The role of overstorey proteoid shrubs in maintaining species richness in a southern Cape mountain fynbos community.

Abstract

This study was conducted to determine if, and how, over storey proteoid shrubs affect the species richness of a southern Cape mountain fynbos community. Protea eximia, P. lorifolia and P. repens were the dominant overstorey shrubs in the community studied. The percentage canopy cover and density of overstorey protea shrubs before a fire were regressed against the a-diversity of understorey species after a fire, for spatial scales ranging from 1 - 100m². High canopy cover percentages (≥ 50%) and high densities (≥ 30 plants per 100m²) of overstorey proteas before a fire enriched the a-diversity levels of understorey species after a fire. The spatial scale at which α-diversity was measured affected results. The number of understorey species at a site, where overstorey proteas were absent for several fire-cycles, was compared with those where overstorey proteas persisted. The number of understorey species was least where the overstorey proteas were lacking for several fire-cycles, but results also depended on the spatial scale at which α-diversity was measured. The basal cover percentage and density of sprouting understorey species of two sites, burned at several short (6 year) fire-cycles and where overstorey proteas were lacking, were compared with those of an adjacent site which was not burned for 28 years and where the overstorey proteas persisted. Where several short fire-cycles eliminated the overstorey proteas, the basal cover percentage of understorey sprouters was approximately 32% higher than where the overstorey shrubs persisted. The number of understorey species in dense clumps of understorey sprouters was contrasted against those on 0.25m² quadrats located in the open and under burned skeletons of overstorey protease In dense clumps of sprouters the mean number of understorey species was less than half of that for quadrats located in the open, or for quadrats located under the burned protea skeletons. Species specific competitive interactions amongst overstorey protea and understorey sprouter species were examined for several pyric successional stages. Competitive interactions between overstorey proteas and understorey sprouters were evident in all pyric successional stages. Results indicate that the overstorey proteoid shrubs are important to restrain the competitive ability of understorey sprouters, to prevent homogeneity in post-fire regeneration niches and to amplify within-community patchiness of understorey species, which ultimately enhances the species richness of fynbos communities.