

Spatial extent and severity of the 2005 Amazonian drought based on forest canopy water stress (measured by seasonal [JAS] standardized anomaly of QSCAT backscatter data). Courtesy of Saatchi et al 2012.



The impact of a major drought in the Amazon rainforest in 2005 persisted far longer than previously believed, raising questions about the world’s largest tropical forest to cope with the expected impacts of climate change, reports a new study published in the journal Proceedings of the National Academy of Sciences.

The research is based on analysis of rainfall observations and satellite data that measures characteristics of the forest, including water content in leaves and the overall structure of the canopy. The scientists, led by Sassan Saatchi of Caltech, looked at the response of the rainforest to the 2005 drought, which was the worst Amazon drought on record at the time.

They found that more than 70 million hectares of forest in the western Amazon — a region that is generally viewed as less vulnerable to drought than other parts of the Amazon — “experienced a strong water deficit during the dry season of 2005 and a closely corresponding decline in canopy structure and moisture”. Surprisingly the drought’s impact was evident for more than four years after the drought, with lower canopy moisture and density persisting until the 2010 drought, which was even worse than the 2005 event.

The findings lend credence to troubling concerns about the future of the Amazon forest, which appears to be experiencing an uptick in drought conditions due to rising temperatures in the tropical Atlantic.

“If droughts continue to occur at 5–10-year frequency, or increase in frequency,large areas of Amazonian forest canopy likely will be exposed to the persistent effect of droughts and the slow recovery of forest canopy structure and function,” write the authors. “In particular, areas of south and western Amazonia have been shown to be affected severely by

increasing rainfall variability in the past decade, suggesting that this region may be witnessing the first signs of potential large-scale degradation of Amazonian rainforest from climate change.”



The Amazon rainforest in Peru. Photo by Rhett A. Butler







The results add to the growing body of evidence that the Amazon may be vulnerable to a “tipping point” where the ecosystem could shift from closed-canopy rainforest to more open woodlands and savannas should current warming and deforestation trends continue. Other researchers have warned about the dangers of interacting threats to the Amazon, which could drive a feedback loop leading to increased forest conversion, fire, and drought in the region.

CITATION: Sassan Saatchi et al. “Persistent effects of a severe drought on Amazonian forest canopy”. PNAS Dec 28, 2012 http://www.pnas.org/cgi/doi/10.1073/pnas.1204651110

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