Sowing densities for active restoration in Swartland shale renosterveld vegetation, Western Cape

Abstract

Ecological restoration has been recognized as a crucial strategy for mitigating habitat degradation and biodiversity loss, particularly within the Cape Floristic Region (CFR), a global biodiversity hotspot. This study focuses on active restoration of the Swartland Shale Renosterveld (SSR), a critically endangered vegetation type within the CFR, where restoration efforts are hindered by limited seed availability and low recruitment. This thesis aimed to evaluate restoration success over time, determine optimal seed sowing densities for effective SSR restoration, and investigate species-specific limitations affecting plant establishment. Field experiments were conducted at Tygerberg Nature Reserve (TNR), where different seed sowing densities (50%, 75%, 100%, and 125% of the standard sowing density) were tested sowing seed mixes of 41 species. Restoration outcomes were assessed based on species richness, vegetation cover, and species abundance. Results show that similar restoration outcomes could be achieved using half the current standard sowing density, suggesting a more resource-efficient approach to SSR restoration. Additionally, the study examined seed viability, germination, and the influence of smoke treatment on five selected species (Dicerothamnus rhinocerotis, Podalyria sericea, Helichrysum dasyanthum, H. patulum, and H. teretifolium) with poor seeding recruitment. Seed viability was found to be a major limiting factor in this study, particularly for D. rhinocerotis, which exhibited extremely low viability (<2%). Smoke treatment positively influenced germination of P. sericea and D. rhinocerotis but negatively impacted all Helichrysum species tested in this study. These findings highlight the importance of species specific restoration approaches to optimize plant diversity and long-term restoration success in SSR. By integrating optimized seed sowing strategies with targeted species-specific interventions, this study provides practical recommendations for enhancing SSR restoration efforts. The findings provide a framework for improving SSR restoration, informing future research and management strategies. Additionally, the findings contribute to the understanding of ecological restoration practices within Mediterranean-type ecosystems.