Acocks’ Valley Bushveld 50 years on: new perspectives on the delimitation, characterisation and origin of subtropical thicket vegetation

Abstract

We present the approach and results of an intuitive, expert-based mapping exercise to identify subtropical thicket (including Acocks’ (1953) Valley Bushveld, Noorsveld and Spekboomveld) vegetation types as features for conservation planning. The study area comprised 105 500km2 in southern and south-eastern South Africa, the planning domain for the Subtropical Thicket Ecosystem Planning (STEP) Project. We developed a four-tier typological hierarchy based on geography, floristics, structure and grain. This yielded 112 unique thicket vegetation types, 78 of which comprised thicket clumps in a matrix of non-thicket vegetation (mosaics). By identifying mosaics, we expanded the subtropical thicket concept and increased its extent in the study area by between 1.8 and 2.8 times that of earlier assessments. We also compiled a list of plant species that yielded a rich flora of 1 558 species, 20% of which are endemic to our expanded thicket biome. Consistent with previous studies, endemics were strongly associated with succulent members of the Aizoaceae, Asphodelaceae, Euphorbiaceae, Apocynaceae and Crassulaceae. We discuss our results in terms of Acocks’ (1953) typology as well as those of more recent treatments, and comment on the evolution of subtropical thicket vegetation. Although some confusion regarding the delimitation and characterisation of thicket was resolved by this study, much more research is required to develop and test hypotheses on the determinants of thicket boundaries and the origins and evolution of thicket species.