The Harmattan: Part II

Contrary to popular belief, true rainforest is not a tangled, fetid mass of clinging vines and threatening shadows. It doesn’t require a machete and a sense of determination to traverse (though it’s best to watch your step – the snakes camouflage well, and don’t always flee from intruders). Instead, rainforests are cathedrals.

Tall, ancient trees buttressed with tremendous roots shape the architecture of the forest, and overhead the canopy seals out light and sound. The floor of a rainforest can be a surprisingly cool and quiet place, like a monastery cloister in the silent hours before matins. Small breaks in the overhead branches allow shy rays of light. The trees dilute the sun’s tropical power, until only dapples glaze the forest flower – more beautiful than the shapes cast by any stained glass window.

But unlike a cathedral, where life occurs mostly on the stone floors and in musty hallways and uncomfortable pews, life in the rainforest flees the depths and stretches to the sky. Vines and lianas wrap crawl towards light. Monkeys leap from branch to branch 30m above the ground, and hummingbirds zip through the canopy, dodging branches at super-human speeds.

At the top of the forest, in the highest level of the canopy – the emergents – flowers grow. Orchids wrap their roots around the sturdy branches of trees, and unfold their petals to capture sun and rain. Orchids are epiphytes. They grow on trees or rocks, but do not parasitize their hosts. This allows them to grow in places inaccessible to other plants – for instance, 45m above the forest floor – but it is a lifestyle that brings its own set of challenges.

High above the forest floor, orchids can gather water and sunlight, but they are otherwise divorced from the nutrient cycle – they cannot access the circle of growth, death, and decay that infuses the soil with endlessly recycled nutrients. To acquire the nutrients necessary for growth, orchids in the Amazonian rainforest rely on a more fickle source – they rely on the harmattan.

The harmattan can deposit as much as 190kg per hectare of mineral rich dust every year – it is nature’s own fertilizer. Over the course of a year, hundreds of millions of tons of dust are carted around the world. Much of this dust is dumped in the Amazon basin, where it fuels rainforest growth on a massive scale. Looking back through the geological record, palaeontologists and geologists noticed something interesting: fluctuations in rainfall over North Africa correlate closely with the expansion and contraction of the rainforest. High rainfall in North Africa leads to less dust being tossed around the world by the harmattan, and a shrinking in the rainforest; periods of drought in the Sahel cause more dust to be carried to the Amazon, and an increase in rainforest size.

For a long time, this theory relied on climate modeling and inference from the geologic record. The trace minerals deposited in dust plumes are difficult to measure, requiring elaborate and expensive laboratory techniques (I know – I once tried to measure the trace levels of iron in seawater, it was a frustrating experience. But then, I’m a mediocre chemist). However, advances in technology have allowed scientists to collect dust deposited in the Amazon and compare it with North African soil – the two are virtually identical.

If that’s not enough for you, the harmattan sometimes brings life in more direct ways. In 1994, researchers in the Caribbean made a curious discovery – a new species of grasshopper. Or at least, new to the Caribbean. It was well known in its home territory – North Africa. Caught in the harmattan, the grasshopper had been carried across the Atlantic. Despite its stressful week, it lived. Tough little buggers.

The harmattan brings both life and death. It’s dust triggers toxic blooms in the ocean, while at the same time fuelling rainforest growth. The dust of North Africa is a finite resource – researchers estimate that the harmattan will deposit nutrients for another 1,000 years. After that, it’s difficult to tell what will happen. The Amazon may shrink, as it has in historical times, but Florida’s beaches will be safe for swimmers.

The driest landmass on Earth fuels the growth of the planet’s lushest rainforests. Without the desert, the rainforest would die. Two massive ecosystems, separated by an ocean, are intimately connected. Changes in one reverberate around the world to affect the other. The harmattan is a powerful reminder of the interconnectedness of life on Earth.

Neil Griffin

Featured photo from blog.cupio.org

Literature Cited

Bristow CS, N Drake and S Armitage. 2009. Deflation in the dustiest place on Earth: The Bodele Depression, Chad. Geomorphology 105: 50-58.

Bristow CS, KA Hudson-Edwards, A Chappell. 2010. Fertilizing the Amazon and equatorial Atlantic with West African dust. Geophys. Res. Let. 37

Garrison et al. 2003. African and Asian Dust: From Desert Soils to Coral Reefs. Bioscience 53.

Stoorvogel JJ, N Van Breemen and BH Jassen. 1997. The nutrient input by Harmattan dust to a forest ecosystem in Cote d’Ivoire, Africa. Biogeochemistry 37: 145-157.

Swap R, M Garstang, S Greco, R Talbot and P Kallberg. 1992. Saharan dust in the Amazon Basin. Tellus 44B: 133-149.

Prospero, JM. 1996. Saharan Dust Transport Over the North Atlantic Ocean and Mediterranean: An Overview, in The Impact of Desert Dust Across the Mediterranean. Editors: S Guerzoni and R Chester. Kluwer Academic Publishers: Nowell, USA.

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