Role Of Mycorrhizae In Forest Tree Regeneration

Dr. Irungbam Jamuna Devi *

Pine forest in Chingkheiching Reserve Forest :: Pix - Manipur Forest Department

Through the ages of man's existence on the earth, there have been a lot of significant changes in the mode of living as well as in the socio-cultural status and scientific and technological advances. Significant changes in the global climate conditions and reserves of the natural resources of the earth will have been preserved. For example, our natural resources-the forests are greatly damaged by the various diverse human activities such as deforestastion and burning, so, valuable forest areas have been lost day by day. Owing to incessant felling of trees and burning of forests throughout the world there is also a gradual decline in the amount of oxygen liberated by the plants.

Therefore, to maintain the rapid growth of forest trees production especially pine trees there will have such effort. In this context, the role mycorrhizae is quite indispensable to the plants.

The fine feeder roots of trees are associated with highly specific root inhabiting fungi and they play vital roles in numerous physiological and ecological functions. These dynamic processes are mediated by associations of micro-organisms participating in saprotrophic, pathogenic and symbiotic root activities.

The major symbiotic associations on tree roots are mycorrhizae. The word mycorrhiza is used to define a structure that results from the symbiotic association between the fine feeder roots of plants and species of highly specialized, root-inhabiting fungi.

The term was first proposed by A.B. Frank in 1885. Mycorrhizae are active and living components of the soil and have some characters like those of roots and some those of the associated fungi.

Evidence suggests that the mycorrhizal habit evolved as a survival mechanism for both partners i.e., plants and fungi in the existing environments of low fertility, drought, disease and extreme temperatures.

The mycorrhizal fungi derive most of their needed organic nutrients from their symbiotic niche in the primary tissue of the roots. On the other hand, the plants derive a number of beneficial effects from the symbiotic association.

The mycorrhizae are broadly classified into endo and ectomvcorrhizae on endophytic or ectopytic nature of the mycosymbiont. Endomycorrihizae are more widespread among plant kingdom in comparison to ectomycorrhizae. In endomycorrhiza, the fungal hyphae penetrates in to the cortex of the root passing between and into the plant cells. Outside the root tissue loose hyphal strands may be present in the surrounding soil.

These hyphal strands are believed to give beneficial effects on the plants by more efficient absorption of water and nutrients from the soil. Endomycorrhizae are grouped into arbutoid, ericoid, orchidaceous and vesicular arbuscular mycorrhizae (VAM).

Harley (1969) suggested that arbutoid mycorrhiza is a transition between ectomycorrhizae and encomycorrihizae, accounting for the term ectendo-mycorrhizae. These mycorrhizae are characterized by a fungal sheath surrounding the infected roots, intracellular fungal penetration and in some cases, development of an intracellular network of hyphae. Ericoid mycorrhiza occurs on four or five families in the Ericalses. Orchidaceous mycorrhizae are a distinct type that occurs only in the family orchidaceae.

Vesicular arbuscular mycorrhizae (VAM) occur on more plant species than do all the other types of mycorrhizae combined and have been observed in roots of over 1000 genera of plants representing some 200 families. It has been estimated (Kendrick and Berch 1985) that over 90% of the 3,00,000 species of vascular plants in the world form VAM.

VAM are particularly widespread on plants growing in tropical and subtropical savannas and rain forest. They are also common on pteridophytes and bryophytes. Because of this broad host range, VAM fungi are ubiquitous in all natural soils throughout the world except when they have been eliminated by man's activities. VAM increase a plant's uptake of certain nutrients, particularly P, Cu and Zn.

These elements are relatively immobile in soil and zones of depletion near feeder roots. The extrametrical growth of hyphae from VAM fungi can extend beyond the feeder roots and increase the volume of soil from which these elements are absorbed. The additional nutrient absorption due to VAM fungi can result in several-fold growth increases in plants. The degree of plant benefit appears to be related to the plant's requirement, its ability to absorb nutrients from soil, the amount of available P in soil and the species of VAM fungus involved.

