Skip to content
NatureHQ

mechanism

Decomposition

The ability to digest wood appears to have been invented once, by fungi, and everything a forest floor does follows from it.

Decomposition is the other half of photosynthesis. Almost everything a forest builds is taken apart again — mostly by fungi, with insects doing about a third of the work on dead wood — and the rate depends more on what the tissue is made of and which organisms arrive than on the weather.

Every gram of wood in a forest is carbon that was in the air, and nearly all of it is going back. Decomposition is how, and it is not a passive process of things falling apart: it is a set of organisms making a living by dismantling molecules that were built to resist dismantling. Lignin is the crux. It is the compound that makes wood stiff, waterproof and, for most of life, indigestible, and the ability to degrade it is essentially a fungal invention — reconstructed from fungal genomes as having appeared once, deep in the Palaeozoic, in the ancestor of the white-rot fungi. What follows from that single enzymatic capability is most of what a forest floor is. Two rot types leave visibly different wood behind and hand on very different residues to the soil. Insects, it turns out, do about a third of the work on deadwood worldwide, most of it in the tropics. And the rate is set less by climate than anybody expected: identical blocks of wood in the same weather decay at very different speeds depending on which fungi happened to colonise them first.

Developed record · 72% complete · reviewed 2026-08-30

What this page covers

The breakdown of dead plant tissue, chiefly by fungi, bacteria and invertebrates. Weighted towards wood, where the chemistry is hardest and the ecology richest.

Often confused with: Composting; Fermentation

Quick facts

The hard part
Lignin — stiff, waterproof and very difficult to digest
Insect share of deadwood decay
About 29% globally, most of it in the tropics
What sets the rate
Tissue chemistry and which decomposers arrive, more than climate
Deadwood in a forest
Habitat for hundreds of specialist species, for decades

Why wood is so hard to eat

Cellulose is sugar. Lignin is the problem.

A wood cell wall holds two things worth eating and one obstacle. Cellulose is a chain of glucose units — food, in principle, for anything with the right enzyme. Hemicellulose is similar. Packed around and between those fibres is lignin: a dense, irregular, three-dimensional polymer with no repeating unit to attack, no water-soluble products, and a chemistry that resists nearly everything. Lignin is why a leaf disappears in a season and a trunk of the same tree lies on the floor for decades.

Fungi evolved the enzymes that break down wood — and may have ended the age of coal by doing it

Contested

Researchers actively disagree, and the disagreement is substantive.

Reconstruction of class II lignin peroxidase gene families across 31 fungal genomes places their expansion near the end of the Carboniferous, coinciding with the sharp decline in coal formation. Lignin is otherwise almost undegradable, so its enzymatic breakdown by fungi is what allows dead wood to decay rather than accumulate.

Who this applies to
the mushroom-forming fungi in which lignin-degrading enzymes evolved
Studied in
Agaricomycetes
Why we rate it this way, and what the caveats are
ContestedModerate confidence

That fungi are the principal decomposers of lignin is not in doubt. The coal-formation link rests on molecular clock dating with wide intervals against a competing tectonic and climatic explanation, and NatureHQ records it as a compelling hypothesis rather than an established cause.

How far it can be extended

The enzyme families are shared across white-rot Agaricomycetes, though the genome sampling behind the dating is uneven across the fungal tree.

Caveats

  • The decomposition role is established; only the coal-formation link is contested.
  • Genome sampling across the fungal tree is uneven.
  • Some coal continued to form after the proposed transition.

Where researchers disagree

  • A geological explanation for the decline in coal formation — changes in basin tectonics and climate over the same interval — accounts for the pattern without invoking fungal evolution, and molecular clock intervals are wide enough that the timing coincidence is suggestive rather than tight.

Still unanswered

  • Can the timing be constrained tightly enough to discriminate between the fungal and geological explanations?
  • How many times did lignin degradation evolve?

Last reviewed 2026-08-09

The evidence (2 studies)

Degrading lignin requires enzymes that work by oxidising it rather than by cutting it in a specific place — a chemically indiscriminate attack, closer to controlled burning than to digestion. Comparative genomics across 31 fungal species reconstructed those peroxidases as arising once, in the ancestor of the white-rot fungi, and being lost repeatedly since. Almost nothing else on Earth does this. Termites and some beetles get at wood, but chiefly by cultivating fungi or by relying on gut microbes, which is the same trick at one remove.

Words used here
Lignin
A dense irregular polymer packed around cellulose fibres in wood. Stiff, waterproof and extremely difficult to digest.
Peroxidase
An enzyme that attacks a molecule by oxidising it. The class of enzymes white-rot fungi use on lignin, and the reason they can do what almost nothing else can.

Two kinds of rot, and how to tell them apart

Two kinds of rot take wood apart in opposite ways, and you can tell which one has been at work by looking.

