Reproductive biology of the sausage tree (Kigelia africana) in Kruger National Park, South Africa

Abstract

Kigelia africana has large flowers that are vertebrate pollinated and very large fruits that are likely to be vertebrate dispersed. Our field surveys of size–class distributions of K. africana in the southern Kruger National Park (KNP) suggest a lack of recruitment. This is possibly the result of a failure of mutualistic relationships with vertebrate dispersers and/or pollinators. Breeding system experiments indicated that K. africana is an obligate out-crosser. Despite being primarily adapted for bat pollination, in KNP that K. africana is presently mainly pollinated by a diversity of largely facultatively nectarivorous bird species. Fruit-set is high, although trees isolated by > 50 m were found to suffer depressed seed output. Our preliminary investigation of dispersal suggests that fruits are largely ignored and are thus weakly attractive to potential dispersers. Seedlings placed out in the field in KNP suffered high levels (> 50%) of mortality compared to 17.5% in control plots. This threefold difference is the result of herbivory over a 2-month period. In summary, the adult centric population structure is probably not because of pollen or seed limitation but may result from dispersal limitation or excessive herbivory.