The “Death Zone” – Decoding the Secrets of the Namib Desert’s Fairy Circles

A typical fairy circle on the Kamberg, which the researchers investigated during the 2024 rainy season: The clumps of grass – up to 80 cm high – can be seen at the edge of the fairy circle. These clumps use the fallen rain first and have an immense competitive advantage due to their size. With their established roots at a depth of 20 cm to 30 cm, they are able to suck up the soil water in the fairy circle, leaving it dry and then bare of fresh grass. Credit: Stephan Getzin

Researchers describe topsoil as “death zone” for fresh grass in the fairy circle.

Namibia’s legendary fairy circles are mysterious, circular, bald patches in the dry grasslands on the edge of the Namib Desert. Their formation has been researched for decades and has recently been the subject of much debate. With extensive fieldwork, researchers from the University of Göttingen in Germany and Ben Gurion University in Israel investigated how freshly germinated grass dies inside the fairy circle.

Their results show that the grass withers due to a lack of water inside the fairy circle. The topsoil, comprised of the top 10 to 12 centimeters of the soil, acts as a kind of “death zone” in which fresh grass cannot survive for long. The new grass dies between 10 and 20 days after the rain. According to the researchers, the fact that it shows no signs of termite damage disproves a competing theory. The results were published in the journal Perspectives in Plant Ecology, Evolution, and Systematics.

Methodology and Observations

For the study, the scientists analyzed 500 individual grass plants in four regions of the Namib by taking measurements of root and leaf lengths, carrying out statistical analyses, as well as collecting and comparing photographic evidence. They also took several hundred measurements of soil moisture during or after the 2023 and 2024 rainy seasons.

Small grass plant dead in the fairy circle 19 days after the rain: The root does not reach the more moist soil layers below the “death zone” and because of the low transpiration from its spindly leaves, the plant cannot “suck” more moisture from the soil. Credit: Stephan Getzin

This showed that the topsoil is very susceptible to drying out. During and after the rainy season, the soil moisture here is three to four times lower than the soil at a depth of around 20 centimeters. In addition, the topsoil is significantly drier within the fairy circle than outside during the period of grass growth after ample rainfall. Under these conditions, freshly germinated grasses cannot survive in the fairy circle: they dry out because they cannot reach the deeper, more moist layers of soil with their roots, which are on average 10 centimeters long.

Implications for Grass Growth

In contrast, the large, perennial clumps of grass that grow at the edge of the fairy circle benefit from being able to access the soil water to a depth of 20 to 30 centimeters and below. These clumps of grass quickly turn green after the rain.

“With their well-developed root system, these clumps of grass soak up the water particularly well. After the rain, they have a huge competitive advantage over the freshly germinated grasses in the fairy circle. The new grass only loses a small amount of water via transpiration from its small leaves, resulting in insufficient ‘suction power’ to pull new water from deeper soil layers,” explains first author Dr Stephan Getzin, Göttingen University’s Ecosystem Modeling Department.

A video explaining the findings. Credit: University of Göttingen

The measurement data also show that the physical conductivity of the water is high in the first 20 days after the rain, particularly in the upper soil, and decreases with depth. As a result, the clumps of grass primarily draw water from the top 10 to 20 centimeters of the soil.

Getzin says: “This is the cause of the death of the new grass in the fairy circle. Continuous soil moisture measurements over several years support this conclusion. This is because the soil water in the fairy circle only decreases noticeably quickly with the strengthening and regrowth of the surrounding grass after rain.”

Study Conclusions and Termite Theory Discussion

According to the researchers, this testifies to the basic function of the fairy circles as water sources for the drought-stressed grass of the Namib. The round shape of the fairy circles is formed by the grass itself, as this creates the maximum supply of soil water for itself. “This self-organization can be described as ‘swarm intelligence’. It is a systematic adaptation to a lack of resources in arid regions,” say Getzin and his colleague Dr Hezi Yizhaq.

In their study, Getzin and Yizhag also comment on the theory that termites shorten the roots of fresh grass in the fairy circle by feeding on them, causing the new grass to die. “In an extensive discussion of the publications on the sand termite theory, we show that so far not a single field study with systematic measurement data on the root length of dying grasses has shown that termite feeding on the roots of newly germinated grasses create the Namib fairy circles,” the researchers say.

They also highlight a concern in that the supporters of the sand termite theory cite other research as “evidence” for the killing of fresh grass due to termite herbivory on the roots, but in fact the cited articles do not even deal with this specific subject matter.

Reference: “Desiccation of undamaged grasses in the topsoil causes Namibia’s fairy circles – Response to Jürgens & Gröngröft (2023)” by Stephan Getzin and Hezi Yizhaq, 27 February 2024, Perspectives in Plant Ecology, Evolution and Systematics.
DOI: 10.1016/j.ppees.2024.125780

Getzin’s findings from his research into the fairy circles can be found at www.fairy-circles.info.