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

White-rot fungi degrade lignin as well as cellulose using peroxidase enzymes largely restricted to that group, leaving pale fibrous residue; brown-rot fungi remove cellulose and hemicellulose while leaving chemically modified lignin, producing brown residue that cracks into cubes and decomposes slowly thereafter.

Who this applies to
Wood-decay basidiomycetes, the dominant decomposers of woody tissue.
Studied in
Basidiomycota
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

The biochemical distinction is well characterised and has a genomic basis: brown-rot lineages are derived from white-rot ancestors by loss of the peroxidase genes, which shows up directly in genome comparisons.

How far it can be extended

The enzymatic distinction is genomic and has been characterised across many species in both groups.

Caveats

  • Real deadwood is usually colonised by several fungi in succession, so a single trunk can show both.
  • Soft rot, caused by other fungi in wet conditions, is a third pattern not covered here.

Still unanswered

  • How brown-rot fungi break down cellulose without enzymes reaching most of it, which is still being worked out.

Last reviewed 2026-08-30

The evidence (2 studies)

Diagram

What each rot removes, and what it leaves

The difference is visible in the residue: pale and stringy against brown and cracked into cubes.

A wood cell wall holds two things worth eating, and one that fights backcellulose — chains of sugarligninWhite rotbreaks down lignin as well as cellulose, usingperoxidase enzymes essentially unique to fungiWood left pale, soft and stringy: the fibressurvive when the lignin between them does not.Brown rotstrips the cellulose and leaves modified ligninbehind, largely without enzymes reaching itWood left brown and cracked into cubes; theresidue is slow to break down and builds soil.Which rot arrives decides what the deadwood becomes, how fast its carbon returns to the air, andwhich animals can live in it. Lignin is the reason wood outlasts every other plant tissue.
The same explanation in words

A diagram in three parts. At the top, a wood cell wall is shown as two materials: a wide block of cellulose, described as chains of sugar, and a narrower block of lignin beside it. Below and to the left, white rot: it breaks down lignin as well as cellulose, using peroxidase enzymes essentially unique to fungi. The wood it leaves is drawn as pale horizontal fibres and described as soft and stringy — the fibres survive when the lignin between them does not. Below and to the right, brown rot: it strips the cellulose and hemicellulose and leaves chemically modified lignin behind, largely without enzymes reaching most of it. The wood it leaves is drawn as a block broken by vertical and horizontal cracks into cubes, and described as brown, slow to break down further, and a builder of soil. A closing note records that which rot arrives decides what the deadwood becomes, how fast its carbon returns to the air, and which animals can live in it — and that lignin is the reason wood outlasts every other plant tissue.

The consequences run well past the appearance. White rot returns carbon relatively completely. Brown rot leaves a modified lignin residue that resists further decay and contributes disproportionately to long-lived soil carbon — so which fungus arrives at a fallen trunk partly determines how much of that trunk ends up in the atmosphere and how much ends up in the ground. A single trunk is usually colonised by several fungi in succession, competing at visible boundaries, so both patterns can appear in the same log.

Words used here
White rot
Decay that removes lignin as well as cellulose, leaving pale, soft, fibrous wood.
Brown rot
Decay that removes cellulose and leaves modified lignin, leaving brown wood that cracks into cubes.

Fungi do most of it — but not as much as everyone assumed

Roughly a third of the world’s dead wood is taken apart by insects rather than by fungi alone.

Well supported

Good evidence backs this, though some details remain open.

A globally replicated exclusion experiment at 55 forest sites found insects responsible for approximately 29% of deadwood mass loss overall, with the largest contribution in the tropics, and estimated the global deadwood carbon flux at several gigatonnes a year.

Who this applies to
Standardised wood samples in forests on six continents.
Studied in
Insecta, Basidiomycota
Why we rate it this way, and what the caveats are
Well supportedModerate confidence

An unusually well-replicated global field experiment. The main weakness is that mesh exclusion alters microclimate as well as excluding insects, and that a few years of exposure must be extrapolated to full decay.

How far it can be extended

A single protocol run across 55 sites spanning the global climate range, which is what makes the average meaningful.

Caveats

  • Standardised samples are much smaller than real deadwood, where size, bark and ground contact matter greatly.
  • The global carbon figure depends on deadwood stock estimates that carry their own uncertainty.

Still unanswered

  • How much of the insect effect is direct consumption and how much is opening wood to fungi that follow.

Last reviewed 2026-08-30

The evidence (2 studies)

How we know

Wrapping dead wood in mesh, on six continents

How much of the world’s dead wood is taken apart by animals rather than by fungi?

At 55 forest sites around the world, wood from local tree species was cut into standard pieces and left on the forest floor. Half of each set was enclosed in a fine mesh that kept insects out while letting fungal threads, moisture and air through. After several years the pieces were collected, dried and weighed, and the difference in mass loss between caged and uncaged wood was attributed to insects.

