Sarah Lloyd OAM

When plants first colonised the land some 600 million years ago, some developed an extensive fine root system of their own, and some entered into a relationship with fungi, thereby increasing their ability to obtain nutrients and water from the soil via the fungal hyphae, the microscopic thread like structures that are the living components of most fungi.

Plants including sedges, rushes and members of the saltbush and cabbage families rarely form associations with fungi.

However, the vast majority, perhaps 95 per cent, of plants, including ferns, mosses, and most families of flowering plants have a symbiotic (i.e. mutually beneficial) partnership with fungi known as mycorrhizal fungi (mycor = fungus, rhiza = root, mycorrhizal is pronounced mike-or-rise-al).

There are several types of mycorrhizae. Ectomycorrhizal fungi attach to the outside of the plants’ roots, endomycorrhizal fungi penetrate the cells.

In just one teaspoon (one gram) of healthy forest soil there are 160 kilometres (100 miles) of fungal hyphae that together form a massive network of mycelium, often referred to as the ‘wood wide web’.

The same teaspoon of soil also contains one billion bacteria, up to 10,000 protozoa (including slime mould amoebae) and scores of nematodes.

Via this extensive network, fungi can access carbon compounds, amino acids, vitamins, hormones and other nutrients from the host plant which also provides the fungi with habitats that are relatively free from other soil micro-organisms.

The plants’ root zones are effectively extended by the fine microscopic fungal hyphae giving the plant greater access to water and soil nutrients, particularly phosphorus and nitrogen.

These relationships are especially important in Australia, given its nutrient poor soils and dry climate.

Just one tree may have an association with one hundred different species of fungi that can access different areas of the forest.

Furthermore, the fungal network does not just form connections between trees and plants of the same species but between many different species.

Recent research has shown that the underground hyphal threads play a more important role than just the exchange of nutrients.

For instance, older ‘mother’ trees can supply ‘daughter’ seedling trees with sugars via the fungal network. Trees that are old or dying may ‘dump’ their resources into the network for use by other trees. Trees that are under attack can release chemical signals through their roots and ‘warn’ other trees of lurking danger and to increase their defenses.

Some scientists are even regarding forests as one superorganism and comparing the network of fungi, roots, soil and microorganisms to neurons and axions in the human brain

Fungi are also important in breaking down and recycling nutrients in dead plants and animals, so are crucial in making the components otherwise locked up in this material available for other organisms.

These saprotrophic fungi are particularly conspicuous in autumn when large and small fruiting bodies in a variety of colours and forms appear on dead trees, logs, leaf litter and animal dung.