Soil nutrient availability, root symbioses and tree species composition in tropical Africa: a review

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Abstract
Relations between soil nutrient availability, the occurrence of root symbioses and tree species composition in tropical Africa are discussed in the light of new evidence. Transects from humid areas at the equator towards the deserts can be summarized as follows:High availability of nitrogen, low availability of phosphorus, low pH and aluminium toxicity probably restrict the competitiveness of nitrogen-fixing species in the forests on highly weathered and leached soils in humid west and central Africa. Ectomycorrhizal species attain dominance in species-poor forests on the soils poorest in phosphorus, while species with vesicular-arbuscular (VA) mycorrhiza form diverse forests on more fertile soils. Nitrogen-fixing species, which mostly are VA mycorrhizal, are more common in woodlands, where nitrogen is lost in grass fires, than in forests, which have a more closed nitrogen cycle. Low availability of phosphorus, however, limits the growth of nitrogen-fixing species in moist savannas. Ectomycorrhizal species are dominant, possibly because of a competitive advantage conferred by the large storage capacity of the fungal mantles under conditions of discontinuous nutrient supply. Nitrogen-fixing species become more abundant towards drier areas with an increase in soil phosphorus and a decline in soil nitrogen. These conditions culminate at the desert fringe, where nitrogen-fixing species reach their most conspicuous development.It is concluded that soil nutrient availability and root symbioses interact and strongly affect the composition of vegetation communities.