The effect of different fire regimes on plant diversity in southern African grasslands

South African grasslands support a rich flora that is attracting growing conservation interest. Fire has long been used to manage grasslands for livestock production. However, there is very little information on the effects of fire on forb diversity to help guide conservation management. We studied plant diversity at scales of 1 and 100 m² in three long-term burning experiments in mesic, montane and semi-arid grasslands, respectively, to explore forb responses to different fire regimes. Though the dominant grasses were strongly influenced by season and frequency of fire, forb diversity showed no consistent trends. Ordination results showed that forb composition varied less with fire treatment than with local site conditions. Forbs in all three grasslands seem remarkably resilient to fire. However, all three sites showed large compositional changes if fires were excluded for about 10 or more years, with the replacement of many species by a suite of, mostly, woody species. Patterns of beta-diversity sampled in the montane grassland, showed somewhat different patterns, with species turnover increasing with inter-fire interval. Our results indicate that most forb species tolerate a wider range of season and frequency of fires than the dominant grasses. However, to accommodate those species with low tolerance of frequent fires, parts of the landscape will require less frequent fires.