Dr. Stephan Getzin | Scientist | Ecologist

10 Fairy circle facts

In October 2022, we published in PPEES the first thorough test of the plant self-organization hypothesis and the sand termite hypothesis. Based on examining 500 excavated grasses in the southern, central and northern Namib we showed that grasses within fairy circles died due to plant water stress but not due to root herbivory by termites. Jürgens & Gröngröft (2023) then wrote a rebuttal, claiming that “Sand termite herbivory causes Namibia´s fairy circles”. The statements of that paper have caused some confusion about the topic. Therefore, we have now examined the claims of that paper in detail and published a response in PPEES, which presents the current state of knowledge. Below you will find a list of fairy circle facts that briefly summarize the content of our new paper Desiccation of undamaged grasses in the topsoil causes Namibia’s fairy circles – Response to Jürgens & Gröngröft (2023).

Fact 1

Getzin et al. (2022) have shown that the grasses within fairy circles died quickly after rainfall in an undamaged state. Based on measurements of the root lengths, statistical testing, and comparative photo documentations the authors showed that sand termite herbivory did not cause the death of the freshly germinated grasses within fairy circles (FCs) and sand termites were not found at the excavated grasses. Roots of those dead grasses were as long or even significantly longer than those of the living grasses outside in the vegetation matrix, which is contrary to termite herbivory because the consumption of plant material would lead to a measurable reduction of biomass. These new data thus demonstrate that sand termites are no prerequisite to induce the barren patches and that the grasses within fairy circles die independently of such external factors.

Fact 2

The sand termite hypothesis claims that the species Psammotermes allocerus causes fairy circles by “foraging on the roots of freshly germinated grasses” (Jürgens 2013). However, since the first publication of that hypothesis in the year 2013, scientifically relevant data evidence in the form of systematic measurements of plant roots across various regions of the Namib has never been provided. Already in 2013, the termite and fairy circle expert Walter Tschinkel gave a comment on the new sand termite hypothesis: “If Juergens claims termites are killing the grass, he's got to show that they're actually attacking living plants. That's not easy to do, and he didn't do it.” While this data evidence is also more than ten years later still missing, Jürgens & Gröngröft (2023) made the incorrect claim “evidence for the statement that grasses in the bare patch of fairy circles get killed by localized herbivory at the roots has been provided by numerous publications”. The response paper of Getzin & Yizhaq (2024), however, revealed that alone seven of these “numerous publications”, listed as evidence, do not deal with root herbivory at all. In fact, there is not a single study based on systematic grass excavations across the Namib that would have shown that sand termites kill the green grasses within fairy circles via root herbivory.

Fact 3

The initially dying annual grasses within fairy circles had significantly higher root-to-shoot ratios than the vital grasses in the matrix, both of which can be attributed to the same grass-triggering rain event (Getzin et al. 2022, Table 2). This indicates that they died from water stress because the desiccating grasses invested biomass resources into roots, trying to reach the deeper soil layers with more moisture, but they failed.

Fact 4

The upper topsoil of around 10 cm to 12 cm depth is very susceptible to drying out. Measurements of Getzin & Yizhaq (2024) show that this topsoil is about four times drier than the upper 20 cm of soil during the dry season and it is three times drier during the rainy season. These measurements were done with a mobile soil moisture sensor which integrates over the upper 12 cm and 20 cm of the soil, respectively.

Fact 5

The dying grasses within fairy circles had on average a root length of 10 cm across the study sites in the Namib (Getzin et al. 2022, Table 2). Consequently, these small plants cannot reach and utilize the higher soil moisture content, which is only found in deeper soil layers below a depth of 20 cm to 30 cm. These small suffering grasses have a too small leaf and root biomass, hence they cannot strongly pull soil water towards their roots. The higher soil moisture within the fairy circles at greater soil depth is thus irrelevant to the dying grasses. However, the large edge plants growing along the circle periphery can utilize that higher soil moisture because they strongly pull water with their established root system at a depth of around 20 cm to 30 cm.

