Abstract
BackgroundIn 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.ResultsWe 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.ConclusionsIt 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.
Highlights
In this multidisciplinary study we present soil chemical, phytochemical and Geographic information systems (GIS) spatial patterning evidence that fairy circles studied in three separate locations of Namibia may be caused by Euphorbia species
Physical and chemical properties of Fairy circle (FC) soil The aim of this section was to determine if the soil of FCs in areas where euphorbias co-occur with FCs was as sandy as previously reported in FC hotspots
Gas chromatography-mass spectroscopy (GC-MS) analyses were performed on soil from FCs and the matrix, as well as on soil collected from underneath dead euphorbias, to compare the similarity in compounds/fragments of remaining compounds in these soil types
Summary
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. The fairy circles (FCs) of the arid grasslands in the proNamib Desert have puzzled the scientific community for decades. The volumetric soil water content of FCs has been found to be in excess of 53 mm of water stored in the upper 100 cm of the soil (even throughout the dry seasons) [2]. Another significant characteristic of FCs are their faster water infiltration rates [3, 9]. These characteristics have previously been explained by the possible trapping of aeolian and water-borne sediments by plants that could result in soil textural changes beneath the vegetation, which in turn, explains the heterogeneity in hydrological processes such as infiltration and runoff [9]
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