Regeneration dynamics in arid subtropical thicket, South Africa

In arid environments nurse-plants modify localised habitats and create regeneration opportunities for seedlings vulnerable to hostile conditions created by biotic and abiotic factors. Facilitation is thus recognised as an important process structuring plant communities in harsh environments. Here we use spatial patterns of species association and recruitment to infer species replacement patterns in arid subtropical thicket of South Africa. Although our study site was floristically impoverished, all major plant functional groups that characterise subtropical thicket were present. Portulacaria afra clumps comprised approximately 50% of the study site by area. The mean and median clump size was 16.5 and 6.9 m2, respectively, indicating the prevalence of small individuals in the population. Approximately 90% of tree seedlings were recorded under P. afra clumps and 93% of P. afra seedlings were recorded under woody shrubs. P. afra seedlings were recorded more frequently than expected beneath Rhigozum obovatum compared with other woody shrub species. No clear recruitment patterns were recorded for R. obovatum. Lycium cinereum, a woody shrub, and the stem-succulent Psilocaulon absimile were distributed more frequently on nutrient rich patches than expected and both these species are replaced by grass as the nutrient rich patch ages. Mature trees were generally recorded growing to the south of the assumed founding P. afra stem indicating that tree establishment was more frequent on the shaded side of P. afra clumps. However, most trees grew towards the sunny north and east-facing aspects. Plant species replacement patterns are facilitated by nurse-plant effects in arid subtropical thicket. These recruitment patterns together with our inferred species replacement on nutrient rich patches result in a predictable sequence of species replacement that is cyclic in nature.