What happened

Insects accounted for roughly a third of the wood decayed overall, with much the largest effect in the tropics and a small one in cold forests. Scaled up, deadwood was estimated to return carbon to the atmosphere on the scale of several gigatonnes a year.

What it shows

Decomposition is not a purely fungal process, and the animal share is large, climate-dependent and previously unquantified at this scale. It also puts a number on the fate of carbon held in wood.

What it does not show

A mesh cage changes the microclimate inside it as well as excluding beetles, so some of the difference may be humidity rather than insects. Standard blocks are also not whole trunks — bark, diameter and contact with the ground matter enormously for real deadwood — and a few years of exposure has to be extrapolated to full decay, which is where much of the uncertainty in the global figure sits.

The controls — what makes this evidence rather than a story
  • Caged and uncaged wood sat side by side at the same site, so climate, soil and fungal community are shared.
  • The same protocol ran across a very wide climate range, so the result is a global pattern rather than a local one.
  • Mesh let fungi and water through, isolating the animal contribution rather than excluding decay in general.

From The contribution of insects to global forest deadwood decomposition

The insect contribution is not simply chewing. Beetles and termites open wood to air and to spores, carry fungi in with them — some in dedicated pouches — and create the tunnels along which decay spreads. Separating the animal share from the fungal share is therefore slightly artificial, which is part of why nobody had put a number on it before somebody wrapped wood in mesh at 55 sites on six continents.

Words used here
Deadwood
Standing or fallen dead woody material. Ecologists call the larger pieces coarse woody debris.

What actually decides how fast something rots

Less about the weather than anybody expected.

How fast something rots depends more on what it is made of and which fungi arrive than on the weather.

Well supported

Good evidence backs this, though some details remain open.

Within biomes, litter traits explain more variation in decomposition rate than site climate does, and in standardised wood placed across a climate gradient, decomposer community composition predicted decay better than temperature or moisture.

Who this applies to
Leaf litter across biomes and standardised wood within one region; the two lines of evidence are separate.
Studied in
Tracheophyta, Basidiomycota, Ascomycota

You may have heard

“Things rot faster where it is warm and wet.”

On average, and across very different climates, yes. Within a forest the larger differences come from what the material is made of and which fungi got there first — two identical blocks in the same wood can go at very different rates.

Why we rate it this way, and what the caveats are
Well supportedModerate confidence

Two independent lines of evidence pointing the same way, one a large meta-analysis and one a designed field experiment. The scope conditions differ, which is why the claim is stated with them attached.

How far it can be extended

The litter result is explicitly within-biome; across biomes climate regains a larger role. The wood result is from one region.

Caveats

  • Litter-bag methods exclude large decomposers and change the microclimate inside the bag.
  • Across very different biomes, climate is a larger factor than either study addresses.

Still unanswered

  • Which decomposer community will colonise a given piece of wood, which neither study can predict.

Last reviewed 2026-08-30

The evidence (2 studies)

How we know

Identical blocks of wood, left in very different weather

Is how fast wood rots a question about the climate, or about which fungi happen to turn up?

Blocks of wood cut to the same size from the same material were placed at many sites spanning a wide range of temperature and rainfall. Because the wood was identical everywhere, any difference in how much of it disappeared had to come from the site rather than from the substrate. After a fixed period the blocks were collected, dried and weighed, and the decomposer communities in them were characterised.

What happened

Climate explained far less of the variation than expected. Blocks at similar temperatures decayed at very different rates, and which fungi had colonised a block predicted its fate better than the weather did.

What it shows

Wood decay is carried out by organisms whose arrival is partly a matter of chance, and treating decomposition as a function of temperature — as many carbon models do — leaves out the largest source of variation.

What it does not show

It does not identify which community will colonise a given piece of wood, so it diagnoses the gap rather than filling it. Standardised blocks are also much smaller than real deadwood, and size, bark and ground contact change decay substantially, so the size of the effect in a fallen trunk is not established by this design.

The controls — what makes this evidence rather than a story
  • The substrate was standardised, removing wood traits — the largest single control on decay — from the comparison.
  • Many sites across a long climate gradient, so temperature and moisture varied widely and systematically.
  • Decomposer communities were identified in the recovered blocks, so community composition could be tested as an explanation rather than assumed.

From Climate fails to predict wood decomposition at regional scales

For leaf litter the dominant control turns out to be the leaf. Across 818 species, within a biome, which plant the litter came from explained more of the variation in decay rate than where it was lying. Tough, lignin-rich, nutrient-poor litter decays slowly wherever it falls — so the same chemistry a tree built to deter herbivores goes on shaping its soil for years after the leaf has dropped. What a plant is made of is an ecosystem-level decision, not only a defensive one.

