Autumn with its mild damp weather is the best time to see fungi in woodland habitats. However, the fruiting bodies, mushrooms and toadstools, which we see are only a tiny part of these organisms. The vast majority of a fungus is found underground; a fine network of microscopic strands known as hyphae make up the main body of fungi, the mycelium. The mycelial networks of woodland fungi play a hugely important role in forest ecosystems, particularly because of their decomposing abilities.

Fungi have a complex relationship with many woodland plants, forming what can only be described as an extension of their root systems known as mycorrhizas (literally "fungus roots"). Mycorrhizal associations link the decomposer and plant systems within the forest ecosystem, closing an important loop in the nutrient cycle. In fact it is estimated that over 75% of known plant species have mycorrhizal fungi. Mycorrhizal associations benefit both the plant and the fungus. The fungus gains energy (carbon) from the plant in the form of sugars and in return the plant gains access to important nutrients such as phosphorus and nitrogen. The diameter of the fungal hyphae is much less than the plant roots so providing a greatly increased surface area for absorption of nutrients and water. There are 3 types of mycorrhizas; ectomycorrhizal fungi, arbuscular micorrhizal fungi and ericaceous mycorrhizal fungi. Each type invades the root tissue of the plant in different ways. For example, ectomycorrhizal fungi are generally Basidiomycetes that sheath tree roots such as oak and beech, and only invade the outer root tissues.

In addition to the important role of fungi in woodland plant associations, they also are crucial for the breakdown of dead plant matter in woodlands. Fungi have the ability to produce enzymes that can breakdown tough plant compounds such as lignin and cellulose. Wood is particularly high in these substances and so fungi is hugely important for decomposition in forests. The fungal hyphae can penetrate into the dead wood and secrete extracellular enzymes to breakdown the wood, releasing nutrients that are then absorbed. Tree species vary in their susceptibility to decomposition by fungi. Whilst alive they produce aromatic compounds in their wood, capable of disrupting fungi metabolism. In some species only certain fungi can invade the woody tissues effectively. Some of the least resistant trees include poplars and birch whereas yew and juniper are some of the most resistant.

So when you are in a woodland this autumn, marvelling at the weird and wonderful mushrooms, just remember there is much, much more to fungi that we cannot see!