A Dry Year in the Amazon Rainforest

by Laura Klappenbach in Nature Notes

The Amazon rainforest is a moist broadleaf forest that blankets 5,400,000 square kilometers of the Amazon River basin in South America. The shear vastness of this forest is difficult to comprehend. It stretches across the boundaries of nine nations—Brazil, Colombia, Peru, Venezuela, Ecuador, Bolivia, Guyana, Suriname, and French Guiana. Its biodiversity is unparalleled—an estimated one in ten animals on the planet inhabits the Amazon rainforest.

The staggering proportions of the Amazon rainforest earns it high rank among the planet's most significant biological repositories of carbon. The old-growth forests of the Amazon basin store an estimated 120 Pg of carbon in their biomass—that's 1.2 x 1017g of carbon neatly locked-up in rainforest roots, trunks, branches, and leaves (Malhi 2008). The Amazon rainforest is for this reason a key stockpile of carbon, it is an immense carbon sink. But its storage of carbon is anything but stagnant.

Like all forests, the Amazon rainforest breaths. It inhales sunlight and carbon dioxide through photosynthesis. It exhales carbon dioxide through respiration and decomposition. This cycle is quietly comforting if you envision the forest as a large, slumbering organism. Comforting, that is, until you realize that this living organism, this carbon behemoth, is capable of losing its breath. If stressed, it might inhale less or exhale more. It may transform from a carbon sink into a carbon source, pumping carbon dioxide skyward at rates that exceed its uptake of the notorious greenhouse gas.

But for a moment, let's put talk of vast forests and carbon sinks aside and simply consider a single leaf. The leaf I would like to consider is pictured at the right. It is a moisture-stressed leaf and appears to be the only leaf left clinging to a young but fast-fading sapling. It was photographed in November 2005 in the Columbian Amazon during the worst drought to strike the Amazon basin in 100 years.

When this leaf inevitably dropped to the ground and the sapling died, it ceased taking up carbon dioxide from the air to photosynthesize. It decomposed and gradually returned the carbon stored in its cells back into the atmosphere in the form of carbon dioxide. When a single, tiny sapling parches and fades in this manner, the carbon dioxide it releases is minute. But when extreme drought causes widespread leaf loss and tree death, the volume of carbon dioxide that is released may be enough to swell atmospheric carbon dioxide concentrations.

The drought that descended upon the Amazon River basin in 2005 was one such extraordinary event. Its more visible effects were well documented in the mainstream media (see here, here, here, and here for a few examples). There were reports of rivers that had turned to mudflats, boats that became stranded whilst waterways evaporated, and thousands of fish that perished amidst receding currents. At the time, the media attributed the lack of rainfall in the Amazon to climate change, deforestation, and abnormally warm sea surface temperatures in the North Atlantic Ocean. But from a scientific perspective, the true causes and effects of the drought would require methodical analysis to understand.

In 2008, a careful analysis of climatic data surrounding the 2005 drought event was published in Environmental Research Letters by a team of scientists lead by Ning Zeng of the University of Maryland. The figures below (from Zeng 2008) illustrate two key events that occurred in 2005, an abnormally dry Amazon River basin and an abnormally worm tropical North Atlantic Region.

Figure (a) above illustrates just how dry the Amazon basin was in 2005 as compared to mean rainfall measurements for the region over the preceding 25 years (1979-2005). Where the shaded areas are the darkest brown, rainfall deficits were the most pronounced. The regions where the drought was the most devastating were in the western and southern parts of the Amazon basin. Figure (b) above shows the abnormally warm sea surface temperatures in the North Atlantic region, with the more elevated temperatures marked by increasingly darker shades of red.

These two images—one of a dry Amazon basin and another of elevated Atlantic sea surface temperatures—provided the backdrop against which Dr. Oliver Phillips from the University of Leeds and his colleagues ventured into the depths of the Amazonian rainforest. They visited 55 study sites—plots that had been monitored for 25 years as part of the long-term research project RAINFOR. They collected data (such as tree diameter and wood density) at each location and compared it to that of past years. What they found was sites that had previously acted as carbon sinks became carbon sources under drought conditions. The diagram below (from Phillips 2009) articulates this reversal.

Figure A shows that in the years leading up to 2005, the above ground biomass (AGB) at many of the sites was on the increase (they were acting as carbon sinks). Then in 2005, shown in Figure B, there was a reversal of AGB accumulation at many sites (they had transformed to carbon sources). Figure C further isolates the situation in 2005 by showing the difference between 2005 and pre-2005 AGB accumulation. The shaded areas on the map indicate rainfall data (the darker areas reveal regions of more intense drought).

One additional part of the Amazon basin drought story is the atmospheric carbon levels. In 2005, carbon dioxide concentrations crept to the third highest level since records began. Phillips and his colleagues make mention of this in the concluding remarks of their paper (Phillips 2008) but remain cautious about what to conclude from it:

"The exceptional growth in atmospheric CO2 concentrations in 2005, the third greatest in the global record, may have been partially caused by the Amazon drought effects documented here. However, our findings do not translate simply into instantaneous flux estimates because carbon fluxes from necromass will lag the actual tree death events. Drought can suppress respiration, so the system as a whole might even contribute a temporary net sink even though the live biomass was in negative mass balance." ~ Dr. Oliver Phillips, University of Leeds (Phillips 2008)

Although the role that the Amazon rainforests play in the global carbon cycle remains cryptic, the droughts of 2005 shed light on how this vast forest ecosystem might respond to and recover in a changing climate.

Y. Malhi, J. T. Roberts, R. A. Betts, T. J. Killeen, W. Li, C. A. Nobre (2008). Climate Change, Deforestation, and the Fate of the Amazon Science, 319 (5860), 169-172 DOI: 10.1126/science.1146961

O. L. Phillips et al. (2009). Drought Sensitivity of the Amazon Rainforest Science, 323 (5919), 1344-1347 DOI: 10.1126/science.1164033

Ning Zeng, Jin-Ho Yoon, Jose A Marengo, Ajit Subramaniam, Carlos A Nobre, Annarita Mariotti, J David Neelin (2008). Causes and impacts of the 2005 Amazon drought Environmental Research Letters, 3 (1) DOI: 10.1088/1748-9326/3/1/014002

Photo (top) © Peter van der Steen. The Amazon forest canopy from above, blanketed in a dawn mist. Photo (bottom) © Peter Vitzthum. Moisture-stressed leaf. Diagram (top) from Malhi 2008. Diagram (bottom) from Phillips 2009.... Read more »