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 coverage · 90% complete · reviewed 2026-08-31
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
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 wood-decay fungi actually reach and exploit dead wood, which is the ecological work the enzymes make possible.
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 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.
Harmon et al., 1986 · Advances in Ecological Research
Documents the contrasting residues and their consequences for nutrient release and habitat in forest deadwood.
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.
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.
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.
Shows that decomposer identity rather than climate governs much of the variation in wood decay, which is the same lesson from the fungal side.
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.
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.
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.
Held the wood constant across a climate gradient and found decomposer community outweighing climate.
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.
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.
Fifty years of address changes, in the same piece of wood.
Decay is usually pictured as a single process running slowly. It is closer to a succession: a sequence of communities, each of which alters the wood enough to make it suitable for the next and unsuitable for itself. A fresh trunk and a crumbling one are different habitats, and the species in them barely overlap.
The stages are a description rather than a schedule. In a warm wet forest the whole sequence can run in a few years; in a cold or dry one a single stage can last decades.
Stage
What the wood is like
Who is working on it
Freshly dead
Bark intact, wood firm, still holding much of its original moisture and its soluble sugars.
Bark beetles and their fungal passengers, moulds and yeasts taking the easy sugars, first colonising decay fungi arriving at wounds.
Early decay
Bark loosening, sapwood softening, the first pale or brown patches appearing inside.
White-rot and brown-rot fungi establishing territory and fighting over it; wood-boring beetle larvae; ants and termites where they occur.
Advanced decay
Structurally soft, holding water like a sponge, crossed by dark boundary lines, breaking into fragments or cubes depending on the rot type.
Late-stage fungi, large invertebrate communities, cavity-using birds and mammals in standing trunks, mosses and lichens on the surface.
Merging with the soil
No longer recognisably wood; a dark friable mass continuous with the forest floor.
Soil fungi and bacteria, earthworms and other detritivores, plant roots growing into the residue.
Two things follow from the sequence being a sequence. The first is that a wood containing only fresh deadwood, because everything older has been cleared, holds a fraction of the species a wood with all four stages does — and the specialists of the late stages are the ones most often missing. The second is that the order is not fixed. Which fungus arrives first shapes what can follow it, which is why identical blocks of wood in identical weather decay at very different rates.
And in much of the warm world the sequence starts differently, because the animals get there before the fungi do.
Warmth speeds up microbial decay of wood. It speeds up termite decay several times faster.
Well supported
Good evidence backs this, though some details remain open.
In a standardised global field experiment, microbial wood decay approximately doubled per ten-degree Celsius rise in mean annual temperature, while decay attributable to termites increased by close to sevenfold over the same interval, and was additionally constrained by precipitation.
Who this applies to
Identical blocks of a single wood type at 133 sites spanning six continents, over a fixed exposure period.Do not extend this beyond the taxa listed — the popular version over-reaches.
Studied in
Isoptera, Fungi
Why we rate it this way, and what the caveats are
Well supportedModerate confidence
A large, well-controlled global experiment with an unusually clean design for separating animal from microbial decay. One wood type over one exposure period, so the magnitudes are more provisional than the direction.
How far it can be extended
The comparison across climates is the strength of the design; the single standardised wood type is its limit, and real forest deadwood varies enormously in size and chemistry.
Caveats
Cages exclude termites and also change the microclimate inside them, which is the standard difficulty with exclusion designs.
A block of wood on the ground is not a fallen trunk; size changes everything about how something decays.
Still unanswered
Whether termite ranges expand with warming as the temperature response implies, and how quickly.
The experiment itself: paired caged and uncaged blocks at sites across the world climates, which is what allows the two decay pathways to be separated.
Independently establishes the global share of deadwood decay done by insects, which this study localises to the warm, seasonally dry parts of the world.
How we know
The same block of wood, at 133 sites, half of it behind mesh
How much of wood decay is done by animals rather than microbes, and where?
Blocks cut to identical dimensions from the same wood were placed at sites across six continents, spanning most of the world climates. At each site, half the blocks were enclosed in mesh fine enough to exclude termites while leaving fungi and bacteria free to colonise, and half were left open. After a fixed period every block was recovered, dried and weighed. The difference between the caged and uncaged blocks at a site is the termite contribution; the loss from the caged blocks is the microbial one.
What happened
Microbial decay roughly doubled for each ten degrees Celsius of warming. Termite decay rose almost sevenfold across the same range, and was additionally limited by rainfall, concentrating the effect in warm, seasonally dry regions.
What it shows
Wood decay has two pathways with sharply different sensitivities to climate. A warming world does not simply decay faster; the balance shifts towards the animals, and towards the places where they thrive.
