Description:
<p>The few surveys made and taxonomic considerations suggest that African dry forests are commonly diverse and dominated by vesicular-arbuscular mycorrhizal, VAM tree species. Ectomycorrhizal, ECM tree species are usually absent or occur in small numbers, but occasionally dominate. Nitrogen-fixing species (which also mostly form VAM) are, in general, few. Theoretical considerations and the limited data available suggest that phosphorus is the limiting nutrient. This probably explains the paucity of N<sub>2</sub>-fixing species. It is more difficult to explain why VAM-species and not ECM-species dominate, or vice versa, in situations where P is limiting. A dominance of either type will be reinforced by processes mediated by hyphal networks. Because few taxa form ECM, such processes will promote the development of low diversity stands, whereas very many species can share the benefits of a VAM-network. Fertilizer trials are needed to identify precisely the limiting nutrient(s), and the reactions of species with different root symbioses to additions of N and P. In connection with such studies the fractional contribution of N<sub>2</sub> to N<sub>2</sub>-fixing spp. can be estimated by 15<sup>N</sup>-methodology. Analogous possibilities do not exist for work on mycorrhizas, but comparative studies of A-values, or phosphatase and protease activities would be worthwhile. A promising approach would be studies of ECMand VAM-seedlings transplanted into a variety of sites, including each other's habitats.</p>