Holes in the grass

Water stress on plants causes fairy circles

Fairy circles in the Marienfluss Valley in Namibia

(Bild: Wikipedia/Dr. Stephan Getzin (CC BY 2.5))

The famous so-called fairy circles in Namibia - circular and bare gaps in the grassland - have been the subject of research for several decades. Now a German-Israeli team of researchers has been able to unravel the phenomenon.

Scientists from the University of Göttingen and Ben-Gurion University in Israel have investigated how and why freshly germinated grasses die in the fairy circle. Their results show that they wither due to a lack of water in the fairy circle.

No traces of termite damage found
According to the researchers, the topsoil, i.e. the top ten to twelve centimetres of the soil, acts as a kind of "death zone" in which young grasses do not survive in the long term. Instead, they die between ten and 20 days after the rain. According to the scientists, the fact that they show no signs of termite feeding disproves a competing theory.

Fairy circles in the Marienfluss Valley in Namibia

(Bild: Wikipedia/Dr. Stephan Getzin (CC BY 2.5))

For the study, the scientists examined 500 individual grass plants in four regions of the Namib using measurements of root and leaf lengths, statistical tests and comparative photo documentation. They also took hundreds of measurements of soil moisture during and after the 2023 and 2024 rainy seasons.

A typical fairy circle that the researchers investigated: The eyrie grasses at the edge, which are up to 80 centimetres tall, are the first to use the fallen rain and have an immense competitive advantage due to their size. With their established roots at a depth of 20 to 30 centimetres, they suck up the soil water in the fairy circle, leaving it bare.

(Bild: 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 in the top 20 centimetres of the soil. In addition, the topsoil is significantly drier in the fairy circle than outside during the period of grass growth after heavy rainfall (see animation below).

Roots of freshly germinated grass too short
Under these conditions, freshly germinated grasses cannot survive in the fairy circle: They dry out because their roots, which are only ten centimeters long on average, cannot reach the deeper, moister soil layers (see animation below).

Schematic representation of plant growth and soil water distribution in the fairy circle over the first few weeks after heavy rainfall.

(Bild: Bauhaus-University Weimar, Faculty of Art & Design/Animation Art & Design)

In contrast, the large, perennial grasses, which grow at the edge of the fairy circle and quickly turn green after the rain, benefit from the soil water below a depth of 20 to 30 centimetres. After the rain, they have an immense competitive advantage over the newly germinated grasses in the fairy circle.

As the freshly germinated grasses only release a small amount of water through evaporation via their still small leaves, "their suction power is too low to absorb new water from deeper soil layers," explains Stephan Getzin from the Department of Ecosystem Modeling at the University of Göttingen.

The round shape of the fairy circles is designed by the grasses themselves, which can thus supply themselves with as much soil water as possible. "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 Israeli colleague Hezi Yizhaq.

Millions of bare patches in the Namib Desert
About 80 to 140 kilometers from the coast, there are millions of so-called fairy circles in the Namib Desert, each only a few meters wide and together forming a distinctive pattern in the landscape that can be seen for miles around.