What it does not show
A mesh cage excludes termites and also shades what is inside it and holds moisture differently, which is the standing difficulty with every exclusion design. The blocks are small and uniform, and a fallen trunk is neither. The temperature response is measured across space, between sites that differ in many ways at once, and used to infer a response over time.
The controls — what makes this evidence rather than a story
One standardised wood type everywhere, so climate rather than substrate drives the differences between sites.
Paired caged and uncaged blocks at each site, so the two decay pathways are separated within the same conditions.
A climate gradient wide enough for a temperature response curve to be fitted rather than assumed.
The termite result is worth dwelling on because it changes what warming means for a forest floor. Microbial decay roughly doubles for every ten degrees; termite decay rises almost sevenfold over the same range, and is limited instead by rainfall. A warming forest therefore does not simply decay faster. Where termites can reach, decay shifts towards them, and the sequence above is compressed at the start into something that looks much less like rot and much more like removal.
Words used here
Succession
An orderly turnover of communities in one place, where each occupant changes the conditions in a way that favours the next.
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.
Quantifies the insect community’s contribution to breaking deadwood down, which is the biota this claim is about doing measurable work.
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.
Most of it goes back to the air. The part that does not is not what almost everyone assumes.
A decomposer is not disposing of wood. It is eating it, and what an organism does with food is respire most of it and build the rest into itself. That is the whole accounting: the majority of the carbon in a fallen trunk leaves as carbon dioxide, breathed out by fungi, bacteria and insects over years to decades, and a minority stays behind in the soil.
The carbon that stays in soil for a long time is mostly the remains of the microbes that did the eating, not the parts of the plant nothing could eat.
Emerging evidence
Real findings exist, but too few or too recent to be settled.
The persistent, mineral-associated fraction of soil organic matter derives substantially from microbial biomass and necromass produced during the efficient assimilation of labile litter compounds, rather than from the selective preservation of chemically recalcitrant plant polymers such as lignin.
Who this applies to
Soil organic matter formation, chiefly from leaf litter, in soils with mineral surfaces available for binding.Do not extend this beyond the taxa listed — the popular version over-reaches.
Studied in
Fungi, Bacteria
Why we rate it this way, and what the caveats are
Emerging evidenceModerate confidence
The direction of the field has moved decisively away from selective preservation, on isotope and fractionation evidence. The framework organises that evidence rather than testing a new prediction, and the proportions remain argued over.
How far it can be extended
The relative contribution of microbial and plant residues varies with soil mineralogy, and the underlying data comes mostly from leaf litter rather than from wood.
Caveats
This does not say lignin is irrelevant. It says lignin governs how fast litter disappears, not how much carbon the soil keeps.
Wood is under-represented in the underlying data, which is mostly leaf litter.
Proportions differ between soils; the mechanism is better established than any single number.
Still unanswered
How much of the picture holds for deadwood, where the chemistry and the decomposer community are both different.
Whether soils can be managed for microbial efficiency in a way that measurably increases stored carbon.
Establishes that litter chemistry sets decomposition rate, which is the premise this reframes: rate and long-term storage turn out to be different questions with different answers.
The minority that stays is where the intuitive picture fails, and it fails in an interesting direction. The story most of us were taught is selective preservation: the easy compounds are eaten, the hard ones — lignin above all — are left over, and the leftovers accumulate as humus. Isotope work has largely dismantled that. What persists in soil is mostly the remains of the microbes themselves, stuck to mineral particles where enzymes cannot easily reach it. Lignin governs how fast litter disappears. It does not much govern how much carbon the soil keeps.
That inverts the expected relationship between the two questions. The compounds that decompose fastest are the ones converted most efficiently into microbial bodies, so easily digested litter can contribute more to the long-lived fraction than resistant litter does. The framework is still argued over and most of the underlying data comes from leaves rather than wood, so NatureHQ carries it as the current best account rather than as settled — but the direction the field has moved is not in doubt.
Decay also returns everything that is not carbon. Nitrogen, phosphorus and the rest were taken from the soil to build the tissue, and are released back into it as the tissue is dismantled — which is why a forest with a functioning decomposer community needs no fertiliser and a forest without one starves.
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.
How complete this page is, and what it is still missing
NatureHQ publishes its own gaps. This page is at 90% completeness against what we would call a finished subject, and was last reviewed on 2026-08-31. It carries 18 claims and answers 4 mapped search questions.
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.
The stage table is a description of a temperate log. How the sequence runs in the tropics, where termites often start it, is covered by one claim rather than by the table.
How much of the microbial-origin soil carbon account holds for wood, as opposed to leaf litter, is genuinely unknown and the page says so rather than extrapolating.