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The winter‐rainfall region of southern Africa, covered largely by the fynbos and succulent karoo biomes, harbours the world's greatest concentration of geophyte species. Species diversity is greatest in the south‐west, where more than 500 species co‐occur in one quarter‐degree square; in the south‐east the values are generally around 100, and in the arid north‐west, always less than 50 (more often less than 10). In at least three species‐rich genera (Moraea, Eriospermum and Oxalis), the size of storage organs (bulbs, corms, tubers) varies inversely, with the largest average values occurring in the species‐poorer areas — both in the north‐western, and in the south‐eastern parts of the region. This negative correlation between average storage organ size and species diversity is, however, only observed at relatively large spatial scales, which suggests that there is no direct relationship between storage organ size and species diversity. More likely, both these measures are driven by winter rainfall amount and reliability, both of which peak in the south‐western Cape. We suggest that reliable winter rainfall makes large storage organs unnecessary and depresses extinction rates, thus leading to the accumulation of species.

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Aim The sub-escarpment coastal plains of South Africa provide remarkable opportunities to study the determinants of biome boundaries as numerous biomes are found closely juxtaposed, including the Nama-Karoo semidesert shrubland and Albany subtropical thicket. The Nama-Karoo shrubland is centred on the semi-arid and frosty high-elevation interior plateau of South Africa, whereas the Albany subtropical thicket inhabits the comparatively warmer sub-escarpment coastal plains. We examined the role of winter frosts in determining the boundaries between these two biomes on the coastal plain.Location Kaboega, Eastern Cape, South Africa.Methods We determined the relative freezing tolerance of thicket and Nama-Karoo communities by sampling dominant species from each biome in a small study site (c. 50 ha) spanning a clear vegetation and minimum temperature boundary. Freezing-induced stress on leaf photosynthesis was measured using chlorophyll fluorescence imaging across a range of subzero treatments. ResultsIn general, largely irrespective of any possible effects of temperature acclimation, freezing exposure significantly reduced photosynthetic efficiency (F v /F m ) values in thicket species relative to those from the Nama-Karoo shrubland across all treatments.Main conclusions As reduced photosynthetic efficiency is generally associated with leaf damage, and species from both these biomes are largely evergreen, we interpreted our results in terms of species-level frost resistance. Therefore, our results support the hypothesis that frost occurrence is a primary driver of the boundary between the subtropical thicket and Nama-Karoo shrubland in South Africa. This has implications for both regional-and landscape-level planning of restoration efforts and predicting boundary shifts under altered climates of the past and future.

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