Do high soil temperatures on Namibian fairy circle discs explain the absence of vegetation?

Abstract

Evenly-dispersed enigmatic bare discs known as ‘fairy circles’ occur within grasslands of the pro-Namib Desert. In spite of their conspicuous appearance, their nature and origin is still debated. The possible inhibitory effects of high surface and sub-surface soil temperatures on grass germination and seedling development on fairy circles have not yet been investigated. We measured maximum, mean daily (24 hour) and mean daytime (sunrise to sunset) temperatures of fairy circles and matrices in the NamibRand Nature Reserve (southwest Namibia). Optimum germination and growing temperatures, and thermal maxima of Stipagrostis ciliata, a grass commonly associated with fairy circle grasslands, were determined experimentally in growth trials. Seeds and seedlings were exposed to temperatures of 35 °C, 37 °C, 41 °C, 44 °C and 47 °C for 10 days. The optimum growth temperature range of S. ciliata seedlings was determined to be 35 °C to 37 °C, with depressed growth above 47 °C. Seed germination was also depressed at 47 °C, and optimal germination occurred between 35 °C to 37 °C. Circle soils were consistently 2 °C cooler than matrix soils at both surface and 15 cm depths, and though the soil surface achieved daily temperatures of 45 °C and 47 °C for the circle and matrix respectively, mean daily temperatures at 15 cm depth were 36 °C and 38 °C respectively, coinciding with the optimum germination and growing temperature of S. ciliata. Circle soil temperature is thus unlikely to limit S. ciliata germination and seedling growth and contribute to the maintenance of a bare disc, as both thermal conditions and the presence of higher soil moisture on circles provide a more favourable growing environment than the matrix.