The Heat Is Online

CO2 Changes Rainforest Composition

CO2 Wreaking Havoc with Amazonian Forest

Agence French Presse, March 11, 2004

Carbon dioxide (CO2) disgorged by fossil fuels is silently causing a dramatic change in the composition of tree species in the Amazonian forest, the world's most precious wildlife haven, a study says.

The CO2 is causing some tree species to grow faster and dominate in the forest and this in turn is forcing other species into decline, a long-term change which is bad news for biodiversity and the fight against global warming.

Previous research has already established that rising levels of CO2, the gas pollution that drives man-made global warming, have a big effect on forests.

Trees "breathe" in CO2 as part of the natural process of photosynthesis, storing the carbon in their trunks, branches and leaves.

But the rate at which trees absorb the gas and convert it into growth depends on the species. These factors help determine which species come to dominate in the forest, and how useful the forest is as a "sink" that can soak up the pollution.

In research published on Thursday in Nature, a U.S.-Brazilian team of biologists describe the long-term effects of rising CO2 pollution on virgin Amazonian forest.

They painstakingly marked out 18 one-hectare (2.5-acre) plots in central Amazonia, tagged nearly 13,700 trees with a trunk diameter of more than 10 centimeters (4.5 inches), and monitored the growth of each species' population over the next 20 years.

Of the 115 most abundant species, 27 showed spectacular changes in population density and "basal area"  the amount of land occupied by the trunk of that species, which is 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 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.

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.

"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 authors warned.

"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 subcanopy trees."

Carbon that is taken out of the atmosphere and stored in forests is a contribution to fighting climate change, although the carbon is released when the tree dies and rots.

CO2 hangs in the atmosphere like an invisible blanket, trapping the sun's heat instead of letting it radiate safely back out into space.

The authors added that "serious ecological repercussions" are likely to be in store for Amazonia's fabled biodiversity.

A major reshuffling of the arboreal community could have a disastrous impact on the plants, funghi and animals that live off specific tree species and cannot adapt to other types.

The study is led by William Laurance of the Smithsonian Tropical Research Institute in Panama and the National Institute for Amazonian Research in Manaus, Brazil.

The authors look at other potential factors that could have driven the change in tree composition, such as recovery from human disturbance, vulnerability of some trees to the El Niño phenomenon, or a long-term change in rainfall patterns. But the likeliest cause is rising atmospheric CO2 levels, they believe.