Effects of density on the reproductive output of Protea lepidocarpodendron

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. Hierdie artikel verduidelik die effek van digtheid op die vrugbaarheid van Protea lepidocarpodendron (L) L. Plante behorende tot hierdie spesie besit onvertakte stamme en is serotinies. Dit is endemies tot die fynbos van die Suidwestelike Kaap. Dit kom natuurlik as die dominante kruinstratumspesie voor in populasies waarvan digthede baie kan verskil: dit is derhalwe geskik om huidige opvattings te toets wat van die veronder-stelling uitgaan dat die ‘voorbrand’-populasiedigtheid ’n belangrike bepaler is van die ‘nabrand’-populasie-grootte. Die invloed van digtheidafhanklikheid op reproduksie-uitset en saadparasitering voor saadvrystelling is vir P. lepidocarpodendron bepaal deur die jaarlikse blomkop en saadproduksie, saadgrootte, kiemkragtig- heid en saadparasitering voor saadvrystelling aan te teken in digte Stande (22 480 plante ha− 1), matig digte Stande (2 080 plante ha− 1) en yl Stande (160 plante ha− 1) wat 16 jaar oud is. Op die vlak van individuele plant, het reproduktiewe uitset ’n aansienlike digtheidafhanklikheidsafname getoon wat geassosieer word met kleiner kruinvolumes in digter areas. Die voorkoms van saadparasitering voor saadvrystelling het ’n digtheid- afhanklikheidstoename getoon, waarskynlik vanweë ’n korter ‘soektyd’ van die parasiet. Ons data het egter aangetoon dat ‘digtheidonderkompensasie’ effektief was op die populasievlak wat ’n groter kiemkragtige saadproduksie per hektaar en gevolglik potensieel hoër ‘nabrand’-saailinggetalle in digte Stande as in yl Stande tot gevolg het. Dit is teenstrydig met onlangse navorsing op protea-gemeenskappe wat in herfs gebrand is en laer saailinggetalle in digte Stande as yl Stande aantoon. Daar is afgelei dat die seisoen waarin gebrand is, die bepalende faktor was in ‘nabrand’-aanwins van saailinge in Stande van P. lepidocarpodendron wat ons bestudeer het. Die aantal struikstratumspesies m− 2 het aansienlik afgeneem met ’n toename in digtheid van die Protea kruinstratum. Hierdie interaksie kan lei tot plaaslike uitsterwing van struikstratumspesies as gevolg van brand wat ’n vermeerdering van plante en gevolglik boskasievorming van kruinstratum proteas tot gevolg het.