Reproductive ecology offers some answers to the pepperbark tree persistence puzzle in the Kruger National Park, South Africa

Abstract

Warburgia salutaris (pepperbark tree) is an endangered species greatly valued in South Africa for use in traditional medicine. In one of the few remaining W. salutaris conserved sites, the Kruger National Park (KNP), fruit production is extremely low, with a relatively low proportion of juveniles in the population. This study aimed firstly, to understand the reproductive ecology of W. salutaris and, secondly, to investigate factors that may contribute to the limited fruit (and seed) output in KNP by comparing results from KNP with a reproductively successful orchard in Eston, KwaZulu-Natal Province. In the first part of the study, potential pollinators were observed and collected for further analysis; four pollination treatments (i.e. autonomous self-, geitonogamous self-, cross- and natural pollination) were set-up to elucidate the breeding system; pollen viability was assessed; and pollen tube growth for self-, cross- and natural pollination was compared. In the second part of the study, fruit production and maturation was assessed; seed dispersers were identified; and germination success of seeds from natural and cultivated populations were compared. Pollination trials revealed that W. salutaris is likely self-incompatible as self-pollinated flowers in neither KNP nor Eston resulted in mature fruit. In addition, the majority of flowers that began to form fruit in the natural-bagged treatment (i.e. naturally pollinated flowers bagged to protect from seed predation) were aborted. Pollen viability was high across both the KNP sub-populations and Eston, while percentage pollen tube occurrence for self- and cross-pollination was higher in Eston than KNP. Mature KNP fruit were only present on three trees in one sub-population during the two-year study period. Ceratitis cosyra , the marula fly, was the only seed predator in W. salutaris fruit, and 20% of collected KNP fruit were affected by seed predation. Frugivore observations in a supplemental population at Leshiba Mountain Retreat (Limpopo) revealed that baboons ( Papio ursinus ) and vervet monkeys ( Chlorocebus aethiops pygerythrus ) eat W. salutaris fruit (but not the exocarp) and possibly orally discard the seeds at a later stage. Seeds from KNP fruit were viable and had the highest percentage germination, followed by Eston, and Leshiba. Overall, W. salutaris has many floral visitors, with Lepidoptera the most likely pollinators, pollen viability is high, and seeds that are produced germinate successfully. Therefore, the impaired reproductive output in KNP appears to be in the high occurrence of aborted fruit, supporting the hypothesis that the species relies on clonal reproduction and resprouting to persist in KNP. However, long-term reliance on clonal reproduction may impair the ability to reproduce sexually, which is ultimately necessary to establish genetically diverse genets that can adapt to environmental changes.