Ecological drivers of Seriphium plumosum encroachment: Implications for management and conservation

Abstract

• Burning and grazing pressure promote the proliferation of Seriphium plumosum . • Juvenile S. plumosum individuals thrive in recently burned and heavily grazed habitats. • Grasses compete with S. plumosum throughout its distribution. • Site-specific management is needed to control S. plumosum . Seriphium plumosum is recognised as a highly encroaching species in the Grassland and Savanna Biomes of South Africa, particularly in degraded areas. This study investigated the impacts of biotic and abiotic factors on the density, height, and population structure of S. plumosum across two sites: Amathole Hogsback Plantation (HP, Eastern Cape) and Klipriviersberg Nature Reserve (KNR, Gauteng), which are both subjected to significant disturbances. Field sampling using the Point-Centered Quarter method along 9 (100 m) transects at each site identified 928 plants. At the HP site, S. plumosum density was the highest in recently burned middle slopes, followed by less recently burned bottom and top slopes. In contrast, at KNR, shrub density peaked in the middle slope with intermediate grazing, followed by slopes with heavy and minimal grazing, respectively. The population structure at KNR consisted predominantly of mature plants with few juveniles, whereas the HP site exhibited high densities of juveniles and young adults, indicating recent disturbances facilitating shrub proliferation. Our findings highlight the significant influence of disturbance (i.e., fire and grazing) on the distribution patterns of S. plumosum by creating favourable ‘open niches’. Structural equation modelling identified disturbance and tree cover as critical factors at HP, while grass biomass dominated at KNR, highlighting site-specific biotic and abiotic influences on shrub proliferation. This study provides valuable insights into the dynamics of S. plumosum in response to varying environmental conditions, which is essential for informing effective land management strategies aimed at mitigating shrub encroachment and preserving biodiversity in similar ecosystems.