The effects of vesicularndash;arbuscular mycorrhizae (VAM) and ectomycorrhizae (ECM) on plant ecology are considered, with emphasis on four areas. The first of these is mycotrophy in wild plants. We suggest that many natives have lower nutrient demands than crop plants and may therefore be less responsive to mycorrhizal infection. Pot experiments must be interpreted with caution because their outcome is strongly influenced by such factors as soil type and pot size. The second topic is the role of mycorrhizae in competition. Mycorrhizae have been shown to figure in plantndash;plant competition for limited resources. Several lines of indirect evidence suggest that mycorrhizae are involved in allelopathic interactions between plants. Mycorrhizae could aid in competition between plants and microorganisms. Mycorrhizal roots are sometimes found in humus layers of the forest floor. It has been postulated that mycorrhizal fungi influence the decomposition of plant detritus. The most important role of mycorrhizal roots in the organic layer may be, in many cases, a close physical association with the most nutrient-rich microsites of the heterogeneous soil volume. The ability to allocate absorbing tissue to localized organic matter zones has been documented for roots of forest trees and for hyphae of VAM fungi. The advantageous physical placement may result in greater nutrient acquisition for the plant than would occur if root and hyphal distribution were random. Third, the occurrence of mycorrhizae in natural communities has not frequently been documented in ways that show the abundance of infection in the whole community. Future surveys in natural ecosystems should include a measure of the relative abundance of the major plant species in the community. The final topic is carbon allocation to the symbiosis. We suggest that estimates for VAM from a few percent to ~50 of gross photosynthesis can be reconciled to some extent by considering the dynamic processes of root and mycelial turnover.
展开▼
