Population Structure of an African Cycad: Fire May Stimulate the coning phenology of encephalartos lanatus (Zamiaceae) and also predispose its cones to damage.

Abstract

Cycads are the most threatened group in the plant kingdom. Fire is identified as one of the major factors heightening cycad extinction risk. However, compared to South American cycads, we know little about how fire negatively affects the demography of African cycads. Here, we collected a snapshot of demographic data on the largest known population of South Africa’s cycad species, Encephalartos lanatus, in unburnt and regularly burnt habitats. We fitted several statistical models to investigate the effects of fire on the population structure of E. lanatus. First, we found that the population follows a ‘J’ structure with more adults than any other life stage. Contrary to popular belief, this ‘J’ structure may not necessarily imply the future of the population is at risk, given that E. lanatus is a long-lived species. Second, we found that the abundance of adults explains 25% of the abundance of seedlings but does not predict the abundance of suckers, perhaps suggesting the adults ensure preferential seedling rather than clonal recruitment. Third, irrespective of life stages, the subpopulation in fire-prone habitats is, in term of size, proportionately lower than the subpopulation in unburnt areas, suggesting that fire may negatively affect the dynamic of the population. However, fire is not linked to differences in sex ratio across the population; not only do fire-prone subpopulations have more cones, but they also tend to have more damaged cones than unburnt populations. Overall, although we raised some limitations of the present study, we also inferred that fire may shape the observed ‘J’ structure of the population of E. lanatus, but, contrary to traditional belief, the ‘J’ structure is not enough to raise concern about the future of the population. A population dynamics study is required to determine if the future of the population is at risk.