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Universität zu Köln (2002)

The quartz fields of Southern Africa - flora, phytogeography, vegetation, and habitat ecology

Schmiedel, Ute

Titre : The quartz fields of Southern Africa - flora, phytogeography, vegetation, and habitat ecology

Auteur : Schmiedel, Ute

Université de soutenance : Universität zu Köln

Grade : Doktorgrade 2002

The subject of this thesis is the flora, vegetation, and habitat ecology of quartz-covered stone pavements, i.e., quartz fields. These present unusual habitat islands which are found in five areas distant from each other in the arid regions of southern Africa (western South Africa and southeastern Namibia). The areas are situated in two biomes, which are also subjected to different rainfall regimes, the Succulent Karoo in the winter rainfall zone and the Nama Karoo in the summer rainfall zone. Despite the contrasting ecological conditions in the two biomes, the vegetation of the quartz fields in all areas is characterised by the dominance of compact, leafsucculent dwarf-shrubs. The vegetation thus contrasts significantly with the surrounding, mainly shrubby, vegetation. These dwarf-shrubs which derived from distantly related lineages or even different plant families, indicate convergent evolution in adaptation to special habitat conditions. Moreover, 155 plant taxa (species and subspecies) are entirely restricted to these quartz fields, the majority being local or regional endemics. 67 % of the quartz field taxa are Mesembryanthema (Aizoaceae). In order to get insight into the composition and history of the obligate quartz-field flora, its floristic and structural composition, phytogeographical pattern and ecological and phylogenetic background were analysed. It revealed that the obligate quartz-field flora has a strong correspondence to the Succulent Karoo Flora but overemphasises the characteristic floristic and structural features of the latter. Based on the phytogeographical patterns of the quartz-field taxa six quartz-field phytochoria were defined, i.e., the Little Karoo, Knersvlakte, Riethuis-Wallekraal, Southern Richtersveld, Northern Richtersveld, and the Bushmanland-Warmbad Phytochorion. The regional floras of these phytochoria showed general correspondence with respect to floristic and structural composition, thus supporting the hypothesis of the convergent evolution. But also regional differences emerged. Based on preliminary phylogenetic data and palaeo-environmental reconstruction, an interpretation of the floral history of the quartz-field flora is given. The six phytochoria seem to originate from four centres of origin, i.e., the Little Karoo, the Knersvlakte, the Richtersveld, and the BushmanlandWarmbad area. The Riethuis-Wallekraal Phytochorion consists mainly of taxa which originate from the Knersvlakte or the Richtersveld area. The palaeo-environmental reconstruction indicates that the quartz-field flora of southern Africa evolved only during the post-glacial warming and drying (15000 years PB). For the vegetation-ecological investigations, vegetation relevés and soil samples from quartz fields and adjacent zonal habitats from all phytochoria were analysed. 72 plant communities of the vegetation were described and their distribution along edaphic and topographical gradients analysed by employing Correspondence and Canonical Correspondence Analysis (CA, CCA). In general, the soils of quartz fields were shallower compared to those of adjacent zonal habitats. In all areas, except the Bushmanland-Warmbad area, two different groups of quartz-field habitats, representing extremes of a continuum, were identified. One group was characterised by high salt content, neutral to slightly acid soil pH, and low stone content. The other group was characterised by low salt content, low soil pH, and high stone content. The two groups house a completely different flora and vegetation and were thus separated in all analyses. Analyses of vegetational and edaphic data of quartz fields and adjacent, zonal habitats were carried out using multivariate direct gradient analysis (CCA) in order to identify those factors that control the peculiar composition of growth forms on quartz fields. The results revealed highly similar patterns of growth-form composition in relation to similar edaphic gradients in all areas. Microclimatic measurements inside and outside quartz fields were conducted. Quartz fields showed lower daily maximum temperature for the air near the ground than surrounding zonal soils. This has a measurable impact on the dwarf plants growing on quartz fields. This microclimatic regime which enhances the general characteristics of the winter rainfall conditions is assumed to be responsible for the overemphasising of the characteristic features of the Succulent Karoo Flora in the obligate flora of the quartz fields even outside the winter rainfall zone.


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