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Last Updated: Monday, 24 September 2007, 13:53 GMT 14:53 UK
Amazonian forest 'more resilient'

Terra satellite. Image: Nasa
The Modis instrument scans the Earth's surface every eight days
The Amazon rainforest may be more resistant to rising temperatures than has been believed.

Researchers found that during the 2005 drought, many parts of the rainforest "greened", apparently growing faster.

This finding contrasts with some computer models of climate change, which forecast that the Amazon would dry out and become savannah.

Writing in the journal Science, the researchers say it is unclear how the forest would respond to a long drought.

"We measured the changes between the drought (of July to September 2005) and an average year," explained study leader Scott Saleska from the University of Arizona, Tucson, US.

Some of the models, in particular the Hadley Centre group, became famous for predicting collapse of the Amazon and a change into savannah
Scott Saleska
"And what we saw was that there was more photosynthesis going on, more capacity to take up carbon dioxide than in an average year," he told the BBC News website.

The scientists used the Modis (Moderate Resolution Imaging Spectroradiometer) instrument on the US space agency's (Nasa) Terra satellite to make their observations.

Some areas of the Amazon had seen reduced growth during the drought, but these were regions heavily impacted by human activities.

Clear skies

It has been thought that stressed trees in drought conditions would try to preserve their water by reducing loss through leaves (transpiration), with this shut-down having a consequent knock-on for photosynthesis.

This, in turn, would be expected to exacerbate the drought by interrupting the supply of water into the atmosphere, a supply which contributes to rainfall.

Aerial view Anama lake, Brazil, amid drought

"Some of the models, in particular the Hadley Centre group (part of the UK Met Office), became famous for predicting collapse of the Amazon and a change into savannah," said Dr Saleska.

"There's a prompt response to the initial drought: trees down (transpiration), they release less water to the atmosphere so there's less to recycle as rain; and in that model world, it pushes the forest over the edge.

"We've tested whether that mechanism is there, and found it's not there on a short timescale. That doesn't mean the forest won't collapse, but it says that the scenario in that model is not right in that particular [situation]."

Although increased photosynthesis in drought conditions might appear counter-intuitive, the group said it could be explained if the trees were still able to access water reserves with deep root systems.

Persistent bright skies during a drought would allow more sunlight through to the leaves, driving photosynthesis and leading to the "greening" seen from space.

Model refinement

Chris Jones from the UK's Hadley Centre for Climate Change commended the study, and said it demonstrated the importance of using real-world observations to challenge and fine-tune the models.

He added that the Hadley and Saleska teams were sharing data.

The British researcher agreed that the satellite images showed up the short-term constraints of the Hadley model, but did not overturn the long-term predictions of his group.

"The key thing here is that the tree roots access water deeper than is often represented in models," he told BBC News.

"In most models, including ours, this goes down to about three metres, which for most of the world's vegetation is fine. But these (Amazon) trees can clearly go deeper than that, so while our model predicts they would suffer during a couple of months of drought, in reality they have access to a much deeper store which doesn't respond on that timescale."

He said, however, that a climatic shift to longer and more frequent drought conditions would eventually diminish the deep-water stores, and make the trees suffer.


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