September 21, 2007
Amazon Forest Unexpectedly Resilient to Drought
The extensive forests of South America's Amazon are turning out to be tougher than expected when it comes to withstanding the onslaughts of a changing climate. A team of U.S. and Brazilian scientists using the insightful eyes of two NASA satellites has shown that one of the worst droughts in decades could not stop the undisturbed regions of the Amazon forest from "greening up."
The Amazon drought of 2005 reached its peak just as the region's annual dry season was beginning, from July through September. Although the double whammy of the parched conditions might be expected to slow the growth of the forest's leafy canopy, in much of the drought-stricken areas the canopy became significantly greener -- an indication of increased photosynthetic activity.
"Instead of "Ëhunkering down' during a drought as you might expect, the forest responded positively to drought, at least in the short term," said study author Scott R. Saleska, an assistant professor of ecology and evolutionary biology at the University Of Arizona. "It's a very interesting and surprising response."
The new finding contradicts a prominent global climate model that predicts the Amazon forest would begin to "brown down" after just a month of drought. The model also predicts an eventual forest collapse, shifting the ecosystem permanently from a thick, evergreen, broad-leaved forest to a grassy savanna.
"No one had looked at this issue with observations that are available from satellites," said co-author Kamel Didan, an associate research scientist in the University of Arizona's department of soil, water and environmental science. "We took the opportunity of the most recent drought, the 2005 drought, to do so."
"A big chunk of the Amazon forest -- the southwest region where the drought was severest -- reacted positively," said Didan. "The forest showed signs of being more productive. That's the big news." The new study, which was supported by NASA research funding, was published on Sept. 20 in Science Express, the online version of the journal Science.
The researchers and their colleagues already knew the Amazon forest took advantage of the annual dry season's relatively cloudless skies to soak up the sun and grow. From a previous study that used NASA satellite data combined with additional field measurements, the researchers found that intact Amazon forest increases photosynthesis, actually "greening up," during the dry season.
The severe 2005 drought and the detailed, long-term observations from two NASA satellites -- one that maps the greenness of vegetation, one that measures rainfall in the tropics -- gave the researchers what they needed to see how the Amazon forest responds to a major drought.
One of the instruments on NASA's Terra satellite, launched in 1999 -- the Moderate Resolution Imaging Spectroradiometer, or MODIS, "“ provides month-to-month maps of changes in vegetation status across the Amazon (and the world). The one-of-a-kind Tropical Rainfall Measuring Mission spacecraft, launched in 1997, collects observations of rainfall.
The scientists used the seven-to-nine years of observations from these satellites to map "normal" rainfall and greenness conditions in non-drought years. When they compared those conditions to the same months of the 2005 drought, the researchers found that areas of Amazon's intact forests that had received below-normal rainfall in 2005 also had above-average greenness.
Global climate models predict the Amazon forest will cut back photosynthesis quickly when a drought starts. That slowdown in plant growth would create a positive feedback loop: as the forest shuts down more and more, it removes less and less carbon dioxide from the atmosphere. The carbon dioxide ordinarily sequestered by growing trees would remain in the atmosphere, increasing global warming and further accelerating the forest's decline and additional carbon-dioxide-fueled warming.
By contrast, the research team's findings suggest the opposite happens, at least in the short-term. The drought-induced flush of forest growth would dampen global warming, not accelerate it. During the 2005 drought, Amazon forest trees flourished in the sunnier-than-average weather, most likely by tapping water sources deep in the forest soil. To grow, trees must take up more carbon dioxide, thus drawing down the levels of atmospheric carbon dioxide.
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