Origin of the enigmatic, circular, barren patches ('Fairy Rings') of the pro-Namib

Albrecht C.F. ; Joubert J.J. ; De Rycke P.H. (2001)

Article

We have studied the enigmatic, circular, barren patches on the edge of the pro-Namib Desert in Namibia. An aerial photograph of the Wolwedans area (25°4.5′S, 15°59.5′E) showed that the patches had an average diameter of 6.1 m and an ordered distribution with an R-value of 1.68, suggesting a termite-associated origin. We postulate that there is an active evolutionary process of circular patch formation, involving genesis, growth, maturation and extinction. Stipagrostis uniplumis seedlings growing in the moist inner soil of barren patches, after rains, appeared to lack root side-hairs when compared to similar plants growing outside the patch and these seedlings invariably died when the rains subsided. Growth experiments with Cynodon dactylon in inner and outer soil samples showed that these grass seedlings could survive cycles of dehydration and hydration only when growing in outer soil. Seedlings in the inner soil collapsed and died under similar conditions. This suggested that there is a biological factor in the inner soil, which inhibits resistance to dehydration, possibly through inhibition of side-root growth and/or maintenance. We postulate that termites are linked to this biological factor in some unknown manner and that the barren patches have an evolutionary advantage for the termites by acting as water traps. Direct experiments showed five-fold more water in soil samples from the barren patches than between them. Preliminary attempts to indicate or isolate the putative abiosis factor with HPLC were not successful and more sophisticated analytical techniques are called for.

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