Rainforest enjoys a dry spell - and confounds scientists

Rather than suffering from a lack of water, the drought-stricken Brazilian Amazon appeared to have flourished during an unusually dry summer.

An Amazon drought led the forest to flourish - in the short term

An Amazon drought led the forest to flourish - in the short term

A team of scientists from Brazil and the USA have been looking at data collected by NASA satellites during 2005 when one of the worst droughts in decades affected the Amazon.

While parched conditions had been expected to slow growth, in drought stricken areas the canopy appeared significantly greener.

"Instead of 'hunkering 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 widely-used global climate model, and intuition, which predict 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 savannah.

"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 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.

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 absorbed 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.

Sam Bond


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