Additions of available P to soil may eliminate the plants dependence on VA (Baylis 1970, 1972). However, it takes from 50 to 100 times the normal P levels found in forest soils to eliminate the VAM dependency of hardwood trees (Kormanik et al 1982). The fungi forming VAM belong to the class Zygomycetes and the family Endogonaceae, which includes the general Glomus, Gigaspora, Acaulospora, Sclerocystic, Entrophospora and Endogone.

Ectomycorrhizae occur on about 10% of the world flora. Trees belonging to the Pinaceae (pine, fir, larch, spruce, hemlock), Fagaceae (Oak, Chestnut, Beech), Betulaceae (Alder, Birch), Salicaceae (Poplar, Willow), Juglandaceae (Hickory, Pecan), Myrtaceae (Eucalyptus), Ericaceae (Arbutus), and a few others form ectomycorrhizae.

Numerous fungi have been identified as forming ectomycorrhizae. In Northe America alone it has been estimated that more than 2100 species of fungi form ectomycorrhizae with forest trees. Worldwide there are over 5000 species of fungi that can form ectomycorrhizae on some 2000 species of woody plants.

Among the basidiomycetous fungi, species of Hymenomycetes (mushroom) in the genera Boletus, Cortinarius, Suillus, Russula, Gompidious, Hebeloma, Tricholoma, Laccaria and Lactarius and species of the Gasteromycetes (Puffballs) in the general Rhizopogon, Scleroderma and Pisolithus form ectomycorrhizae. Certain orders in the Ascomycetes such as Eorotiales (Cenococcum geophilum), Turberales (Truffles) and Pezizales have species that form ectomycorrhizae on trees.

Ectomycorrhizal fungi aid the growth and development of trees. For some trees, such as Pinus, they are indispensable for growth under natural conditions. The obligate requirement of Pine for ectomycorrhizae in a natural environment has been clearly shown by numerous workers in tree regeneration trials in former treeless areas and in countries without native ectomycorrhizal trees. (Marx 1980, Mikola 1973).

Forest trees and ectomycorrhizal fungi have evolve together by adapting to natural and man-made stress. A major benefit of ectomycorrhizal associations is that they aid the host tree in managing those stresses.

The forests of Manipur which are distributed in the hilly and valley regions are the offshoots of the Himalayas. The areas of the forest are 17,418 sq. km. consisting about 78.9% of the total geographical area of the state (Statistical Abstract, Manipur, 2007). The forest areas have elevations between 100 and 2700m ASL. Most of the forests of Manipur are developed through natural regeneration. The Pine forests of Manipur are equivalent to Assam sub-tropical pine forests (Champion and Seth, 1969).

It is found in elevation between 900m-1600m and rainfall 1800mm. The dominant spps. Found in these forest areas are Pine with Quercus spp., Castanopsis spp., Alnus, Magnolia, Schima, etc. These forests are found in North-Eastern and southern parts of Manipur. The total area is about 2442.77km2. The indigenous species Pinus Kesiya is extensively found in these forests of Manipur.

Thus, pine is not only one of the most important indigenous timber producing tree species but also one of the few species which have been chosen for extensive afforestation programmes.

For Pinus tree ectomycorrhizal fungi are indispensable for growth under natural conditions. Thus, proper management of mycorrhizal association in forest nurseries has greater implication at the present situation of continuous degradation of forest areas.

The ectomycorrhizal fungi have a very important role in the nutrient cycling of forest ecosystem. According to Reed (1993), mycorrhizae are important in preserving species diversity by providing host trees with necessary nutrients from soil minerals and organic matter.

The occurrence of the natural ectomycorrhizal fugal populations or artificial introduction of suitable fungal species and proper management of the soil nutrients are some of the most important factors for abundant development of ectomycorrhizae in forest areas.

Root symbioses (mycorhizae) are as common on roots of plants in forest ecosystem as are chloroplasts in leaves of plaints in the ecosystem. A forest ecosystem, a stand of mixed tree species, a plantation of tree, a single tree or a small segment of lateral root on a tree can have many types of symbiosis with multiple species. Ectomycorrhizal fungal succession apparently occurs during the development of forest stands.