Are Namibian “Fairy Circles” the Consequence of Self-Organizing Spatial Vegetation Patterning?

Causes of over-dispersed barren “fairy circles” that are often surrounded by ca. 0.5 m tall peripheral grasses in a matrix of shorter (ca. 0.2 m tall) grasses in Namibian grasslands remain mysterious. It was hypothesized that the fairy circles are the consequence of self-organizing spatial vegetation patterning arising from resource competition and facilitation. We examined the edaphic properties of fairy circles and variation in fairy circle size, density and landscape occupancy (% land surface) with edaphic properties and water availability at a local scale (<50 km) and with climate and vegetation characteristics at a regional scale. Soil moisture in the barren fairy circles declines from the center towards the periphery and is inversely correlated with soil organic carbon, possibly indicating that the peripheral grass roots access soil moisture that persists into the dry season within fairy circles. Fairy circle landscape occupancy is negatively correlated with precipitation and soil [N], consistent with fairy circles being the product of resource-competition. Regional fairy circle presence/absence is highly predictable using an empirical model that includes narrow ranges of vegetation biomass, precipitation and temperature seasonality as predictor variables, indicating that fairy circles are likely a climate-dependent emergent phenomenon. This dependence of fairy circle occurrence on climate explains why fairy circles in some locations may appear and disappear over time. Fairy circles are only over-dispersed at high landscape occupancies, indicating that inter-circle competition may determine their spacing. We conclude that fairy circles are likely to be an emergent arid-grassland phenomenon that forms as a consequence of peripheral grass resource-competition and that the consequent barren circle may provide a resource-reservoir essential for the survival of the larger peripheral grasses and provides a habitat for fossicking fauna.

纳米比亚草原上那些常被约0.5米高的外围草本环绕、处于更矮（约0.2米高）草本植被基底中的过度离散贫瘠"仙女圈（fairy circles）"的成因至今仍是未解之谜。此前有假说提出，仙女圈是由资源竞争与促进作用催生的自组织空间植被格局的产物。我们分别在局域尺度（<50千米）下结合土壤理化性质与水分可获得性，以及在区域尺度下结合气候与植被特征，对仙女圈的土壤理化性质，以及仙女圈的大小、密度与景观占比（土地表面百分比）的变化展开了研究。贫瘠仙女圈内部的土壤湿度从中心向边缘逐渐降低，且与土壤有机碳呈负相关关系，这或许表明外围草本根系可获取仙女圈内存留至旱季的土壤水分。仙女圈的景观占比与降水量和土壤氮含量呈负相关，这与仙女圈是资源竞争产物的假说相符。借助包含植被生物量、降水量与温度季节性的窄区间作为预测变量的经验模型，可高度精准地预测区域尺度上仙女圈的存在与否，这表明仙女圈大概率是一种依赖气候的涌现现象。仙女圈发生对气候的这种依赖性，解释了为何部分区域的仙女圈会随时间推移出现或消失。仅当景观占比较高时，仙女圈才会呈现过度离散的分布特征，这表明圈间竞争可能决定了它们的空间间距。我们的研究结论为，仙女圈大概率是一种由外围草本资源竞争催生的干旱草原涌现现象；由此形成的贫瘠圈体不仅可为体型更大的外围草本的存活提供必不可少的资源库，还能为觅食性动物提供栖息生境。