burning, researchers have noticed dramatic differences in the growth patterns of trees over the past 20 years.
That could distort the forest's fragile balance, affecting rare plant and animal species, researchers from Panama and Brazil reported in Thursday's issue of the journal Nature.
"The changes in Amazonian forests really jump out at you," said lead author Dr William Laurance of the Smithsonian Tropical Research Institute in the Republic of Panama and the National Institute for Amazonian Research in Manaus, Brazil.
"It's a little scary to realise seemingly pristine forests can change so quickly and dramatically."
The researchers marked out 18 plots of 2.5 acres in central Amazonia and tagged nearly 13,700 trees with a trunk diameter of more than 10 centimetres. They then monitored the growth and each species' population over the next 20 years.
Of the 115 most common species, 27 showed spectacular changes in population density and basal area, the amount of land occupied by the trunk of that species, a reliable indicator of biomass.
Thirteen species gained in population density and 14 declined; 14 species occupied a greater portion of the land, while 13 species retreated.
The big winners in the fight were spindly canopy trees and shrubs, such as the manbarklak, sclerobium and parkia, which are fast growing and whose wood is of light density.
The losers were slow-growing, dense tropical hardwoods, such as the croton and oenocarpus, that live in the dark forest interior.
"The changes in Amazonian forests really jump out at you. It's a little scary to realise seemingly pristine forests can change so quickly and dramatically"
Dr William Laurance,
Smithsonian Tropical Research Institute
Their decline is significant, because these slow growers are by far the biggest absorbers of carbon. They are the species that give the Amazon its reputation as a vital "sink" that can suck up CO2.
Levels of CO2 have risen by 30% in the past 200 years because of emissions from vehicles and industry, and rapid forest burning, particularly in the tropics.
Much of the increase in CO2, which plants use from the air as a carbon source for photosynthesis, has occurred since 1960.
The scientists suspect the rising CO2 levels are fertilising the rainforests and increasing competition for light, water and nutrients in the soil. So the big fast-growing trees have an advantage and are outpacing the smaller ones.
The researchers believe the odd change in growth patterns could also be a signal for an overall change in rainforest ecology.
The authors looked at other factors that could have driven the change in tree composition, such as recovery from human disturbance, vulnerability of some trees to the El Nino phenomenon, or a long-term change in rainfall patterns.
But they believed the most likely cause was rising atmospheric CO2 levels.