Adult Amazon Trees Gain Mass, Puzzle Scientists

John Roach
for National Geographic News
December 13, 2001

Research has shown that mature forest trees in the Amazon have gained in size over the last 20 years, but scientists aren't sure what's causing it. Nor do they know what effect it might have on global warming, although tropical forests in the Amazon are an important component in the global climate and water cycle.

An international team of researchers has formed a partnership to try to find out some of the answers.

The issue has been a subject of intense interest since 1998, when Oliver Phillips of the University of Leeds in the United Kingdom published the results of a study showing that mature forests in the Amazon have become more massive over the last 20 years.

Phillips hypothesizes that the Amazon forests have become more massive as a result of increased amounts of carbon dioxide in the atmosphere, which the trees and vegetation absorb.

The burning of fossil fuels such as gas and coal have greatly increased the amount of carbon dioxide released into the atmosphere over the past 50 years. Carbon dioxide is a major greenhouse gas, which contributes to global warning by trapping heat in the atmosphere.

The increased mass of Amazonian forests suggests they are acting as a huge "carbon sink," absorbing more carbon from the atmosphere than they release. The discovery was unexpected because mature forests were thought to be "carbon neutral," giving off as much carbon as they absorb each day.

Since Phillips' paper was published, many researchers have been eager to determine the relationship between the carbon-sink role of the Amazon and global warming.

The task is difficult because of the size and variation of the Amazonian forests.

One of the limits to Phillips' 1998 study was that the research plots where data was collected were in only one part of the Amazon. "Of the whole Amazon area, we had one figure [of carbon absorption] per hectare," he said. "The ecological reality is there is tons of variation."

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