Key takeaways
- Wood is made of cellulose, hemicellulose and tough lignin.
- Fungi release enzymes that break wood down; white rot fungi digest lignin.
- Decomposition recycles nutrients and creates habitat.
- Fungi are important players in the carbon cycle.
Without fungi, forests would be buried in their own dead wood. This article explains why wood is so hard to break down, how fungi do it, why it matters for soil, wildlife and the climate, and how growers harness the same ability.
Why wood is hard to break down
Wood is made mainly of three materials:
- Cellulose: long, strong chains of glucose, bundled into fibres.
- Hemicellulose: shorter, branched chains of various sugars, linking the fibres.
- Lignin: a complex, irregular, highly cross-linked material that fills the spaces between the fibres, waterproofing and stiffening the wood.
Lignin in particular is extremely tough. It has no regular repeating structure for enzymes to latch onto, and it shields the cellulose from attack. Few organisms can break it down effectively, and fungi are the most important of them.
How fungi do it
Fungal hyphae grow into wood, through the cell cavities and pits, and release enzymes that break its components into smaller molecules the fungus can absorb. Different fungi take different approaches:
- White-rot fungi, including Reishi, Oyster, Lion’s Mane and chaga, produce powerful oxidising enzymes that attack lignin itself, alongside enzymes that break down cellulose. The decayed wood is pale, soft and stringy.
- Brown-rot fungi take a shortcut, using chemical reactions that generate highly reactive molecules to break open the wood’s structure and get at the cellulose, while leaving much of the lignin behind. The remaining wood is brown and crumbles into cubes.
See Parasitic and Wood-Decaying Fungi Explained.
Fungi and the coal age
One intriguing idea links wood decay to geology. Much of the world’s coal formed around 300 million years ago, in a period called the Carboniferous, from vast quantities of undecayed plant material. Some scientists have suggested that part of the reason so much wood accumulated was that fungi able to break down lignin had not yet become widespread, and that the evolution of white-rot fungi helped bring that era of coal formation to a close. It is a debated hypothesis, but it shows just how much wood decay matters on a planetary scale.
Recycling nutrients
As fungi decompose wood, they release carbon and nutrients back into the ecosystem, enriching the soil and feeding new growth. A fallen tree may take decades to disappear completely, passing through a succession of different fungi along the way.
Habitat for wildlife
Decaying wood is one of the richest habitats in a forest. In Britain, a large share of woodland invertebrates depend on dead and decaying wood at some stage of their lives, including stag beetles, whose larvae feed on rotting wood for years. Woodpeckers excavate nest holes in fungus-softened trunks, and bats and birds use cavities created by decay. Leaving dead wood in place, rather than tidying it away, is one of the simplest things gardeners and land managers can do for wildlife.
A key part of the carbon cycle
Wood decay is a major route by which carbon locked in trees returns to the atmosphere and soil, making fungi important players in the global carbon cycle. How fast wood decays, and how much carbon ends up in the soil rather than the air, depends partly on which fungi do the work.
Fungi you might see
On a woodland walk you may spot brackets such as the artist’s bracket, turkey tail or birch polypore, or Oyster mushrooms on fallen logs: visible signs of this hidden work. See Artist’s Bracket.
Harnessing it
Growers use the same ability to cultivate mushrooms on wood, sawdust and agricultural by-products, turning waste into food. Fungal enzymes are also used in industry, from paper-making to textiles, and researchers are exploring fungi for breaking down pollutants. See Mushroom Substrates Explained.