Changes of woody plant interaction and spatial distribution between rocky and sandy soil areas in a semi-arid savanna, South Africa

Abstract

We examined the impact of shrub–shrub interactions and soil type (rocky or sandy) on growth and spatial distribution of the two savanna shrub species Tarchonanthus camphoratus and Acacia mellifera. To explore plant interactions, we compared the size of juvenile and mature T. camphoratus shrubs between different locations (under the subcanopy of A. mellifera and in the open). Juvenile T. camphoratus shrubs had similar sizes regardless of location; however, in rocky soil, mature shrubs in the open were larger than those near A. mellifera, implying an inter-specific competitive effect of A. mellifera on T. camphoratus. Juvenile T. camphoratus shrubs grew faster in the sandy than in the rocky area. Furthermore, we used the Wiegand–Moloney O-ring statistics to explore the spatial distribution of T. camphoratus. T. camphoratus showed spatial aggregation, but in the rocky area T. camphoratus juveniles were positively associated with A. mellifera (indicating facilitation as the pattern-creating process), whereas in the sandy area they were positively associated with mature T. camphoratus shrubs (indicating seed dispersal as the pattern-creating process). T. camphoratus exhibited encroachment potential in the sandy area. We showed how spatial pattern analysis can help to explore processes determining woody plant spacing and recommend its further use.