Skip to content
NatureHQ

phenomenon

Forests

Most of the species in a forest live on the parts of it that are already dead.

A forest is not a collection of trees. It is a structure in layers, most of its species living on the dead parts, and it is shaped by disturbance — fire, wind, drought, insects — which is how much of it regenerates rather than an interruption to it.

The most useful correction to make about a forest is that the living trees are not where most of the biology is. A very large share of forest species depend on dead or decaying wood, on the litter layer, or on the fungi that connect the two; the standing trees are the structure those things happen in. The second correction is about disturbance. A forest is not a stable end-state that fire and windthrow interrupt: many forests are what they are because of fire and windthrow, some species cannot release seed without heat, and the gaps that a fallen giant opens are where the next generation gets its light. That is why an old-growth forest looks untidy — a range of tree ages, standing dead trunks, fallen logs at every stage of decay, and gaps — while a plantation of one species of one age looks like a forest and functions like a crop. What is changing is not that disturbance exists but how often it comes and in what combinations, with drought weakening trees that insects then kill and fire then takes. And underneath all of it, in most forests, sits the largest carbon store on the site: the soil.

Developed record · 76% complete · reviewed 2026-08-30

What this page covers

Forests as ecosystems: structure, disturbance, regeneration and carbon. Weighted towards temperate and boreal systems, where the long-term measurements are.

Often confused with: Plantations; Savannas; Wooded grasslands

Quick facts

Where the species are
Disproportionately in deadwood, litter and soil
Disturbance
A structuring process, and intensifying
Old stands
Still taking up carbon, and holding centuries of it
Largest carbon store
Usually the soil, not the trees

A forest is a set of layers, and most life is in the lower ones

A forest is usually described from the top down — canopy, understorey, shrub layer, floor — and that ordering is misleading about where the biology is. The canopy takes the light and does the photosynthesis; almost everything else in the system runs on what falls out of it. Beneath is a shaded understorey of young and suppressed trees waiting for a gap, then a floor of litter, deadwood and soil in which the great majority of the forest’s species live: fungi, invertebrates, bacteria, and the roots of everything above.

  • Light falls off sharply through a canopy, so leaves at different depths in the same tree are built differently — small and thick at the top, broad and thin below.
  • The understorey is largely a queue. Many species can persist for decades in deep shade, growing very little, waiting for a canopy gap.
  • The litter layer is where nutrients are returned, and its depth reflects how fast the local decomposers work.
  • Soil holds the largest carbon store in most forests and is the least visible part of the system.
  • Deadwood, standing and fallen, is a structural component in its own right, persisting for decades.

The floor, in detail

  • Decomposition

    What takes the litter and the fallen trunks apart

  • Roots

    How thoroughly the ground beneath a wood is occupied

Words used here
Canopy
The layered upper surface of leaves. Light falls off sharply through it, so each depth is a different environment.
Understorey
The shaded layer beneath the canopy, containing young and suppressed trees, shrubs and herbs.

Disturbance is how a forest renews itself

Fire, wind, drought and insects are not interruptions to forest ecology. They are part of it.

Fire, wind, drought and insects are how many forests renew themselves — and they are getting more frequent.

Well supported

Good evidence backs this, though some details remain open.

Disturbance is an intrinsic component of forest dynamics and a driver of structural diversity and regeneration. Synthesis of the evidence finds disturbance regimes intensifying across many forest types under climate change, with amplifying interactions between drought, insect outbreak and fire.

Who this applies to
Forest ecosystems generally, with the strongest evidence from temperate and boreal Europe and North America.
Studied in
Tracheophyta

You may have heard

“A forest fire is a disaster for the forest.”

Many forests are shaped by fire and regenerate through it; some species need it to release seed at all. What is changing is how often and how severely it burns, and that is a different concern from fire itself.

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

That disturbance structures forests is long established. That regimes are intensifying is supported by observation and projection, with regional variation large and model uncertainty substantial.

How far it can be extended

The role of disturbance is general; the direction and rate of change differ substantially by region.

