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The disenchantment of the fairy circles

In some places, the area in western Namibia looks as if it has freckles: thousands of red dots on a green background.

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The disenchantment of the fairy circles

In some places, the area in western Namibia looks as if it has freckles: thousands of red dots on a green background. The dry grasslands here in southern Africa are dotted with bare rings that expose soil. Most a few meters tall, some more than 20 meters in diameter, all surrounded by a ridge of sturdy grass. These so-called fairy circles have always puzzled people.

Some believe they are dance floors for elves, witches, figures with magical powers - similar to witch rings made of mushrooms in Europe. For almost 50 years, the rings have also invited scientists to speculate: How can such mysterious structures arise in nature? One of the most popular theories has long been that the spots are the work of termites. The insects nibbled at the roots of the grass, killing them.

Others blamed hydrocarbons from below for the bald patches. According to the theory, the gases would rise to the surface like in a chimney and kill the vegetation. Still others assumed that the plants form the patterns themselves. There was even speculation about plant toxins or meteorites. The problem with all of these hypotheses: For a long time nobody was able to watch the structures being created on site – until today.

A team from the University of Göttingen has now succeeded in doing just that. Two exceptionally heavy rainfall phases in the Namib Desert over the past two years played into the hands of the ecologists. They observed how the grasses within the circles began to die off just ten days after the watering, after 20 days they were completely dead, not a stalk germinated - even though the precipitation should actually have triggered growth. Outside the circles he did the same; it grew green and sprouted around them. The surrounding grasses, the researchers concluded, sucked the water out of the circle's interior.

They came to this conclusion by examining the plants in ten places in the desert immediately after the rains. They measured their roots and shoots, looking for possible damage caused by termites. Sensors recorded the soil's moisture levels on either side of the rims. It turned out that the roots inside the circle were initially just as long or even longer than the roots outside. They were looking for water.

The reason for this could be the fluctuating soil moisture. Immediately after casting, this initially decreased equally on both sides of the ring. However, once the grasses grew outside, it quickly dried up everywhere - although almost nothing grew inside to absorb the water.

"Under the intense heat in the Namib, the grasses perspire constantly and lose water," explains Stephan Getzin, biologist from the University of Göttingen and first author of the underlying study. Their roots therefore form a vacuum, and moisture is drawn towards them. In the dry soils, the water flows fast and horizontally, even over distances of more than seven meters.

However, the researchers ruled out that termites had anything to do with the patterns. The roots showed no traces of insects that might have been feeding on them. Getzin: "Most of the interior areas remained bare from the start, so there wasn't even any biomass for the termites to feed on."

The reason for this suction mechanism? Scarce water. The plants benefit from the resources released by the bare gaps. For many grasses, such geometric formations are the only way to survive in an extremely dry environment, says Getzin. "It's pure math." A circle has the smallest ratio of circumference to area. Each plant gets the maximum amount of water from the center.

"Ultimately, the grasses are forced to arrange themselves in this way because of the competition for scarce resources." For comparison: in a square, the border would be larger in relation to the area. More specimens would try to gather around the water source, each would have less water available - too little.

A few years ago, computer simulations by another team led by Getzin found that the circles represent the optimal distribution of water for the plants. Very similar patterns appeared on the screen as in real aerial photos from Namibia. The theory at that time was now actually proven.

"One always thinks plants are stupid," says Getzin. They also had a kind of swarm intelligence. "They change their landscape for their own benefit." Especially in times of climate change, this could help the plants to buffer the increasing drought.

"Aha! Ten minutes of everyday knowledge" is WELT's knowledge podcast. Every Tuesday and Thursday we answer everyday questions from the field of science. Subscribe to the podcast on Spotify, Apple Podcasts, Deezer, Amazon Music, among others, or directly via RSS feed.

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