Community structure on unusual habitat islands:: quartz-fields in the Succulent Karoo, South Africa

Quartz fields are edaphically arid, azonal habitats occurring under different macroclimatic conditions in several arid regions of southern Africa. They are the exclusive home of 142 plant species of which ca. 70% are local or regional endemics. This paper is an analysis of the quartz-field floras and growth form-soil relationships in two quartz field regions: the Knersvlakte in the Namaqualand-Namib Domain of the Succulent Karoo, and the western Little Karoo in the Southern Karoo Domain. The Knersvlakte supported 52 quartz-field specialists of which 39 were endemic to the region. Corresponding data for the Little Karoo were 11, and 10 species, respectively. In both regions, the average canopy cover on the quartz-field releves was ca. 8%, and more than half of this comprised contracted, succulent nanochamaephytes. Cover and vegetation stature were markedly higher on adjacent zonal habitats. Quartz fields in both regions supported a similar array of compact, subglobose and subterranean nanochamaephytes, as evidenced by convergent patterns in two distantly related genera (Argyroderma N. E. Brown and Gibbaeum (Haworth) N. E. Brown, both Mesembryanthemaceae), endemic to the Knersvlakte and largely restricted to the Little Karoo, respectively. Analyses of vegetational and edaphic data of quartz fields and adjacent, zonal habitats were carried out using multivariate direct gradient analysis (Canonical Correspondence Analysis) 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 both regions. In general, the soils of quartz fields were shallower compared to those of adjacent zonal habitats. In both regions, two different groups of quartz-field edaphic habitats, representing extremes of a continuum, were identified. Group 1 was characterized by high salt content, neutral to slightly acid soil pH, and low stone content. Group 2 was characterized by low salt content, low soil pH, and high stone content. Group 1 quartz fields are the most edaphically arid habitats and support the highest relative cover and diversity of subglobose and subterranean chamaephytes. The combination of reduced competition from larger growth forms, shallow soils and high soil salinity, represents a regionally unusual selective regime. Some succulent lineages in the Mesembryanthemaceae have undergone diversification which has resulted in the fine-scale discrimination of subtle edaphic gradients within the saline quartz-patch habitats. Reliable seasonal rainfall and reduced thermal stress have also played a role in the evolution of quartz patch specialists.