The allelopathic, adhesive, hydrophobic and toxic latex of Euphorbia species is the cause of fairy circles investigated at several locations in Namibia

Abstract

Background: In this multidisciplinary study we present soil chemical, phytochemical and GIS spatial patterning evidence that fairy circles studied in three separate locations of Namibia may be caused by Euphorbia species. Results: We show that matrix sand coated with E. damarana latex resulted in faster water-infiltration rates. GC-MS analyses revealed that soil from fairy circles and from under decomposing E. damarana plants are very similar in phytochemistry. E. damarana and E. gummifera extracts have a detrimental effect on bacteria isolated from the rhizosphere of Stipagrostis uniplumis and inhibit grass seed germination. Several compounds previously identified with antimicrobial and phytotoxic activity were also identified in E. gummifera. GIS analyses showed that perimeter sizes and spatial characteristics (Voronoi tessellations, distance to nearest neighbour ratio, pair correlation function and L-function) of fairy circles are similar to those of fairy circles co-occurring with E. damarana (northern Namibia), and with E. gummifera (southern Namibia). Historical aerial imagery showed that in a population of 406 E. gummifera plants, 134 were replaced by fairy circles over a 50-year period. And finally, by integrating rainfall, altitude and landcover in a GIS-based site suitability model, we predict where fairy circles should occur. The model largely agreed with the distribution of three Euphorbia species and resulted in the discovery of new locations of fairy circles, in the far southeast of Namibia and part of the Kalahari Desert of South Africa. Conclusions: It is proposed that the allelopathic, adhesive, hydrophobic and toxic latex of E. damarana, E. gummifera, and possibly other species like E. gregaria, is the cause of the fairy circles of Namibia in the areas investigated and possibly in all other areas as well.

Allelopathic Potentiality of Euphorbia hypericifolia L. on Germination and Seedling Development of Sympatric Crops and Weeds

“…The observed higher potency of the leaf relative to the roots and the stem is in synergy with the finding of Kamal [58] who opined that the leaf is the most important source of allelochemicals and a major site for the photosynthetic pathway that accounts for the upshot of the synthetic pathways for secondary metabolite production. The dosage-dependent nature of allelopathic interactions observed in this study collaborates the works of [59], [60], [61] and [62], which attributed this to the high concentration of allelochemicals gradients as extract concentration increases. Elsewhere, different plant parts extracts of Alternanthera species have been shown to delay the germination of rice seeds.…”

Revisiting Theron’s hypothesis on the origin of fairy circles after four decades: Euphorbias are not the cause

“…Nonetheless, to better understand the fairy-circle phenomenon, we include this region into our final analysis on spatial patterns. This enables us also to put into perspective the results of Meyer et al [ 14 ], who present alleged support for the hypothesis of Theron for E. damarana in the Giribes and of Meyer et al [ 13 ], who proposed a causal link between E. gummifera and FCs at Garub.…”

“…Microbial activity has also been brought in context with the potential formation of FCs [ 12 ]. Finally, the occurrence of triterpenoids, potentially resulting from Euphorbia gummifera , has been found in FCs of southern Namibia near Garub and suggested to be toxic for plant growth [ 13 ], a theory that has now been extended to other Euphorbia species like E. damarana in other regions of Namibia [ 14 ]. Given the wealth of different hypotheses, Getzin et al [ 2 , 15 , 16 ] have emphasized the spatial patterns, asking which of the causal agents is able to produce the extraordinary regularity of FCs?…”

“…Both Theron [ 6 ] and Meyer et al [ 14 ] suggested that the dead and decaying Euphorbias are the causal agents of FCs. In contrast to the joint analysis of all live and senescent Euphorbias lumped together [ 14 ], we show size comparisons between FCs and dead Euphorbias only.…”

“…Both Theron [ 6 ] and Meyer et al [ 14 ] suggested that the dead and decaying Euphorbias are the causal agents of FCs. In contrast to the joint analysis of all live and senescent Euphorbias lumped together [ 14 ], we show size comparisons between FCs and dead Euphorbias only. This is because during our field observations we found that small Euphorbias are typically young and vital while the Euphorbias are typically dying when they reach large sizes as mature individuals.…”