The origin of the elongated fairy circles in the Giribes Plains, northwest Namibia

 

Fairy circles are exceedingly regularly spaced barren circular patches in arid landscapes, typically encircled by a ring of taller grasses. These vegetation patterns occur in Southwestern Africa and Australia and have also been suggested to occur in North Africa, Middle East and Madagascar. The enigmatic origins of fairy circles in arid landscape shave intrigued ecologists and sparked heated debate about the two main competing hypotheses: the termite origin and vegetation self-organization hypotheses.

In the southern part of the Giribes Plains, Kunene region, northwest Namibia, fairy circles form in a distinctive, chain-like arrangement along drainage lines that run from north to south, closely aligned along a slope. These fairy circles are unusual in their extreme elongation, with the most extreme case measuring 32.5 meters long and only 7.7 meters wide. In contrast, the fairy circles in the rest of the Giribes outside the drainage lines are typically circular and exhibit a highly ordered, hexagonal pattern. Based on field work, remote sensing and mathematical modeling we explain the formation of these unique fairy circles.

The soil in the matrix between the circles is covered by physical crust, with some areas featuring a thin biocrust. This is the only place in Namibia where soil crust developed in the matrix. This crust causes the matrix soil to be nearly four times more compact than the soil within the fairy circles. The sand within the fairy circles is coarser (D₅₀ ~600 µm) compared to the matrix soil (D₅₀ ~300 µm), which supports the formation of the crust. Interestingly, sand in fairy circles not aligned with the drainage lines is also coarser (D₅₀ ~450 µm). Hydraulic conductivity, measured using a mini-disk infiltrometer, is three to four times greater within the fairy circles than in the surrounded matrix.

Building on these field observations, we hypothesize that the elongated shape of the fairy circles results from anisotropic soil water diffusion. Water diffuses more readily along the drainage lines than in the surrounding matrix, causing the fairy circles to expand more rapidly along the watercourses than laterally. To test this hypothesis, we used the mathematical model of Zelnik et al. (2015), which simulates biomass and soil water densities under varying water-soil diffusion coefficient ratios, r (r=1outside the drainage lines and r>1 inside the fairy circle) and precipitation rates .

The simulations indicate that, for moderate diffusion ratios and varying precipitation rates, elongated fairy circles form along the drainage lines, while circular fairy circles emerge when the diffusion ratio is lower. The results agree with remote sensing analysis of images take from a drone.  The stability of the pattern to different precipitation rates and r values was also studied. These results support the hypothesis that anisotropic soil water diffusion contributes to the elongated shape of the fairy circles in the Girbies plain, although other factors may also play a role. Indirectly our work supports the self-organization hypothesis for the origin of fairy circles.  The formation of the crust in the matrix remains is still an open question for future research.