Soil factors and competition as determinants of fynbos plant species distributions in the South-western Cape, South Africa
The Cape Floristic Region is one of the most species-rich regions of the world. Fynbos, the Mediterranean climate shrubland which comprises the largest part of this region, is characterized by high beta diversity and high edaphic endemism. However, very few quantitative studies of the factors controlling species turnover and community boundaries exist. I used an integrated approach of broad correlation techniques and detailed field experiments, to investigate this. The study site was a landscape on the southern slopes of the Soetanysberg hills, near Cape Agulhas. This area is exceptional in its floristic and edaphic complexity. The first part of this work involved an investigation of the patterns of soil characteristics and the relationship between these and the vegetation patterns. This showed a strong vegetation-environment correlation, particularly with regard to communities. Community boundaries were strongly related to patterns of soil nutrients and physical factors (relating to moisture availability). The second part consisted of detailed studies of the factors controlling the distribution of three pairs of key species (all Proteaceae). Each species is dominant in part of the landscape and, for each species pair, the replacement of one species by the other across the study site, is very distinct. Protea susannae and P. compacta occur in deep and shallow sands respectively. Root morphology, water relations and phenology of adult ( 15 yr-old) plants were studied over two years in the field. Differences between these species constituted habitat-specialization for soils of different depth. A laboratory study of seedling morphology and water-use showed that species differences also exist at the seedling stage and these would be important in determining distribution patterns. Very little is known about the importance of interspecific competition in fynbos communities. A three-year field experiment was set up to investigate its role in determining species distribution patterns across community boundaries and soil gradients. Each of the three pairs of Proteaceae species was grown from seed in cleared plots at three sites along a transect crossing a community boundary. Their growth and survival were studied in relation to site (soil factors), density and interspecific competition (monoculture/mixture). For all species, site factors had an overriding influence on survival. Average individual biomass was determined primarily by site for three species and, for the other species, by density (irrespective of monoculture or mixture). It was concluded that soil factors are a major influence on the distribution of these species and that, while competition has an important role in determining spacing patterns within communities, it has a minor role in determining species distributions.
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