HELLIS

Introduction

There is no doubt about the symbiotic i.e. mutually beneficial, relationship between mycorrhizal fungi and trees. The benefits of mycorrhizal inoculation of seedlings in the plant nursery has been proved, however, the benefits of further applications of mycorrhiza, prior to planting out, is open to debate.

Mycorrhiza (The term mycorrhiza comes from the Greek words myco, meaning fungus, and rhiza, meaning root).

Pl: mycorrhizae or mycorrhizas

A mycorrhiza is a symbiotic association between a fungus and the roots of a plant. The fungus may colonize the roots of a host plant either intracellularly or extracellularly (see types of mycorrhiza below).

This mutually beneficial association provides the fungus with relatively constant and direct access to carbohydrates, such as glucose and sucrose produced by the plant in photosynthesis. These carbohydrates are translocated from their source location (usually leaves) to the root tissues and then to the fungal partners. In return, the plant gains the use of the mycelium's (the vegetative part of a fungus) very large surface area to absorb water and mineral nutrients from the soil, thus improving the mineral absorption capabilities of the plant roots. Mycelia are much smaller in diameter than the smallest tree root and can explore a greater volume of soil, providing a larger surface area for absorption. The cell membrane chemistry of fungi is different from that of plants hence mycorrhizae are especially beneficial for the plant partner in nutrient-poor soils. Plant roots alone may be incapable of taking up phosphate ions that are immobilized, for example, in soils with a basic pH. The mycelium of the mycorrhizal fungus can access these phosphorus sources and make them available to the plants they colonize.

The mechanisms of increased absorption are both physical and chemical.

Mycorrhizal plants are often more resistant to diseases, such as those caused by microbial soil-borne pathogens and are also more resistant to the effects of drought. These effects are perhaps due to the improved water and mineral uptake in mycorrhizal plants.

Plants grown in sterile soils and composts often perform poorly without the addition of mycorrhizal fungi to colonise the plant roots and aid in the uptake of soil nutrients.

Occurrence of Mycorrhizal Associations

Mycorrhizae are present in 92% of plant families (80% of species), with endomycorrhizae being the ancestral and predominant form.

Types of mycorrhiza

Mycorrhizas are commonly divided into a) ectomycorrhizas and b) endomycorrhizas.

a) Ectomycorrhiza

Ectomycorrhizae are normally found on alder, aspen, beech, birch, eucalyptus, hickory, lime, oak, sweet chestnut, fir, larch, pine and spruce. The fungi grow between rather than within root cells.

They form a structure known as the 'Hartig net' between the cells, as well as a fungus mantle or cover on the surface of feeder roots.

Silver Birch with ectomycorrhizal Fly Agaric

b) Endomycorrhiza

The endomycorrhizae are found on ash, elm, horse chestnut, maple, some poplars, sweet gum, walnut, cypress, juniper and yew.

These fungi grow into the root cells.

Research Findings

Based on the work of Lena Jonsson of the the Swedish University of Agricultural Sciences in Uppsala in her PhD (Community Structure of Ectomycorrhizal Fungi in Swedish Boreal Forests, Doctoral Thesis, Swedish University of Agricultural Sciences, Uppsala, 1998).

• Individual mycorrhizae are not perennial structures and it has been shown that 60% of the individual mycorrhizae last for longer than 6 months and 25% last more than 16 months.

• When seedlings regenerate in a forest, they form their ectomycorrhizas with whatever fungi there are in that area. The ectomycorrhizas that form do not depend on the age of the tree.

• As ectomycorrhizas mainly form in the upper layers of soil, if the soil is disturbed by digging trenches, this drastically reduces the ectomycorrhizas available to the seedling and hence its chances of survival. This is an important thought in terms of forest management.

• In old forests, the ectomycorrhizal community is often made up of large fungal individuals. In the genus Suillus, individuals can often extend over 100m2. The Scandinavian sites suggest that most forests are dominated by a few species that dominate under ground with most other species having low densities.

• It is thought that high ectomycorrhizal diversity is important in the healthy functioning of a woodland. Different fungi appear to have different roles. Some may be better at helping the tree take up particular nutrients, others may be specialised at protecting against pathogens, others in enzyme production.

Mycorrhizae: Benefits and Practical Application in Forest Tree Nurseries

Donald H. Marx - Director, Institute of Mycorrhizal Research, USDA Forest Service, Athens GA,
Charles E. Cordell - Plant Pathologist, USDA Forest Service, Asheville, NC, and
Paul Kormanik - Principal Silviculturist, USDA Forest Service, Athens GA.

Cordell C.E., Anderson R.L., Hoffard W.H., Landis T.D., Smith R.S. Jr., Toko H.V., 1989. Forest Nursery Pests. USDA Forest Service, Agriculture Handbook No. 680, 184 pp.

Studies have shown that when nursery seedlings, notably pine, are inoculated with the Pisolithus tinctorius (Ps) fungus, numbers of cull seedlings are reduced, and survival and growth in field plantings are increased.

The increases have been most apparent on adverse forestation sites like strip mine spoil banks but consistent and significant improvements have also been observed on many routine sites.

In general, spore inoculum has not been as effective as mycelial inoculum in early establishment of Pt ectomycorrhizae on seedling roots.

b) Endomycorrhizae

Most major hardwood species form endomycorrhizae and require them for normal development in forest plantings. Adequate development of endomycorrhizae in the nursery has two benefits.

• seedling quality in the nursery is improved.

• hardwood seedlings that have good lateral roots and good endomycorrhizal development before planting out do not die back as much after planting out as those with poor lateral roots and endomycorrhizal development.

Hardwood seedlings with endomycorrhizae are better able to compete with undesirable vegetation on the planting site.

Numerous scientific tests have demonstrated that inoculating seedlings can and does;

• reduce losses

• protect against pathogens

• promote crop uniformity

• reduce fertilizer use

• reduce stunting

• increase survival rates

• increase growth rates

The Successful Establishment of Newly Planted Trees

This depends upon a number of factors, including;

• the provenance of the tree stock – local trees for local conditions

• nursery practice – including the use/addition of mycorrhiza

• quality of tree stock – a reflection of nursery practise

• the age of the tree stock – younger trees establish quicker than older trees

• stock type – bare root, root balled, container grown

• soil type – chalk, clay, heathland etc.

• soil fertility

• soil pH – acid loving trees on acid soils, wetland trees on wetlandf1 soils

• topography – lowland, upland, sheltered, exposed

• location – street, garden, park, pasture, woodland, coastal, industrial etc.

• the presence of existing mycorrhizae in the soil

• plant handling – root pruning, dessication of roots, physical damage

• transportation – time, temperature, humidity

• planting techniques – notch planted, tree pit

• post planting maintenance – weed control, watering

Conclusion

There are numerous factors effecting the successful establishment of newly planted trees some of which are highlighted above.

Reforestation surveys indicate that the most common problems facing seedling survival are;

·         moisture stress

·         poor handling and physical damage before planting

The addition of supplementary mycorrhizae, prior to planting out, may have some benefits but to date there is no conclusive evidence.

References: http://en.wikipedia.org/wiki/Mycorrhiza

       http://www.forestpests.org/nursery/mycorrhizae

       http://www.nifg.org.uk/ecto.htm

       http://www.arthurclesen.com/resources/Rhizanova.ppt#271,15,Mycorrhizal preferences