Caveats

  • Disturbance being natural does not make its intensification harmless; frequency and combination are what is changing.
  • Projections rest on climate and disturbance models with wide uncertainty.

Still unanswered

  • Whether forests will regenerate as before under more frequent disturbance, or shift to different states.

Last reviewed 2026-08-30

The evidence (3 studies)

When a large tree falls it opens a gap, and the gap is where regeneration happens: light reaches the floor, the suppressed saplings that have been waiting for decades grow, and one of them takes the space. A forest with a range of ages in it is a forest that has been disturbed repeatedly at a range of scales. Fire does something similar at a larger scale, and some species depend on it outright — cones that open only when heated, seeds that germinate on cues in smoke.

What is changing is frequency and combination rather than the existence of disturbance, and the combinations are the dangerous part: drought weakens trees, weakened trees are killed by insects, dead stands burn, and burnt ground is harder for the same forest to reoccupy. That sequence is why the question is no longer whether forests regenerate after disturbance but whether they regenerate into the same thing.

What a cue like smoke does to a seed

  • Germination

    Fire cues, dormancy, and why some seeds wait for a burn

  • Seed dispersal

    How the next generation reaches the gap in the first place

Words used here
Gap dynamics
Regeneration driven by the openings left when large trees die. The main mechanism by which many closed forests renew themselves.
Disturbance regime
The characteristic frequency, size and severity of disturbance in a given forest. It shapes which species can persist there.

What makes an old forest different from an old plantation

Old forests are still absorbing carbon, and they are holding centuries of it already.

Well supported

Good evidence backs this, though some details remain open.

Compiled flux measurements from boreal and temperate stands over about 150 years old found net carbon uptake continuing rather than reaching balance, alongside very large accumulated stocks in wood and soil that are released if the stand is disturbed.

Who this applies to
Old boreal and temperate forest stands; tropical old growth is under-represented in the data.
Studied in
Tracheophyta

You may have heard

“An old forest is carbon-neutral, so there is no carbon reason to protect it.”

Even where uptake has slowed, the stock is what matters. An old forest holds centuries of accumulated carbon in wood and soil, and disturbing it returns that carbon on a timescale of years rather than centuries.

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

A substantial compilation with a clear average result, but flux measurements at a limited number of sites are extrapolated widely, and the variation between stands is large.

How far it can be extended

The measurements are concentrated in the northern hemisphere, and stands that are net sources exist within the same dataset.

Caveats

  • The stock is the larger number, and the more consequential one: it took centuries to build and can be released in a season.
  • Averages conceal stands that are net sources, particularly after disturbance.

Still unanswered

  • How tropical old-growth forests behave, where flux measurements remain sparse.

Last reviewed 2026-08-30

The evidence (2 studies)

Age alone does not make an old-growth forest. What distinguishes one is structural: trees of many ages and sizes rather than one cohort, large old individuals, standing dead trunks, fallen wood at every stage of decay, gaps, and a soil that has not been disturbed for a very long time. Those features are what the specialist species need, and a plantation of one species planted in one year has none of them however long it stands.

Two things that both look like forest from a distance.
FeatureOld-growth forestEven-aged plantation
Age structureMany cohorts, from seedlings to veteransOne cohort, planted together
DeadwoodAbundant, standing and fallen, all decay stagesUsually removed or absent
CanopyUneven, with gaps of varying ageClosed and uniform until thinning
SoilUndisturbed, with deep accumulated carbonOften disturbed at establishment
Specialist speciesSupported, including deadwood and cavity dependantsLargely absent, whatever the tree species

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)
Words used here
Old growth
Forest with the structural features that develop over a long time without stand-replacing disturbance: mixed ages, large veterans, abundant deadwood, undisturbed soil.

What a forest does with carbon

The world’s forests take up an enormous quantity of carbon, and clearance cancels much of it.

Established

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

Global forest carbon accounting for 1990–2007 found established forests taking up carbon at a large and sustained rate, with tropical deforestation releasing a comparable quantity, so the net forest sink was substantially smaller than the gross uptake.

