Canopy gaps in subtropical forest in South Africa: size of the species pool and not the number of available niches limits species richness

Abstract

We consider whether species richness in canopy gaps in coastal scarp forest (South Africa) is constrained by competition for a limited number of available niches or is a chance effect of recruitment and dispersal limitation on species derived from the surrounding species pool. In so doing we confirm findings from relatively species-poor New Zealand Nothofagus forest. We investigate the contrasting roles of determinism and chance, and the putative assembly of woody plants in gaps, using a technique in which the species richness of small, replicate local assemblages (0.25–m² quadrats) are sampled within regions (the surrounding gap) that vary in total species richness. The form of the regression of local species richness against regional species richness indicates the extent to which community assembly is under local-ecological control (niche limited). Species richness of local assemblages was not niche limited and increased as a constant proportion of the size of the species pool in a gap. We argue that, in general, species assembly in these forest gaps is chance-driven, and discuss the management implications this has for selective pole-harvesting in these forests.