Fungi are a cornerstone of woodland ecosystems. Yet they are often overlooked, as so much fungal activity carries on unseen beneath the surface of the forest floor. Even the more conspicuous fruiting bodies, mushrooms and toadstools, represent only a tiny glimpse of the main body of the fungus. There is a vast network called the mycelium, made up of fungal threads (hyphae) carrying out their essential work in wood and soil. Yet the lives of plants, those more obvious members of the woodland community, are intimately bound up with, and dependent on, those of fungi.
Mycorrhizal partnerships are symbiotic, or mutually beneficial, relationships between plants and fungi, which take place around the plant's roots. While there are many species of fungus which do not form these partnerships, the vast majority of land plants have mycorrhizas (from the Greek mykes: fungus and rhiza: root), and many plants could not survive without them. Fossil records show that roots evolved alongside fungal partners and that fungi may have been crucial in helping plants evolve to colonise the land, hundreds of millions of years ago.
Broadly speaking, there are two main kinds of mycorrhiza: Arbuscular mycorrhizas penetrate the cells of their host's roots, and most plants develop this type. Ectomycorrhizas surround the roots without penetrating them. Trees may form either type, and some form both. In each case there is cell-to-cell contact between the plant and the fungus, allowing nutrient transfer to take place.
The fungus within a mycorrhiza receives sugars from its plant host. Since fungi do not photosynthesise (produce their own energy from the sun) and most plants do, the plant becomes a vital source of energy for the fungus. Certain mycorrhizas will actually live inside the plant's root, so the plant can also help isolate them from competitors.
Fungi transfer essential minerals such as nitrogen and phosphorus from decaying organic matter to the plant. They are able to use chemicals (enzymes) to break down substances which plants cannot use unaided, such as cellulose, thus making more nutrients available to the plant. The countless hyphae extending from the plant's roots also increase the overall area of nutrient and water uptake. These fungal threads can be very fine, branching between soil particles, and even exploring the shells of dead insects!
Some mycorrhizas may speed plant growth, stimulate fine root development and lengthen the life of the roots. They can also protect plants from drought, predators (such as nematode worms), and pathogens (micro-organisms that cause disease). Furthermore, in areas polluted by toxic heavy metals, fungi can buffer their plant partners against harm. A diversity of fungi is valuable, as different fungi will specialise in the various functions mentioned above, so one species may be good at taking up particular nutrients, while another will be better at producing enzymes.
To give an idea of the scale on which mycorrhizal partnerships take place, research in Swedish boreal forests found that between 60,000 and 1.2 million ectomycorrhizas were present in one square metre of forest! In old forests, individual fungi can be particularly large - the mycelia of some extend over 100 square metres.
The effects of mycorrhizas are not limited to the fungus and its host. One of the many ways in which they maintain the health of the soil is by acting as a 'safety net', preventing nutrients from being leached away. The overall ecosystem can benefit from higher plant diversity and improved soil structure. Research in temperate forests in the Pacific Northwest of North America has revealed that the networks of mycorrhizas in the forest allow the transfer of significant amounts of carbon between trees, even those of different species.
Carbon is the 'energy currency' within ecosystems, and it was discovered that when Douglas fir (Pseudotsuga menziesii) was shaded, there was an increase in the amount of carbon it received from birch (Betula spp.). This insight calls for a shift from the emphasis on competition to one on resource distribution in plant communities, and illustrates how mycorrhizas can promote coexistence and biodiversity.
Moreover, while the trees in forests are recognised as an important carbon sink, the fact that mycorrhizas also store large amounts of carbon means that they may have a crucial role to play in dealing with global warming.
Native pinewoods have an incredible diversity of mycorrhizas. The species list in one Caledonian Forest site showed that the number of new species being found was not tailing off after twenty years!
