Earth Science and Climate Monitoring | New Study Reinforces Findings on How Natural Tree Loss Affects Carbon Balance

SAN FRANCISCO — Scientists using a combination of satellite, airborne and ground-collected data have concluded that naturally occurring, tree-killing events in South America’s Amazon tropical forests do not seriously undermine the forests’ ability to absorb carbon from the atmosphere.

The new study reinforces previous findings that had been called into question because they were based on studies that did not take into account large-scale events that kill hundreds of thousands of carbon-absorbing trees.

“This is the first time that the full range and scale of forest disturbance has been investigated for old growth forests in the tropics and perhaps anywhere,” said Michael Keller, U.S. Forest Service research scientist and co-author of the study, “Size and Frequency of Natural Forest Disturbances and the Amazon Forest Carbon Balance,” published March 18 in Nature Communications. 

Researchers have known for decades that healthy trees in the Amazon basin absorb carbon dioxide from the atmosphere while dead trees emit carbon dioxide. What they did not know with a high degree of confidence was the relative importance of natural disturbances, including drought, fire and storms, to the overall carbon balance.

Tropical forests are so complex that a team of 21 researchers from five nations spent seven years conducting the study. Each scientist understood a piece of the puzzle. By bringing together all their expertise, the research team was able to produce a comprehensive report on the impact of natural disturbances, including major storm-related events known as blow-downs that can flatten hundreds of thousands of trees, said Fernando Espírito-Santo, lead author of the study and a postdoctoral scholar at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

For many years, the amount of biological material accumulated in Amazon forests has been studied using observations of individual plots of trees. Those observations indicated that the forests took in more carbon dioxide than they produced.

In recent years, however, researchers expressed concern that the plots might not provide an accurate representation of major events taking place across wider swaths of territory.

The new study used a variety of techniques to compare long-term observations of tree plots with satellite imagery and data drawn from airborne lidar. That information was then used to design sophisticated models, which helped researchers make sense of the myriad sources of data.

The study concluded that although blow-downs often destroy a large number of trees at once, they have much less impact on the overall ability of forests to absorb carbon dioxide than far more frequent events affecting smaller numbers of trees. In fact, the study found that these smaller events — confined to forest areas measuring less than 10 hectares — occur so much more frequently that they account for approximately 88 percent of overall trees loss, Espírito-Santo said. 

Researchers used Landsat imagery to determine the impact of blow-downs, such as the severe storms in January 2005 that sent winds as high as 90 kilometers per hour through forests near Manaus, Brazil. That blow-down killed an estimated 300,000 to 500,000 trees, according to a study published in 2010 by researchers at JPL and Tulane University in New Orleans.

The Landsat imagery was combined with information drawn from the Carnegie Airborne Observatory, a Dornier 228 twin-engine turboprop equipped with lidar and imaging spectrometers, which was used to pinpoint smaller gaps in the Peruvian forest canopy caused by dead trees.

Researchers also used high-resolution imagery from DigitalGlobe’s Ikonos commercial satellite to study forest canopies as well as observations from the Moderate-Resolution Imaging Spectroradiometer on NASA’s Terra and Aqua Earth observing satellites to detect the impact of forest fires. In addition, researchers relied on data acquired by the SeaWinds scatterometer on NASA’s QuickScat Earth-observing satellite, which was designed to measure sea surface wind speed and direction, to analyze the water content in forest canopies. 

By combining those varied data sets, researchers were able to refine their understanding of tree growth and death in the Amazon basin. Forests are one of Earth’s most important terrestrial carbon storage systems. One-half of the carbon dioxide produced by natural and man-made sources remains in the atmosphere while the other half is absorbed primarily by oceans and forests. 

When trees are lost to logging, deforestation, fire or blow-downs, their deaths send additional carbon dioxide into the atmosphere and undercut the ability of the forest to absorb carbon dioxide. Since the Amazon basin is home to the world’s largest tropical forest, “we have to make sure we understand it so we can preserve it,” said Sassan Saatchi, co-author of the study and a senior researcher in JPL’s Radar Science and Engineering Section. 

The researchers, many of whom have spent decades studying various aspects of Amazon forests, plan to continue their work. “Our team and other researchers have proposed work to refine the estimates of disturbed areas through more thorough and better designed geographical coverage,” Keller said by email. For example, the researchers are interested in using airborne lidar to study tree mortality in other parts of the Amazon basin, including the forests of Brazil, Espírito-Santo said.