Data from: Edaphic properties enable facilitative and competitive interactions resulting in fairy circle formation

Millions of generally regularly spaced, roughly circular barren patches called fairy circles occur in a narrow band ca. 100 km inland of the south-west African coast. These generally have conspicuously taller peripheral grasses in a shorter grass matrix. The origins of these fairy circles are controversial, but one possibility is that they are self-organizing emergent vegetation patterns that are the consequence of interplay between positive (facilitative) and negative (competitive) interactions between grasses. We hypothesized that the coarse textured sand on which fairy circles occur creates a hydraulically and nutritionally connected landscape, in which neighbouring fairy circles competitively influence each other over several metres, while providing opportunity for focusing of resources around the peripheral grasses. To test our hypotheses we conducted three main groups of analyses: 1) We measured grass biomass to assess facilitative and competitive effects of the component grasses; 2) Across a region with fairy circles we measured the size and density of fairy circles and correlated that with water infiltration rates into soil; 3) We measured the capacity of soil to conduct water pulses and 15N tracers. We found evidence of facilitative interactions in the periphery of the fairy circles and competitive suppression of the matrix grass proximal to the periphery. Across the region, fairy circle size was positively correlated with soil infiltration rates and negatively with precipitation. This suggests that fairy circles emerge in soils with high capacity for water flux that enables landscape hydraulic connectivity. Water- and 15N-pulse experiments showed that edaphic resources were highly mobile, moving up to 7.5 metres over a period of 1–3 weeks. We concluded that the evidence is consistent with an emergent vegetation pattern explanation for the origins of fairy circles and that the circles are more closely associated with a highly connective edaphic environment, rather than with particular biota.数以百万计的、排列大致规整的近似圆形裸斑——被称为仙女环（fairy circles）——出现在非洲西南海岸内陆约100公里的狭长地带内。这类裸斑的外围通常生长着明显高于周围低矮草本群落的禾草。仙女环的成因至今存在争议，其中一种假说认为它们属于自组织涌现植被格局，其形成源于禾草间正（促进）、负（竞争）相互作用的共同影响。本研究提出假说：仙女环所处的粗质地砂土构建了一个水力与营养连通的景观，在此系统中相邻仙女环可在数米范围内产生竞争性相互影响，同时为外围禾草的资源聚集提供了条件。为验证上述假说，本研究开展了三大类分析：1）通过测定禾草生物量，以评估群落内禾草的促进与竞争作用；2）在分布有仙女环的区域内，测定仙女环的大小与密度，并将其与土壤水分入渗率进行关联分析；3）测定土壤传导水脉冲与15N示踪剂的能力。研究结果显示，仙女环外围存在促进性相互作用，且紧邻外围的基质草本受到竞争抑制。在研究区域内，仙女环的大小与土壤水分入渗率呈正相关，与降水量呈负相关。该结果表明，仙女环形成于具备高水流通量、可实现景观水力连通性的土壤中。水脉冲与15N示踪实验结果显示，土壤资源具有极强的移动性，可在1至3周内扩散至7.5米范围。本研究得出结论：现有证据支持仙女环的成因属于涌现植被格局假说，且仙女环与高度连通的土壤环境关联更为紧密，而非特定的生物群落。