Although mycorrhizas are often thought of primarily in association with trees, many different plants have these relationships with fungi. For example, orchids in general are very dependent on mycorrhizas for all or part of their life cycle, and cannot germinate fully without being infected by a fungus. These plants have thousands of tiny seeds which contain no food reserves for growth (unlike a nut for instance), and therefore depend on help from fungi in order to obtain enough food to survive. Creeping ladies tresses (Goodyera repens) is an elegant rhizomatous orchid which occurs in native pinewoods, and two species of fungi (Rhizoctonia goodyera-repentis and R. lanuginosa) are associated with it. These live within the plant's roots, and are therefore given protection by the plant. Before the orchid has begun to photosynthesise, the fungus provides it with carbon. When the plant is mature, the mycorrhiza will mainly provide nutrients.
Heathers (Calluna vulgaris and Erica spp.) also have characteristic mycorrhizas, as do typical pinewood plants such as blaeberry (Vaccinium myrtillus). In Abernethy, scientists have looked at soil samples along a transect running from open moor into mature forest and found that changes in the plant community were accompanied by changes in the mycorrhizas present.
Trees in the Caledonian Forest that have arbuscular mycorrhizas include aspen (Populus tremula) and rowan (Sorbus aucuparia). Trees which form ectomycorrhizas in our native forests include Scots pine (Pinus sylvestris), birch (Betula spp.), oak (Quercus spp.) and willow (Salix spp.). Ectomycorrhizas form fruiting bodies, which are some of the more familiar woodland fungi.
Investigations into mycorrhizas really began in the 1880s with studies to promote truffle production. While these highly prized edible fungi are not a feature of the Caledonian Forest, there are a number of well-known mycorrhizal fungi that are.
The chanterelle (Cantharellus cibarius) is a delicious, orange-yellow, almost trumpet-shaped mushroom, with a pleasant smell of apricots. It is found in many kinds of woodland, including pinewoods. The cep or penny bun (Boletus edulis) tends to associate with oak, and is also a delicacy. Among the poisonous fungi is the fly agaric (Amanita muscaria), which forms mycorrhizal partnerships with birch. This is the familiar toadstool of storybooks, with its vivid red cap and white spots. Although potentially lethal, it is also known for its psychoactive properties and these were apparently utilised by European witches and Siberian shamans.
Less well-known, and often quite rare, are a group called the tooth, or hedgehog fungi, so named because of the spine-like bristles on the undersides of their caps. A number of these, such as the greenfoot tooth fungus (Sarcodon glaucopus) associate with Scots pine. Tooth fungi in the Caledonian Forest are often found growing on bare ground, such as eroded river gravels, and possibly play a role in assisting trees to colonise these areas. Like many woodland fungi, some of the soft-bodied tooth fungi provide food for a range of invertebrates and small mammals. Interestingly, the fruiting bodies of ectomycorrhizal fungi are rarely found growing directly beneath their host tree, (although they may still be beneath a tree of the same species) This shows that tree root systems can be surprisingly large.
Mycorrhizas can be damaged by soil compaction and disturbance, as well as by the use of certain chemicals, all of which occur in intensive agriculture and forestry. Applying large amounts of inorganic fertilisers to young trees, can suppress the development of mycorrhizas. Excess nitrogen, whether from fertiliser use or atmospheric pollution, chiefly affects the reproductive parts of the fungus, and so could have long term effects on fungal populations.
Many foresters and farmers are developing an awareness of the importance of looking after mycorrhizas, and the soil generally. Organic methods, and low impact forestry techniques, such as the use of horses for small scale timber extraction, all help to maintain the integrity of the soil.
Mycorrhizas are so crucial to tree health, that the practice of inoculating the soil with fungi at the time of planting is becoming increasingly common. Because of their ability to buffer trees from toxic minerals, inoculation is particularly useful in restoring vegetation to heavily contaminated land, such as former mining sites. There is no doubt that the restoration of wild, native woodland could not take place without these complex, fascinating and hidden partnerships.
Sources & further reading