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.

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."

Broad-Range Study

To further their understanding of the carbon sink and global warming connection, the scientists need a much broader array of data. Tree species found in hundreds of forest plots throughout the region must be identified.

"There is an awful lot of work to do before we know how many species we have in our plots," said Tim Baker, a colleague of Phillips' at the University of Leeds.

Baker should know. He spends a few months each year climbing thousands of trees in the Amazon to collect and identify leaves. "With over 300 species in one hectare (2.5 acres), that's not so easy," he said. "Every other stem is a new species. The biodiversity is huge." To aid the research efforts, Phillips called for scientific institutions working on the Amazon to collaborate by pooling their data. The suggestion led to the establishment of the Amazon Forest Inventory Network (RAINFOR). Scientific information about a broad range of forest types in the Amazon—wet, dry, with rich soil or poor soil—will be compiled into the RAINFOR system.

Last year the network received funding support from organizations in the European Union and the United States, including the National Geographic Society.

Conflicting Theories

The carbon-sink theory is not the only possible explanation for the growth of forest mass. Another theory suggests that the forests are receiving more nutrients than previously as the result of human activities. Forest fires, for instance, are releasing increased amounts of nitrogen into the atmosphere. The nitrogen is absorbed by clouds and returned to Earth as rain, which fertilizes the forests.

Still a third possibility arises from research led by Stephen Pacala, an ecologist at Princeton University in New Jersey. He has studied a similar carbon-sink effect in the United States, which he attributes to the re-growth of forests that were logged at the turn of the 20th century. A similar effect has been seen in China. In the same vein, the Amazon forests may be recovering from earlier logging.

Given the assumption that the Amazon forests are gaining mass as a result of the carbon-sink phenomenon, an important question is how long the effect will last.

Controlled experiments conducted in forest plots at Duke University in North Carolina have shown that the increased carbon absorption benefit of forests resulting from increased carbon dioxide lasts only a few years. The effect becomes limited by the amount of nitrogen in the ground, which trees also need for growth.

Nitrogen supply is unlikely to be a problem in tropical forests, however, because nutrient breakdown from leaf litter is far more rapid than in cooler northern forests, said Baker. "For tropical forests, currently most interest lies in what will happen if dry seasons become longer and more severe," he said.

A computer modeling study done recently by Peter Cox, a scientist at the Hadley Center for Climate Prediction in the United Kingdom, shows that as temperatures rise, forests could dry out and become a source rather than a sink of carbon.

Need for Conservation

For now, data collected by the RAINFOR team suggest that Amazonian forests are acting as a carbon sink as a result of the carbon-fertilization effect. As the project goes forward, the scientists hope to determine how long the effect will last.

"I dont think anyone expects it will go on forever," said Phillips. "This is like a bonus, an unexpected bonus."

Phillips is concerned that the results could lead to complacency and a failure to address the causes of global warming. Like many other scientists, he and Baker believe that if humans want to avoid the worst-case scenario of global warming, they must reduce the burning of fossil fuels.

"The message from this work is that it is just another reason to conserve old-growth forests," said Phillips. "The danger is that it could lull us into a false sense of security. Once that stops, the rate [of global warming] could double."

The research results heighten the importance of what many people agree is a critical need to prevent the loss of Amazonian forests, which have been called "lungs of Earth."

Since the early 1990s, conservationists have called for protecting the Amazon in part because of its abundance of plant and animal species, which may have as yet unknown medicinal value.

The research of Phillips, Baker, and their colleagues suggest another major reason to conserve Amazon forests: They may be acting as a brake against the pace of global warming by absorbing carbon dioxide from the atmosphere.

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