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.
Find out more:
- 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 © Peter van der Steen.