The megadrought in the Amazon rainforest during the summer of 2005 caused widespread damage and die-offs to trees, as depicted in this photo taken in western Amazonia in Brazil. Photograph: JPL-Caltech/Nasa
With little time for the trees to recover between what the authors describe as a "double whammy", 70m hectares of forest have been severely affected, the analysis of 10 years of satellite microwave radar data revealed.
The data showed a widespread change in the canopy due to the dieback of branches, especially among the older, larger trees that are most vulnerable because they provide the shelter for other vegetation.
"We had expected the forest canopy to bounce back after a year with a new flush of leaf growth, but the damage appeared to persist right up to the subsequent drought in 2010," said study co-author Yadvinder Malhi of Oxford University.
The Amazon is experiencing a drought rate that is unprecedented in a century, said the agency. Even before 2005, water availability had been shrinking steadily for more than 10 years, which made the trees more vulnerable. Between 2005 and 2010, localised dry spells added to the problem.
The leader of the research team, Sassan Saatchi of Nasa's Jet Propulsion Laboratory, said forests will find it increasingly difficult to recover if climate change makes droughts more frequent and severe.
"This may alter the structure and function of Amazonian rainforest ecosystems," he warned.
Although the speed of forest clearance has slowed, the Amazon continues to shrink in area. The latest study suggests the quality as well as the quantity of forest is declining due to extreme climate conditions.At left, the extent of the 2005 megadrought in the western Amazon rainforests during the summer months of June, July and August as measured by Nasa satellites. The most impacted areas are shown in shades of red and yellow. The circled area in the right panel shows the extent of the forests that experienced slow recovery from the 2005 drought, with areas in red and yellow shades experiencing the slowest recovery. Photograph: GSFC/JPL-Caltech/Nasa