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.
Highlights
Forest tree species of the Afrotemperate landscape persist on extremely small (10–100 s m2) fire-protected topographic refugia situated in the grassy matrix
We evaluate the proposition that the forest tree assemblage associated with refuge sites in the matrix is a non-random subset of the assemblage found in forest[14, 21]
Because of the dual filtering effects of fire and the severe conditions associated with exposed rocky sites, we predict that the forest tree assemblage on extremely small topographic refugia should comprise fire-adapted generalist and/or ruderal species8, 9 - the ‘matrix-refuge hypothesis’
Summary
Forest tree species of the Afrotemperate landscape persist on extremely small (10–100 s m2) fire-protected topographic refugia situated in the grassy matrix. The refuge sites are located up-slope of large rocks, on rocky outcrops and above cliff-lines on isolated grassy spurs These sites are protected from fire, they are exposed to extreme conditions that characterise the grassland matrix. Because a pool of colonising ruderal species persists in the matrix, refuge sites may have a profound influence on the structure and composition of habitat fragments[9, 12]. Because of the dual filtering effects of fire and the severe conditions associated with exposed rocky sites, we predict that the forest tree assemblage on extremely small topographic refugia should comprise fire-adapted generalist and/or ruderal species8, 9 - the ‘matrix-refuge hypothesis’. Diaspores of species in refugia should be bird dispersed because this is the most reliable and targeted mode of seed dispersal to sites within a generally inhospitable matrix[22]
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