Mycorrhizae Influence Tropical Succession

Most vascular plants depend to some extent on vesicular-arbuscular (V-A) mycorrhizae for mineral uptake, although a few species do not. Some mycotrophic plants cannot grow without mycorrhizae; others grow better without mycorrhizae in fertile soils, but benefit from mycorrhizae in poor soils. Thus, the mycorrhizal fungus content and the fertility of soil influence the occurrence of plant species. Because V-A mycorrhizal fungi depend on mycotrophic plants for carbon, and because they produce few spores which may survive only a short time in the lowland humid tropics, plant community composition in turn affects the mycorrhizal fungus content of soil. This influences the further course of plant succession by affecting which species of plants can subsequently occur in the community. Pool soils may favor alternate stable communities. If mycorrhizal fungi are present, obligately mycotrophic species are the best competitors and should dominate communities. They support an abundance of mycorrhizal fungi, ensuring the continued superiority of obligate mycotrophs. If mycorrhizal fungi are absent, non-mycorrhizal species will comprise the community. Even if mycorrhizal fungi are subsequently introduced, these plants will not support them, favoring persistence of the non-mycorrhizal species. On fertile soils, facultative mycotrophs are most common. They will support infection if mycorrhizal fungi are available when the soils age and fertility declines, paving the way for their replacement by obligate mycotrophs. If mycorrhizal fungi are unavailable, facultatively mycotrophic species will be replaced by non-mycorrhizal plants. Non-mycorrhizal plants may also derive competitive advantage where mycorrhizal fungi are present, from not needing to await infection to start growing. Thus, pioneer species are often non-mycorrhizal. Seral species tend to be facultative mycotrophs, and most lowland tropical forest trees tend to be obligately mycotrophic. Ectomycorrhizal individuals are likely to occur in abundance in nutrient-starved or disturbed habitats, although ectomycorrhizal species are rare in the lowland tropics. Ectomycorrhizal associates may influence tropical succession by lowering populations of V-A mycorrhizal fungi. THIS PAPER SUGGESTS how interactions between vascular plants and mycorrhizal fungi may influence succession. I discuss: 1) how plants depend on mycorrhizae to different degrees for mineral uptake; 2) how different dependence on mycorrhizae might affect the competitive success of plants in soils with different mycorrhizal fungus contents and mineral availabilities, and as a consequence, might influence plant community composition; 3) how plant communities, in turn, may affect mycorrhizal fungus populations; and 4) how further development of a plant community thereby might be influenced. There is evidence for the first three of these interactions; the fourth, consequent successional change, is based on inference. I focus on vesicular-arbuscular (V-A) mycorrhizal associations in the lowland tropics. Ectomycorrhizal associations, rare in the lowland tropics, are discussed as exceptions, because compared to V-A mycorrhizal fungi ectomycorrhizal fungi are more host-specific, disperse better, and some species may not be as dependent on their hosts. PLANTS DIFFER IN THEIR DEPENDENCE ON MYCORRHIZAE Most species of plants are capable of associating with fungi of a single zygomycetous family, the Endogonaceae, to form V-A mycorrhizae (Gerdemanin 1968). By improving mineral nutrition, especially if substrate mineral availability is low or imbalanced, V-A mycorrhizae can increase the growth of host plants (Mosse 1973b), can improve water uptake and resistance to temporary wilting (Safir, Boyer and Gerdemann 1972; Menge et al. 1978), and may improve seedling survival (Baylis 1959; Gerdemann 1965; Kleinschmidt and Gerdemann 1972; Janos 1975a, 1975b, 1977). Mycorrhizae seem to minimize the expense to the host of seeking-out minerals (Harley 1975) even though the hosts supply carbon to V-A mycorrhizal fungi (Ho and Trappe 1973). The external hyphae of mycorrhizae increase mineral uptake, especially uptake of phosphorus which is relatively immobile in soil, probably by providing a large, well-distributed absorbing surface (Mosse 1973b). The advantage of an extensive absorbing surface is diminished in fertile soils (Voigt 1971), however, and continued support of mycorrhizae in fertile soils may decrease host growth (Cooper