Soil Factors and Competition as Determinants of the Distribution of Six Fynbos Proteaceae Species

The importance of competition and soil factors in determining the distributions of fynbos species was assessed by means of a three-yr experiment. Three pairs of Proteaceae shrub species were selected as case studies. For each pair, one species replaces the other along a transect representing an edaphic gradient across the landscape. These species were grown in cleared plots arranged along the corresponding transect, in monoculture and mixture at a range of total densities. Site effects (soil factors), density and monoculture/mixture effects on survival and above-ground biomass were determined after three yr. Site effects were the major determinants of the survival of all species, although the pattern corresponded well with adult distributions of only three species. Biomass was related primarily to site effects for three species (corresponding with adult distributions) and, for the other three, to density effects. Although all species showed reduction of biomass with increasing density, differences between intra- and interspecific competition were generally small. While competition may be important in determining growth and the spatial arrangement of individuals within communities (as indicated by density effects), this work suggests that it may be relatively unimportant in determining fynbos species distributions. As no conclusive evidence was found for one species competitively excluding the other from part of its range, it was concluded that the distribution of these species across this landscape is determined primarily by soil factors.