The effects of frost on Albany subtropical thicket and nama-karoo shrubland in South Africa

Numerous treeless shrublands exist in areas where climates should support tree growth. In South Africa, the predominantly lowland Albany Subtropical Thicket (thicket), and the predominantly upland Nama-Karoo semi-desert shrubland share a boundary that is often abrupt and analogous to a forest-shrubland alpine ‘treeline’. This boundary exists across similar climatic and edaphic conditions, and field observations and leaf-level experiments suggest that certain thicket component species have lower levels of frost tolerance than the Nama-Karoo shrubs. The hypothesis is that the occurrence of sub-zero temperatures and frost dictates the position of this boundary at both the local landscape and broader regional levels. I test this hypothesis by quantifying leaf-level and whole-plant freezing tolerance of thicket and Nama-Karoo shrubland species, in relation to their local- and regional-scale distribution, and by modelling the occurrence of frost in the mountainous areas of the coastal lowlands relation to 1) naturally occurring boundaries between these two vegetation types, and 2) levels of aboveground net primary productivity (NPP) of Portulacaria afra Jacq. (spekboom) cuttings (planted with the aim of restoring livestock-degraded thicket). I found that the observed and modelled occurrence of frost in the local landscape has significant negative influences on the photosynthetic health, survival and growth rates of thicket component species, and that leaf-level freezing tolerance of evergreen thicket and Nama-Karoo shrubland species is strongly related to their broader regional-scale distributional thresholds. These results support the hypothesis that the occurrence of sub-zero temperatures and frost play a major role in determining the local- and regional-scale distribution of thicket and Nama-Karoo shrubland species, via their reliance on frost-defined refugia at a variety of different scales, and that the once-widespread and regionally dominant thicket vegetation contracted into small climatic refugia during the Last Glacial Maximum (LGM). These results also have major implications for landscape- and regional-level planning of livestock-degraded thicket restoration efforts that use planting of spekboom as an ecosystem engineer, and for predicting boundary shifts under the predicted future warming of global climates