Ectomycorrhizae and Their Importance in Forest Ecosystems

Abstract

Ectomycorrhizal (ECM) associations involve the most diverse category of myocrrhizae. The diversity derives from the fungal partners; more than 5,000 species of fungi, mainly Basidiomycetes, with a limited number of Ascomycetes and Zygomycetes, make the relationship very diverse. On the contrary, however, relatively few families of plants such as Fagaceae, Pinaceae, Betulaceae, and Dipterocarpaceae are involved in the ECM associations. These plants, however, are distributed over wide areas of temperate and boreal forests, and are therefore economically important. ECM fungi make associations with plants by forming a sheath (mantle) around fine root tips with hyphae that grow inward between root cells of the cortex and make Hartig net, and emanate outward through the soil, increasing the surface area to absorb nutrients and water. Thus, the mycorrhizal fungi gain photosynthates and other essential substances from the plant and in return help the plant take up water and minerals. Pine wilt disease (PWD) is a globally serious forest disease, and also shows the importance of ectomycorrhizal relationships. Pine trees planted on a mountain slope were killed by PWD, but some trees survived at the top of the slope, where mycorrhizal associations developed far better than on lower slopes. ECM associations, beside fertilization, also increase the supply of water to the pines, and elevate host resistance against disease and parasites. Moreover, inoculation of pine seedlings with ECM fungi under laboratory conditions confirmed the increase in their resistance to PWD. Pine seedlings can tolerate the adverse effects of environmental stress such as acid mist when infected with ECM fungi. These fungi can also make a significant contribution to forest ecosystems by increasing biomass and creating a network among trees through which nutrients may transported. ECM fungi also improve the growth of host plants at the seedling stage. Many pioneer plants in wastelands are facilitated in their establishment by ECM. This association has been successfully applied to reforestation programs in tropical forests by inoculating mycorrhizae on to nursery seedlings.

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