The association between bushclumps of Calcrete Bontveld and the adjacent Valley Thicket

Calcrete Bontveld is a mosaic vegetation that occurs on flat-topped calcareous ridges adjacent to solid Valley Thicket in the Albany Thicket biome of South Africa. It consists of thicket clumps (bushclumps) scattered in a grassy dwarf shrubland matrix. While the bushclumps of Calcrete Bontveld represent the thicket component in the mosaic the extent and nature of similarity between the bushclumps and adjacent solid Valley Thicket remains unresolved. This study considers the relationship between the thicket communities of these two spatially separate but proximal vegetation types. As expected, bushclumps share a substantive compositional similarity and structure with the nearby solid Valley Thicket but minimally so with the surrounding grassy dwarf shrubland matrix. The latter is made up of elements of other biomes with only a few thicket taxa. The Bushclump composition differs from the adjacent solid Valley Thicket in that it is less succulent and less diverse in the number of plant families. A total of 22 Thicket species were absent from the bushclumps – notably a few iconic ones: trees Portulacaria afra , shrubs Cotyledon velutina, Euphorbia mammilaris and Kalanchoe rotundifolia , as well as the geophyte, Gasteria bicolor . By contrast, only 8 taxa were found only in the bushclumps, but not in the local form of solid Valley Thicket (e.g. Ptaeroxylon obliquum and Scolopia zeyheri ). Several of these have a Cape Thicket affinity – e.g. trees Colpoon compressum and Pterocelastrus tricuspidatus and the shrub Diospyros dichrophylla . The absence of many adjacent thicket species from the bushclumps provides evidence for exchange exclusion of propagules of some Valley Thicket taxa from the bushclumps of Calcrete Bontveld. However, the unique bushclumps are a suitable habitat for establishment of the more distant Cape-affinity taxa.