Functional differentiation of biomes in an African savanna/forest mosaic

• Three biomes were identified based on their major growth forms: savannas, ‘thickets’ and forest.
• Different species assemblages occurred in each biome.
• Functional traits of trees were filtered differently across biomes.
• Biomes in Hluhluwe-iMfolozi seem to be alternative stable states.
• These biomes are maintained by positive feedbacks with fire and herbivory.

Tree densities in tropical and sub-tropical vegetation have, until recently, long been understood as increasing proportionally in response to precipitation. Current understanding is that trees are organised into alternative states with divergent properties that are linked, in the case of savannas, to frequent grass-fuelled fires or the absence of fire in non-savannas. In this paper we explored the hypothesis of functional divergence in three biomes, defined structurally by their dominant growth forms: savannas, ‘thicket’ and forest, within the vegetation of a South African park (Hluhluwe-iMfolozi). Thicket and forest both lack a continuous C4 grassy layer and are therefore non-savannas. ‘Thicket’, as defined here, has shorter trees than forests (4–6 m vs. > 10 m) and, often a dense understorey of sub-shrubs. We analysed tree species composition in 253 sites across these three biomes. We then compared herbivore use and fire frequency among the three biomes. Finally we characterised functional traits for 58 tree species including several linked to fire and herbivore responses. In support of the emerging alternative states paradigm for both tree density and phylogenetic composition, we found that the three structurally defined biomes had separate tree species assemblages. Differences in growth form, especially the presence or absence of C4 grasses had key consequences for consumers with high fire frequency in savannas, low in thickets and none in forest and high grazing in savannas, high browsing in thickets and very low herbivory in forests. Trait analysis was consistent with these differences among biomes. Consumers appear to be important environmental filters by admitting some tree species to a biome and excluding others, depending on their functional traits. Positive feedbacks between vegetation and consumers may explain the coexistence of biomes in areas with similar climate and geology.