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

Abstract

a b s t r a c t a r t i c l e i n f o Available online xxxx Edited by WJ BondNumerous 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 frostprone, karoo shrubland dominated valley floor; and secondly, inside and outside a thicket clump in the frostprone valley floor. We quantified the effects of frost on spekboom by measuring photosynthetic efficiency (F v / F m ), 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.

Rare frost events reinforce tropical savanna–forest boundaries

“…This amount of temperature buffering during frost can have large impacts for tropical plants because most tropical plants are vulnerable to temperatures of just a few degrees below freezing (Sakai & Larcher, 1987). Thus protective effect of trees against extreme cold has been recognized to be important for maintaining mangrove forests (Devaney et al, 2017;Osland et al, 2015), subtropical thicket (Duker, Cowling, Preez, & Potts, 2015), and tropical alpine forest (Rehm & Feeley, 2015) near the limit of their cold tolerance. In all these systems, damage to trees is greatest where tree cover is lowest, thereby reinforcing existing patterns of tree density.…”

Site selection for subtropical thicket restoration: mapping cold-air pooling in the South African sub-escarpment lowlands

“…particle size, depth, infiltration rate, sodium and electrical conductivity) are consistent across the valley floors and shale-derived sections of the valley slopes (Becker et al, 2015). Nonetheless, Karoo shrubland replaces thicket on the valley floor-this is largely driven by frost (Duker et al, 2015a(Duker et al, , 2015b. In these valleys, some of the thicket-shrubland treelines are intact and clearly defined due to historically low levels of livestock pressure (Ian Ritchie, 2016, personal communication); this boundary has become blurred in places as livestock has removed thicket, and Karoo shrubland species have subsequently invaded.…”

“…These 'calibration valleys' are located in the Zuurberg mountain range in the north-eastern portion of the Cape Fold Mountains. Temperature loggers and field observations have confirmed that radiative frost (0 C at ground level) occured on valley floors up to 15 times each winter as a result of CAP on still and clear nights (detailed in Duker et al (2015aDuker et al ( , 2015b).…”

“…Our approach, in brief, was to calibrate the CAP model (Lundquist, Pepin & Rochford, 2008) using two adjacent valleys in the sub-escarpment lowlands where a range of CAP-related data have been collected (e.g. during and after frost events, including temperature, transplant experiments and field observations; Duker et al, 2015aDuker et al, , 2015b). Next, we tested the model predictions at six different localities-selected from satellite imagery (via Google Earth Pro)-using the visually distinctive treeline between the thicket and Karoo shrubland.…”

Catchments catch all in South African coastal lowlands: multilocus phylogeography and distribution modelling of Nymania capensis (Meliaceae) reveal topographic and palaeoclimatic connectivity constraints

“…rock agama, dwarf chameleons, and cicadas; Price et al 2007;Swart et al 2009;Tolley et al 2006). The lower temperatures, both globally (Zachos et al 2001) and regionally (Holmgren et al 2003;Talma & Vogel 1992), coupled with the frost sensitivity observed in many of the thicket's component species (Duker et al 2015a;Duker et al 2015b) is suggested to have driven thicket vegetation into fragmented refugia (Cowling et al 2005), as reflected by Nymania capensis.…”