Small fire refugia in the grassy matrix and the persistence of Afrotemperate forest in the Drakensberg mountains

Abstract

Abstract Afrotemperate forests situated in the Drakensberg mountains of South Africa are characteristically small (1–10 s ha) and widely dispersed in a vast fire-prone grassland. Compared with lowland forests, they are typically species poor with low levels of endemism and species turnover, patterns that are to date unexplained. Here we show that the richness, composition and functional traits of tree species distributed on extremely small (10–100 s m2) rocky fire-refugia situated in grassland are indistinguishable from that in forest. Afrotemperate forest tree species in the Drakensberg are widely dispersed and conform to the habitat generalist strategy. Most forest trees are bird dispersed; wind dispersal is rare and is associated only with species that resprout in response to fire. We present the ‘matrix refuge hypothesis’, which proposes that fire and extreme conditions associated with exposed rocky outcrops have filtered the Afrotemperate forest tree composition resulting in convergence in functional traits essential for trees to arrive, establish and persist on fire refugia in the grassland matrix. Most Afrotemperate forest tree diversity in the Drakensberg thus resides in the matrix where it may function as a recolonisation reservoir during climatic bottlenecks.