Abstract Frequent fire is necessary for maintaining sub-tropical and tropical grassland ecosystems, without which they may transform to forest. In the biodiversity-rich Drakensberg mountains of southern Africa, a challenge for conservation is to emulate the natural fire regime driven mainly by lightning ignition which existed before human influence commenced 25,000 BP. Here, a naturally variable fire regime was replaced with a rigid, frequent fire regime about a century ago in order to maintain a vigorous grass sward for sustained water delivery. However, its impact on plant diversity is not known. We tested impacts of frequent and infrequent fire regimes established by 1957 on grassland composition and diversity at the long-term Cathedral Peak research catchments. Historic sampling of grass composition in 1980 and 1986 was repeated in 2014, at which time forbs were sampled in order to examine a potential influence of fire on diversity. Grass composition was remarkably resistant to different fire regimes over the study period, with changes in the relative abundance of individual species limited to ≤6%. Only two decades after implementing return intervals of one to eight years was a proportion of composition variance explained by the fire regime. Forbs contributed 78% of total species richness and were influenced by the effects of the fire regime on grassland structure (e.g. making space available for recruitment). Forb richness corroborated with the Intermediate Disturbance Hypothesis and was greater in areas with intermediate fire regimes. However, forb turnover was explained in part by both frequent and infrequent fire regimes. Therefore, although richness may be greater under intermediate fire regimes, to maintain unique forb assemblages or turnover, a variable fire regime, believed to reflect the historic lightning-driven fire regime, is demonstrated to be important for maintaining grassland diversity.