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Accueil du site → Doctorat → Afrique du Sud → 2001 → Vegetation on the ultramafic soils of the Sekhukhuneland Centre of Endemism

University of Pretoria (2001)

Vegetation on the ultramafic soils of the Sekhukhuneland Centre of Endemism

Siebert SJ

Titre : Vegetation on the ultramafic soils of the Sekhukhuneland Centre of Endemism

Auteur : Siebert SJ

Université de soutenance : University of Pretoria

Grade : Doctor of Philosophy (PhD) 2001

Résumé
The vegetation types of South Africa, and especially those associated with Centres of Endemism, are under increasing human pressure. Habitat loss as a result of open cast mining, mine dumps, population growth, overgrazing, trampling and alien species invasions, is the greatest threat to the Sekhukhuneland Centre of Plant Endemism (SCPE). A flonstic- ecological account of the threatened vegetation of the SCPE is given. This is the first comprehensive botanical study undertaken in this floristically poorly known region situated west of the northeastern Drakensberg Escarpment, between 24°15’ and 25°30’S latitude, 29°30’ and 30°30’E longitude. A brief review of the physical environment, namely topography, geology, soils and climate, as well as the vegetation and flora, are given. A detailed account is given of the vegetation types of the SCPE. Phytosociological data from 415 sample plots were subjected to phytosociological classification using TWINSPAN and refined with table-sorting procedures based on the Braun-Blanquet floristic-sociological approach. The vegetation classification revealed six major vegetation types consisting of 82 syntaxa, interpreted as Fuirena pubescens-Schoenoplectus corymbosus Wetland Vegetation, Themeda triandra-Senecio microglossus Cool Moist Grasslands, Hippobromus pauciflorus-Rhoicissus tridentata Rock Outcrop Vegetation, Combretum hereroense- Grewia vernicosa Open Mountain Bushveld, Kirkia wilmsii-Terminalia prunioides Closed Mountain Bushveld and Acacia tortilis-Dichrostachys cinerea Arid Northern Bushveld. Plant communities of each major vegetation type are described and the diagnostic species highlighted, with the occurrence of rare and threatened plant species indicated. Maize seedlings that were grown in ultramafic soils of the SCPE showed typical symptoms of nickel and chromium toxicity. To further investigate heavy metal accumulation, 20 indigenous plant species were sampled along 13 points of an ultramafic catena. Plant material and soil samples were analysed with recognised analytical methods to determine the concentrations of 33 elements. Thirteen rock types of the SCPE were analysed to highlight the chemical relationship between the rocks of the study area and serpentinite. Nine SCPE endemics, three SCPE near-endemics, and eight common species were analysed. None of the plant taxa were hyperaccumulators of chromium or nickel, but seven indigenous species accumulated more than 1000 mg/kg of iron and aluminium. The accumulators of high heavy metal concentrations were mostly common species. Accumulators included one SCPE endemic form and one SCPE near-endemic. Three of the hyperaccumulators belong to the Asteraceae. Eighty plant species of the SCPE were assessed according to the IUCN categories of threat and 26 met the criteria. A first division of the SCPE into sub-centres is presented to aid future conservation actions. The endemic plant species are listed, as well as the near- endemic and disjunct taxa that are shared with other centres or floristic regions. A checklist is given of the approximately 2000 plant taxa that occur in the 4000 km² of the SCPE and are arranged by family, with the genera and species listed alphabetically within. Major threats to the plant diversity of Sekhukhuneland are considered and a probable conservation solution presented. The available data supports the recognition of the region as an important Centre of Plant Endemism and Diversity requiring conservation attention.

Présentation (NRF)

Page publiée le 13 septembre 2017