Forest trees are in general completely dependent upon a symbiotic association of their roots with ectomycorrhizal fungi. These fungi mobilize minerals from soil and transfer them to the plant. In exchange the trees deliver assimilated C to the fungi. Ectomycorrhizal fungi have a limited capability to enzymatically degrade the complex carbohydrates of most organic detritus and, instead, rely upon their tree hosts for their energy needs. In return, they take up P, N, sulfur, and zinc from soil and translocate them to their host and greatly extend the functional root system of the host (Allen, 1991). An ectomycorrhizal fungus can connect to and integrate roots of several trees, such that fungi and roots grow as one intact unit. Most ectomycorrhizal fungi are basidiomycetes, with Amanita, Cortinarius, Lactarius, Russula, and Suillus among the best known ectomycorrhizal genera (Hacskaylo, 1972). Ectomycorrhizal associations are widespread, particularly in temperate regions, and involve many of the ecologically important tree species such as Pseudotsuga, Picea, Pinus, Abies, Salix, Quercus, Betula and Fagus.
Fundamental knowledge about the biodiversity of soil microbial communities and their functional impact, especially for the ectomycorrhizal fungi, is essentially non-existent. However, maintaining this below-ground biodiversity is essential for the maintenance of a healthy forest and for successful reforestation programs. Ectomycorrhizal fungi are, economically, one of the most important groups of soil fungi. These organisms form a symbiotic relationship with a plant, forming a sheath around the root tip. The fungus then forms a hartig net. The fungus then gains C and other essential organic substances from the tree and in return support the tree in taking up water, mineral salts and metabolites. The fungus also repels parasites, nematodes, and soil pathogens. Indeed, most forest trees are highly dependent on their fungal partners and in areas of poor soil quality could possibly not exist in their absence. Thus, in optimal forest husbandry, a lack of management of mycorrhizal fungi could result in damage to trees and forest crops.
The importance of ectomycorrhiza in forest plantations has received much attention when it was observed that trees often fail to establish at new sites if the ectomycorrhizal symbiont is absent. This effect has been observed in exotic pine transplantation in different parts of the world. In Western Australia, Pinus radiata and P. pinaster failed to establish in nursery beds in the absence of mycorrhizal fungi (Lakhanpal, 2000). Even the addition of fertilizer had no effect on the establishment of seedlings on such sites. Addition of forest soil produced normal and healthy seedlings, however, because the forest soil contained propagules of mycorrhizal fungi.
High ectomycorrhizal diversity is important in the healthy functioning of woodlands. Different fungi appear to occupy different niches. Some may be more proficient at supporting the tree in taking up particular nutrients, others may be specialized at protecting against pathogens, and others may assist in enzyme production. Intensive study is needed to determine the ectomycor-rhizal diversity which will optimize forest husbandry.
Pine wilt disease (PWD) is a globally serious forest disease and demonstrates the importance of tree-ectomycorrhizal relationships. Pines planted on a mountain slope of Japan (Yamaguchi Prefecture) were killed by
PWD, but some trees managed to survive on top of the slope, where mycor-rhizal relationships had developed better than on lower slopes. The abundant mycorrhizae found in the upper slope enhanced water uptake by the pines, mitigated drought stress, and thereby decreased the mortality from pine wilt disease (Akema and Futai, 2005). Moreover, under laboratory conditions, inoculation of pine seedlings with ECM fungi confirmed their enhanced resistance to pine wood nematode infection. Pine seedlings are also known to tolerate environmental stresses such as acid mist, when infected with ECM fungi (Asai and Futai, 2001).
ECM fungi also make a significant contribution to forest ecosystems by increasing their network among trees through which nutrients may be transported. In addition ECM fungi improve the growth of host plants at the seedling stage. Many pioneer plants in barren tips and other waste lands are facilitated in their establishment by ECM. This association has been successfully applied to reforestration in tropical forests by inoculating mycor-rhizae on nursery seedlings (Lakhanpal, 2000). In forest nursery management it is well known that pine seedlings could not be replanted from the nursery to any other location once they start to expand new lateral roots in spring, though it is easy to replant them in winter. This effect can be attributed to damage of the mycorrhizal association to newly developed lateral roots in spring. Thus, mycorrhizal association is essential for pine seedlings. Generally, we ignore the importance of the mycorrhizal relationship, because mycorrhizae occur underground and are invisible. When trees are exposed to biotic or abiotic stresses, the importance of the mycorrhizal association is noticed, as in the case of pine wilt disease. More than 90% of land plants are associated with mycorrhizal fungi, and two thirds of them are arbuscular mycorrhizae. But tree species predominant in temperate forests are ecto-mycorrhizal. Why have ECM fungi established the special mycorrhizal relationship with such trees? Does the ECM relationship bring about the prosperity of the trees, or does the prosperity of the trees ensure the establishment of ECM associations? There are many questions to be resolved on the ECM relationship, but the beneficial effects of ECM on these trees is an established fact.
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