Amazon Droughts and Greening

The sensitivity of Amazon rain forests to dry season droughts is still poorly understood, with reports of enhanced tree mortality and forest fires on one hand, and excessive forest greening on the other. In a current story there is a report that previous conclusions of large scale greening of the Amazon as a result of drought are not reproducible. Approximately 11%-12% of these drought stricken forests display greening, while, 28%-20% show browning or no change, and for the rest, the data are not of sufficient quality to characterize any changes. These changes are also not unique; approximately similar changes are observed in non-drought years as well.

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The Amazon drought of 2005 was the worst ever recorded in the Amazon. The drought has turned rivers into grassy mud flats, killed tens of millions of fish, stranded hundreds of communities, and brought disease and economic despair to the region.  There have been other droughts such as in 1926, 1983, and 1998.  These other droughts can be more linked to El Nino effects.    

Scientists are not certain as to the cause of the 2005 drought, although warmer water temperatures in the Atlantic Ocean are the leading suspect. Wet tropical forests are the most species rich biome, and tropical forests in the Americas such as along the Amazon River are consistently more species rich than the wet forests in Africa and Asia. As the largest tract of tropical rain forest in the Americas, the Amazonian rain forests have unparalleled biodiversity.  However extensive deforestation has occurred in the last few decades and the 2005 drought did not help.

There had been earlier claims that the 2005 drought caused a "greening" of the Amazon.  Tied to this thought was that available sunlight increased in this area.  In the March Geophysical Letters an article was published by several authors entitled:  "Amazon Forests Did Not Green-up in the 2005 Drought".  The authors included Arindam Samanta and Ranga B. Myneni.

In this the authors concluded that only about 10% of the affected area increased in greenness and about three times this ares became browner.  The majority of the affected areas could not be determined.  At the same time sunlight (in the wavelengths most useful for plant life) decreased rather than decreased in most areas.

There was no co-relation between drought severity and greenness changes, which is contrary to the idea of drought induced greening.

Finally the study concluded that the spatial patterns of Enhanced Vegetation Index changes seen in drought year 2005 are not unique in comparison to non‐drought years.

For further information:  http://www.agu.org/pubs/crossref/2010/2009GL042154.shtml