Effects of density on the reproductive output of Protea lepidocarpodendron

Abstract

This paper explores the effects of density on the fecundity of Protea lepidocarpodendron (L.) L. This species is a non-sprouting, serotinous (canopy-stored seed) proteoid, endemic to the fynbos of the south-western Cape. It occurs naturally as the dominant overstorey species in populations encompassing a wide range of densities: it is therefore suitable for investigating current ideas which suggest that pre-fire population density is a major determinant of post-fire population size. Density-dependent effects on reproductive output and predispersal seed predation were determined for P. lepidocarpodendron by recording annual cone and seed production, seed size, viability and pre-dispersal seed predation in 16-year-old dense (22 480 plants ha − 1 ), moderately dense (2 080 plants ha − 1 ) and sparse (160 plants ha − 1 ) stands. At the level of the individual plant, reproductive output showed considerable density-dependent decline, associated with smaller canopy volumes in the denser sites. Incidence of pre-dispersal seed predation showed a density-dependent increase probably due to shorter predator search times. However, our data indicated that density under-compensation was effective at the population level, resulting in greater viable seed production per hectare, and therefore potentially higher post-fire seedling numbers in the dense than in the sparse stands. This is contrary to recent research on proteoid communities burnt in autumn which showed lower seedling numbers in dense than in sparse stands. It was concluded that season of burn would be the overriding determinant of post-fire recruitment in the stands of P. lepidocarpodendron which we studied. Point diversity (number of understorey species m − 2 ) declined markedly with increasing proteoid overstorey density. This interaction could cause the local extinction of understorey species as a result of a fire regime which favours recruitment, and therefore thicket formation, of overstorey proteoids.