What was being defended

  • Plant defence

    The chemistry that deters herbivores and then slows the fungi

  • Wood

    What the material is, and why lignin is the hard part

Words used here
Litter
Dead leaves, twigs and other fine plant material on the ground, as distinct from the coarse woody debris of fallen trunks.

A fallen trunk is an address

Dead wood is one of the richest habitats a forest has, and a tidy wood is a poorer one.

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

Coarse woody debris in temperate forests persists for decades to centuries, supports a large specialist biota of fungi, invertebrates, birds and mammals, stabilises stream channels and provides establishment sites for tree regeneration; its removal reduces those functions.

Who this applies to
Temperate forest and stream systems, where the synthesis is strongest; comparable roles are documented elsewhere.
Studied in
Tracheophyta, Basidiomycota

You may have heard

“Fallen and dead trees should be cleared to keep a wood healthy.”

A fallen trunk is an address for hundreds of species, a slow-release nutrient store and often the only place seedlings can establish. Removing it changes what a wood contains, not just how it looks.

Why we rate it this way, and what the caveats are
EstablishedHigh confidence

Long-established forest ecology with a large observational literature, now supported by a globally replicated experiment quantifying the biological contribution to decay.

How far it can be extended

The functional roles have been documented across temperate forest types and are supported by the global deadwood decay work.

Caveats

  • Deadwood near paths and buildings carries genuine safety considerations, which is a separate question from its ecological value.
  • Quantities vary enormously between forest types, so any figure is illustrative.

Still unanswered

  • How much deadwood a managed forest needs to retain its specialist species, which differs by region and taxon.

Last reviewed 2026-08-30

The evidence (2 studies)

Deadwood in a temperate forest persists for decades and in some cases centuries, and while it does it holds a specialist community that lives nowhere else: fungi that fruit only on particular species at particular stages of decay, beetles whose larvae need wood already softened by a specific rot, the birds that eat those beetles, and the bats and owls that use the cavities left behind. Rotting wood also holds water, which makes it the seedbed on which the next generation of trees establishes in many forests — the nurse log, a fallen trunk carrying a row of young trees along its length.

Conservation

Deadwood is the part of a wood most easily tidied away

Deadwood is among the scarcest resources in managed woodland, because it is the component most likely to be cleared as untidy, unsafe or merchantable. Standing dead trees, fallen trunks left where they fall and old hollow veterans each support different species, and the specialists that depend on them are disproportionately represented on national threatened-species lists. Retaining deadwood is the single cheapest thing a woodland can do for its biodiversity — subject, near paths and buildings, to genuine safety judgement about what might fall.

Where this applies: General ecological principle; retention rules and safety obligations differ by country and by landowner.

When to get help: Decisions about whether a specific tree near people or property is safe belong to a qualified arboriculturist, not to a website.

Where the wood came from

  • Forests

    The structure deadwood is part of, and the disturbances that create it

  • Trees and carbon

    How long the carbon in a fallen trunk stays out of the air

Words used here
Nurse log
A fallen trunk on which tree seedlings establish. In some forests almost all regeneration happens this way.
Saproxylic
Depending on dead or decaying wood at some stage of the life cycle. A large share of forest insect diversity is saproxylic.

What is still unsettled about decay

  • How do brown-rot fungi break down cellulose without reaching most of it?

    Why it matters: Their enzymes are too large to penetrate an intact cell wall, so something smaller and chemically aggressive must go first. The details are still being worked out and they matter for industrial uses as well as for ecology.

  • Which decomposer community will colonise a given piece of wood?

    Why it matters: Community composition predicts decay better than climate does, and nothing currently predicts community composition — which leaves an unsized error in every carbon model that treats decay as a function of temperature.

    What would settle it: Long-term colonisation experiments tracking arrival order and outcome in the same pieces of wood.

  • How much of the insect effect on deadwood is consumption rather than access?

    Why it matters: Insects both eat wood and open it to fungi, and exclusion experiments cannot separate the two.

The research behind this page

10 studies, newest first. Each one has a page explaining what it found and what it could not show.

Where to go from here

Each of these follows from something on this page — a relationship in the evidence, a claim people ask about, or the next mechanism along.

How complete this record is, and what it is still missing

NatureHQ publishes its own gaps. This record is at 72% completeness against what we would call a finished subject, and was last reviewed on 2026-08-30. It carries 7 claims and answers 4 mapped search questions.

  • no popular claim about this subject has been checked yet
  • Bacterial decomposition and soil food webs are treated only in passing; they carry much of the litter breakdown this page attributes broadly.
  • Composting is what most readers searching "decomposition" mean domestically, and is deliberately out of scope.
  • Soft rot and the decay of wood in water are absent.