Fact 6

The quickly drying topsoil where the freshly germinated grasses die is significantly drier within the fairy circle as compared to the matrix outside. This has been shown with several types of data, including 400 new measurements undertaken in the rainy season 2024 (Getzin & Yizhaq 2024, Table 1). Two to three weeks after rain, the soil moisture in the topsoil was below the permanent wilting point of 2%. This is further support for the finding that young grasses with their 10 cm long roots die due to desiccation in the topsoil, which acts as a kind of “death zone” for the new grasses.

Fact 7

The soil physical conditions allow a very high hydraulic conductivity that supports the “uptake-diffusion feedback” during the first weeks after grass-triggering rainfall. During the first two weeks, the soil moisture at 20 cm depth ranged for several rainfall events in 2021 and 2022 between 9% to 18% within the FCs, hence way above the 6% to 8% threshold below which the hydraulic conductivity strongly declines. Even 20 days after rainfall, soil moisture was still above 8% (Getzin & Yizhaq 2024, Fig. 4a). During this biologically active period, new grasses germinate after about five days, the large perennial grasses along the FC edge resprout and strongly draw water from the fairy circle with their established root system at a depth of 20 cm to 30 cm, and the freshly germinated grasses in the FCs desiccate and die within 10 to 20 days.

Fact 8

The soil water diffusion and mobility of water over long distances is directly reflected by biomass gradients (so-called “halos”), which surround the fairy circles. The highest biomass is found at the edge of the fairy circle and then it declines over many meters away from the water reservoir of the circle. This is shown for fairy circles at southern Angola in the area where Jürgens et al. (2015) measured the soil water content at 15 cm depth continuously from the fairy circle up to 24 m away from the circle. Their selected fairy circle (GPS location: -16.918600°S; 12.362910°E) shows in Google Earth imagery from 2013 biomass gradients of more than 10 m away from the edge of the circle and a halo was also present in 2018. Their data show a significant decline in soil water content (p < 0.01) with increasing distance away from the fairy circle, which indicates the long-distance lateral movement of water from the fairy circle into the matrix vegetation. Furthermore, their data also show that the soil water content ranged between 8% to 12%. Hence a high hydraulic conductivity is given to allow the “uptake-diffusion feedback” and self-organization of vegetation to form this emergent landscape pattern of fairy circles. Long-distance water diffusion over several meters has been also empirically proven by Cramer et al. (2017) using stable isotopes.

Fact 9

The continuous soil moisture measurements of Getzin et al. (2022) within and around fairy circles revealed that the active and vigorous growth of the new grasses after rainfall had a very strong effect on the decline of soil water within the circles. The drop in soil water at 20 cm depth was within the fairy circle about three times larger during one week after rainfall, when grasses were well established, as compared to the same duration after rainfall before, when the grasses were not yet established. Given that no grasses were growing within the fairy circle to take up that water, only the surrounding grasses can be responsible for the sudden decline of soil water within the fairy circle. This indicates that the grasses must have pulled the water at 20 cm depth from within the circle and thereby they also strongly depleted the upper topsoil which dries out fastest. Especially the quickly resprouting edge plants with their established root system at a depth of 20 cm to 30 cm pull water strongest to their roots and thereby outcompete their neighbors within the circle.

Fact 10

A circle has the smallest circumference-to-area ratio. Hence, if plants in arid environments form a circle, each individual plant growing along the edge would maximize its access to the water reservoir within the circle. Therefore, a circle is the most logic and stable structure for the plants to utilize the limited water in the environment. By outcompeting germinating grasses from within the circle, the edge plants can provide themselves with a long-term water reservoir. Not only the Stipagrostis grasses form round fairy circles, but there are also other grass and forb species that form circular plant rings along the Namib Desert. Circular growth of plants is thus a universal phenomenon whereby plants adapt to severe water stress. This ecosystem engineering of the plants enables them to form extra sources of water, which are critical to the resilience of the ecosystem facing constantly drought and desertification.

Plant growth and soil water dynamics within and around a typical fairy circle.