Frost, Portulacaria afra Jacq., and the boundary between the Albany Subtropical Thicket and Nama-Karoo biomes

•Portulacaria afra (spekboom) is a keystone species in Arid Thicket.•Spekboom can only survive in frosty areas under a nurse-shrub canopy.•In frosty areas, thicket is unlikely to establish if spekboom is killed.•Frost is probably a major determinant of the Arid Thicket/Nama-Karoo biome boundaries.

Numerous shrublands exist in areas where soil moisture should support tree growth. In South Africa, the dwarf shrublands of the Nama-Karoo biome and tree-dominated vegetation of the Albany Subtropical Thicket biome share a boundary that is often abrupt. This boundary is not associated with edaphic or rainfall transitions. Field observations and leaf-level experiments suggest that the vulnerability of thicket species to frost damage is responsible for this thicket–shrubland boundary. We tested this hypothesis by establishing cuttings of Portulacaria afra (spekboom) – a dominant thicket succulent shrub that is a keystone species in arid forms of thicket – in two separate transplant experiments. Firstly across a topographic gradient from frost-free, thicket clad slopes to frost-prone, karoo shrubland dominated valley floor; and secondly, inside and outside a thicket clump in the frost-prone valley floor. We quantified the effects of frost on spekboom by measuring photosynthetic efficiency (Fv / Fm), leaf number, estimating the percentage of healthy stem before and after frost events in June 2013. Frost-exposed spekboom cuttings rapidly underwent declines in photosynthetic efficiency, followed by severe leaf and stem necrosis; herbivores played no role in these declines. Those planted on the frost-free valley slopes or under the frost-protecting thicket canopy on the valley floor remained largely unaffected. This supports the hypothesis that frost-exposure is likely to be the main factor determining the growth rates, and ultimately survival of spekboom. These results suggest that frost occurrence is an important factor involved in determining the boundaries between the arid, and spekboom-rich subtypes of Albany Subtropical Thicket, and the frost tolerant shrublands of the Nama-Karoo.