The vegetation cover dynamics and potential drivers of habitat change over 30 years in the Free State National Botanical Garden, South Africa

Abstract

As a conservation strategy, the South African National Biodiversity Institute (SANBI) establishes biodiversity gardens in areas with unique vegetation types that are vulnerable to extinction. The study aimed to (1) determine the vegetation cover dynamics of the Free State National Botanical Garden (FSNBG) over a 30-year period (1987–2017), focusing on different vegetation classes; (2) evaluate the ecological integrity of the Critical Biodiversity Area 1 (CBA1) vegetation using species abundance and vegetation cover; and (3) quantify potential conservation threats that may be drivers of vegetation cover changes. The “moderate vegetation cover” and “dense vegetation cover” had increased by 25.1 ha and 8.6 ha respectively in the FSNBG. Woody vegetation cover expanded significantly over the past 30-year period, suggesting “bush” encroachment. Shannon–Wiener diversity indices showed high overall plant species diversity of CBA1 vegetation type (H = 3.5), with the vegetation cover remaining high (79.6 ± 15.9%), 50 plant species no longer existing, suggesting reduced taxonomic richness. Major conservation threats included the presence of 27 alien and invasive plant species interspersed within different vegetation patches and anthropogenic habitat fragmentation in the past 19 years (i.e. covering ~ 18% of the buffer zone). We conclude that increased vegetation cover is associated with bush encroachment and we recommend interventions to reduce the population density of woody plants and establish permanent vegetation monitoring plots.