What are mycorrhizas?
It will not be apparent to the 'army" of enthusiasts who visit our woodlands and forests each summer, that beneath the soil surface there are kilometres of fungal threads forming close intimate links with the roots of each tree species present. Such roots are known as 'mycorrhizas', which literally means 'fungus-root'. We now know that mycorrhizas are formed, not just by trees, but by the majority of plants growing throughout the world. They therefore represent the normal type of root occurring in nature.
We also know that two major types of mycorrhizas exist: arbuscular mycorrhizas, where the fungus penetrates the host's root cells, and ectomycorrhizas, where the fungus surrounds, but never penetrates, the host's root.
Although the majority of plants, including many agricultural crop plants, form arbuscular mycorrhizas, trees may form either or even both types. Tree species that form arbuscular mycorrhizas include many important ornamental and orchard trees, such as sycamore, maple, cherry, poplar, rowan, apple, pear and cypress.
The major economically important forest trees within the UK, such as pine, spruce, fir, cedar, oak, beech, willow, chestnut and birch, are known to form ectomycorrhizas. It is this type that gives rise to so many colourful toadstools in our woodlands each autumn.
What do mycorrhizas do?
Although the two types of mycorrhiza differ in how they look and behave, they benefit their tree partner in exactly the same basic manner. They provide their host with a highly efficient means of acquiring mineral nutrients andwater from the surrounding soil. Fungal threads, or hyphae, spread out from the roots they associate with. This way they act as a second root system for their tree host. For the tree, this is very cost-effective; for the same amount of carbon, a plant can produce 1mm of root against 2500mm of mycorrhizal hyphae, which can explore a much greater volume of soil, a full 10-15 times that of a nonmycorrhizal plant. This is particularly important in many parts of the tropics where mineral nutrients can be severely limiting and fertilizers too costly to purchase.
In Kenya, for example, I have seen a considerable increase in the survival of Acacia tortilis and Termillulia pruilloides seedlings growing with abundant arbuscular mycorrhizas at semi-arid, disturbed sites near Marimanti and Olorgosaillie in Kenya. No fertilizers were added to either site, but the seedlings' survival rate 30 weeks after outplanting was 97% and 89% respectively, compared with only 79% and 57% for seedlings planted with few mycorrhizas, or entirely without.
Mycorrhizas may also, according to type, impart other benefits to their tree hosts. They may for example stimulate fine root development and lengthen the lifespan of roots. They can also increase the tolerance of trees to drought and protect a tree against pathogens. With the help of mycorrhizas a tree can grow in soils subject to extremes of temperature, pH, toxins and heavy metals, in other words under conditions where they would not normally thrive. But mycorrhizas can also help to improve soil structure and mineral recycling by accelerating litter decomposition, which all means that the tree will enjoy improved soil fertility.
Mycorrhizas therefore form the key link in nature between a tree and the surrounding soil.
Mycorrhizas in the nursery
The key role of mycorrhizas within the forest ecosystem makes it important to adopt management practices in the nursery that encourage high mycorrhizal development. Even if the site where a tree is going to be planted already has adequate mycorrhizal inoculum, nursery managers should aim to produce young seedlings with a wellformed myconhizal system. This approach will benefit the young seedling in three different ways. Firstly, the seedlings produced in the nursery will be more uniform than they would have been without mycorrhizas. Secondly, the nursery seedlings will enjoy better protection from pathogens, and thirdly, it may not be necessary to apply as much fertilizers and pesticides as you would otherwise have had to do. The latter not only brings economic benefits, but environmental benefits also.
Where tree seedlings, especially ectomycorrhizal trees (pines, birch, willow), are bound for difficult locations - urban areas, motorway verges, former industrial sites or abandoned coal mines - nursery managers should consider the use of targeted inoculants.
The research done by our group at CEH in Edinburgh has shown that these trees choose different species of mycorrhizal fungi depending on the site where they grow. At disturbed locations, we have noticed that the fungi present are different and much more restricted in diversity than at undisturbed sites. At disused coal mines near Edinburgh for example, we found that birch trees that were in the process of colonising the site established ectomycorrhizas with two fungal species, Paxillus involutus (Brown roll-rim) and Scleroderma citrinum (one of the "puff balls"). This is in sharp contrast to the rich array of ectomycorrhizal fungi normally found on the roots of birch trees growing in undisturbed woodland habitats. Worldwide, there are more than 5000 ectomycorrhizal fungi known to associate with trees.
Nurserymen should consider using growth media that have been mixed with specific fungi, known to be adapted to the sites where the trees will be planted. This is especially important if there are no trees at the site already. It is likely that such a treeless site will lack ectomycorrhizal fungi, or the fungi may at least be very sparse. Pre-inoculation is becoming an increasingly popular planting strategy, and has been taken up by tree growers in the USA, France, Italy, New Zealand and China. The same preinoculation approach is also being adopted for quite different reasons where seedlings are planted in orchards for the production of edible mushrooms and truffles.
In contrast to forest trees with their ectomycorrhizal partners, most ornamental trees form arbuscular mycorrhizas. So do also a wide range of other plants, including grasses and shrubs, and not only that, they all usually share the same fungi. This makes life easy for the nurseryman as it means that virtually all field soils will naturally contain fungi which will form arbuscular mycorrhizas with any ornamental trees he wishes to plant. The exception may be field soils from totally bare sites in the tropics. What we still don't know is whether any of the arbuscular mycorrhizal fungi are specially adapted to difficult locations (like some ectomycorrhizal fungi seem to be), and should be specifically targeted by the nurseryman.
Mycorrhiza and aftercare of newly planted trees
We now recognise that for short- and long-term sustainable development of any tree planting it is important to encourage a fully-functional mycorrhizal system and the hyphal network that emanates from it. This can only be achieved by careful consideration of how best to prepare the site in terms of litter management and organic matter conservation, and only use fertilization and pesticide practices are appropriate to the site and the species to be planted.
Forestry practices, particularly site preparation, can alter or upset the local soil ecology, and the same can happen after excess application of pesticides and fertilizers. Any change in conditions may result in severe damage to the forest ecosystem, either due to soil compaction, soil erosion, removal of organic matter, or to waterlogging.
At present, we see young plantations around the world being fed large amounts of inorganic fertilizers to achieve adequate productivity. We know that this is likely to suppress mycorrhizal development by arbuscular and ectomycorrhizal trees alike, with possible negative implications for the trees' early growth. However, besides a reduction in mycorrhizal development, extensive damage to the hyphal networks also occurs. Within the soil ecosystem, besides searching for nutrients for their tree hosts, the hyphal networks also act as 'safety nets'. By intercepting mineral nutrients released from litter on the soil surface, they prevent dissolved minerals from leaching out of the soil and polluting the groundwater.
Our present understanding of mycorrhizas is that they are key components of the soil environment. They appear to have a considerable economic, ecological and environmental impact on the short- and long-term sustainable development of the forest ecosystem, especially its productivity and stability.
Dr. Philip Mason has been a temperate and tropical microbiologist with the Centre for Ecology and Hydrology at Edinburgh for more than 25 years. At present he is examining the status and role of the mycorrhizal component for the survival of endangered conifers in Viet Nam and involved with a team studying the impact of nitrogen pollution on mycorrhizal development and growth of young spruce plantations.