Who this applies to
Global forest carbon budgets over recent decades.
Studied in
Tracheophyta
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

Forest carbon accounting is a mature field with independent methods — inventories, plots, atmospheric measurement — that broadly agree on the size of the sink, even where they disagree on its attribution.

How far it can be extended

Built from national inventories and plot networks covering the world’s forest area, with the largest uncertainty in the tropics.

Caveats

  • A sink is a balance between uptake and loss in a given period, not a property a forest possesses.
  • Tropical inventory coverage is the weakest part of the accounting and the largest single uncertainty.

Still unanswered

  • How much further the tropical sink will weaken, and whether temperate and boreal uptake will offset it.

Last reviewed 2026-08-30

The evidence (3 studies)

A forest takes carbon in through its canopy and returns it through respiration, decomposition and disturbance. Whether it is a sink in a given year is the balance of those, and the balance moves: the Amazon plot network recorded the regional sink falling by about a third over three decades, driven by trees dying sooner rather than by trees growing less. Meanwhile the store — the accumulation of centuries in wood and, more of it, in soil — is a much larger number than the annual flow, and it is the number at risk when a forest is cleared or burnt.

The full carbon account

The half of a forest nobody photographs

A tree’s roots reach far outside the edge of its branches, and neighbouring trees’ roots overlap thoroughly.

Well supported

Good evidence backs this, though some details remain open.

Excavation and tracer studies of forest and open-grown trees record lateral root extension commonly several times the crown radius, so that the root systems of neighbouring trees interpenetrate rather than occupying separate footprints.

Who this applies to
Temperate forest and open-grown trees, where most excavation has been done.
Studied in
Pinaceae, Fagaceae, Salicaceae

You may have heard

“A tree’s roots stop at the edge of its canopy.”

The drip line is a gardening convention, not a boundary. Roots commonly extend two or three times the crown radius, which is why trees in a wood share soil thoroughly — the precondition for everything about shared fungal networks and below-ground competition.

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

The observations are consistent and come from several methods, but they are opportunistic case studies rather than a designed survey, and are weighted towards temperate species.

How far it can be extended

Recorded across conifers and broadleaves in a range of soils, and consistent with the shallow, spreading architecture found globally.

Caveats

  • Extent varies enormously with soil, spacing and competition.
  • That a root reaches somewhere does not establish what it does there.

Still unanswered

  • How much of the overlap between neighbouring root systems represents competition and how much shared infrastructure.

Last reviewed 2026-08-30

The evidence (3 studies)

Beneath a wood the root systems of neighbouring trees run through one another, and nearly all of those roots are joined to fungi. That is the structure the "wood wide web" describes, and it is real: the physical connections exist, and carbon and nutrients move along them. What is disputed — and disputed within the field rather than by outsiders — is how much reaches a neighbouring plant, whether the neighbour is any better off, and whether anything about it resembles sharing.

The claim, taken apart

What is still unsettled about forests

  • Will forests regenerate into the same forests after more frequent disturbance?

    Why it matters: Regeneration after a single disturbance is well documented. Regeneration after two in quick succession, on a hotter site, is not, and it decides whether a forest recovers or converts to something else.

  • How much forest soil carbon is stable over centuries?

    Why it matters: Soil is usually the largest store on the site and the least well measured, so the uncertainty runs through every forest carbon estimate.

  • How much deadwood does a managed forest need to keep its specialist species?

    Why it matters: Retention thresholds are set by convention rather than by evidence, and they differ by region and by taxon.

The research behind this page

17 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 76% completeness against what we would call a finished subject, and was last reviewed on 2026-08-30. It carries 11 claims and answers 5 mapped search questions.

  • no popular claim about this subject has been checked yet
  • Tropical forest ecology is under-represented here relative to its share of the world’s forest, following the measurement record rather than the importance.
  • Forestry, certification and management practice are deliberately out of scope.
  • Forest hydrology — how a forest changes rainfall, runoff and local climate — is a substantial absence.