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ecological relationship

Lichens

A lichen is a fungus farming a photosynthesiser inside its own body. It has no roots and no waterproofing, which is why it can live on bare rock and why it reports the quality of the air around it so faithfully.

Almost every question people ask about lichens is answered by two structural facts. A lichen has no roots, so it needs a surface rather than soil — which is why it can colonise bare rock and why the lichen on a tree is a lodger rather than a parasite, taking nothing from the bark it sits on. And it has no cuticle, the waxy waterproof layer that lets a plant control what crosses its surface, so it absorbs water and everything dissolved in it directly, without filtering. That makes a lichen an accumulator of whatever is in the air, and it is why the species growing on a churchyard wall can be read as a record of local air quality going back years. The partnership itself is the other half of the story: the fungus builds the body and provides the water relations, the photosynthetic partner supplies sugar, and since 2016 it has been clear that many macrolichens include a third partner nobody had noticed in 150 years of study.

Developed record · 57% complete · reviewed 2026-08-10

What this page covers

Not a taxonomic group but a way of living. A lichen is a fungus — usually an ascomycete — housing a green alga, a cyanobacterium, or both. Lichenisation has arisen independently many times, and around a fifth of all known fungi do it.

Often confused with: Bryophyta

Quick facts

What it is
A fungus housing a photosynthetic partner — not a plant
On a tree
A perch, not a parasite
Air quality
Absorbs everything, so it records pollution
Growth rate
Often under a millimetre a year
Partners
At least two, frequently three

A fungus with a garden inside it

The fungus builds the structure — the crust, leaf or shrub shape you can see — and inside it maintains a layer of algal or cyanobacterial cells that photosynthesise. The fungus gets sugar; the partner gets shelter, water retention and protection from ultraviolet light. Neither is what it would be alone, and the composite has a name, a shape and an ecology of its own.

A lichen is not one fungus and one alga — many have a third organism too

Well supported

Good evidence backs this, though some details remain open.

Metatranscriptome analysis of macrolichens revealed basidiomycete yeasts embedded in the cortex, present across many lichen taxa on six continents, with abundance tracking chemical and morphological differences between lichens previously indistinguishable by fungal and algal markers.

Who this applies to
macrolichens, with the decisive case in two Bryoria species
Studied in
Bryoria fremontii, Bryoria tortuosa
Why we rate it this way, and what the caveats are
Well supportedModerate confidence

Presence and distribution are firmly established by sequencing and imaging. The functional role is inferred from the correlation with chemistry rather than demonstrated, because the three-partner system is hard to culture.

How far it can be extended

The yeasts were subsequently found across a wide sample of macrolichen taxa on six continents, though not in all lichens.

Caveats

  • The yeast’s function is inferred, not demonstrated.
  • Not present in all lichens.
  • Lichens may involve bacteria and additional fungi as well, so "three" is likely also provisional.

Still unanswered

  • What does the yeast actually do?
  • How many partners does a lichen have, once bacteria are counted?

Last reviewed 2026-08-09

The evidence (1 study)

The name of a lichen is, formally, the name of its fungus. That is a historical convention rather than a judgement, and it has become awkward now that the number of partners is known to vary.

The three growth forms
FormWhat it looks likeTypical habit
CrustoseA crust fused to the surface, impossible to remove intactRock, bark, gravestones
FolioseLeafy and lobed, attached at points, edges lifting freeBark, walls, soil
FruticoseShrubby or hair-like, attached at one pointTwigs, heathland, hanging from branches

The forms matter practically: shrubby fruticose lichens have the largest surface area relative to their mass and are the first to disappear when air quality falls, while crusts hang on longest.

Words used here
Thallus
The body of a lichen. Not a leaf, stem or root — it has none of those.
Photobiont
The photosynthetic partner: a green alga, a cyanobacterium, or both.
Mycobiont
The fungal partner, which builds the body and gives the lichen its name.

Is the lichen harming my tree?

Lichens on a tree are using it as a perch, not feeding on it

Established

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

Lichens are epiphytic rather than parasitic. They have no roots and no vascular connection to the substrate, absorbing water and dissolved minerals directly across the thallus surface from rain, dust and air. Heavy lichen growth on a tree indicates suitable bark, light and humidity, and is commonly associated with slow-growing or declining trees rather than being a cause of decline.

Who this applies to
epiphytic lichens on trees and shrubs
Studied in
Lecanoromycetes

You may have heard

“Lichen is killing my tree”

The correlation is real and the causation is backwards. Lichens need light, and a tree with a thinning canopy provides it — so lichens arrive on trees that are already struggling and get blamed for it. They have no means of taking anything from the tree even if they were inclined to.

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

Basic lichen anatomy: there is no structure by which a lichen could draw resources from a host, and none has ever been described.

How far it can be extended

Absence of roots and of any vascular connection is a defining structural feature of lichens, not a property of particular species.

Caveats

  • Very dense growth can shade small twigs, which is a minor effect and not the cause of a tree’s decline.
  • A struggling tree with a thin canopy lets more light through, which favours lichens — the correlation runs that way round.
  • A few fungi genuinely do parasitise trees; those are not lichens.

Still unanswered

  • How much does bark chemistry determine which lichens colonise which tree species?

Last reviewed 2026-08-10

The evidence (1 study)

  • Supports · primary

    Lichen Biology

    Nash, 2008 · Cambridge University Press

    Thallus structure, water relations and the absence of any absorbing connection to the substrate.

No. There is no structure by which a lichen could take anything from a tree — no roots, no penetrating hyphae into living tissue, no vascular connection. It is sitting on the bark using it as a surface, in the same way it would use a rock or a roof tile.

The reason the belief persists is that the correlation is real and points the other way. Lichens need light. A tree with a full canopy shades its own branches; a tree that is thinning, ageing or stressed lets light through, and the lichens arrive to take advantage. So heavy lichen growth is a reasonable sign that a tree is struggling, and a poor guess at why.

Practical

If a tree is declining and covered in lichen

Removing the lichen treats a symptom that is not even a symptom of anything the lichen did. If a tree is genuinely in decline the causes worth investigating are root damage, compaction, drought, disease or changes to the water table. Scrubbing bark damages it and helps nothing.

Where this applies: General principle; a local arboricultural service can assess a specific tree.

Words used here
Epiphyte
An organism growing on another for support, taking no nutrients from it.

Reading the air off a wall

A living instrument, and one that reports years rather than minutes.

Lichens report air quality because they absorb everything that lands on them

Established

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

Lacking a cuticle and roots, lichens absorb water and dissolved substances across the whole thallus without selectivity, and accumulate atmospheric pollutants. Epiphytic lichen communities drop out in a consistent species order as sulphur dioxide rises, a sequence regular enough to have been calibrated into a field scale for estimating pollution from the flora present.

Who this applies to
epiphytic lichens; the calibrated scale is specific to sulphur dioxide in England and Wales
Studied in
Lecanoromycetes
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

A field scale calibrated against measured concentrations across a national pollution gradient, used in practice for decades and corroborated by the return of sensitive species as air quality improved.

How far it can be extended

The physiological basis — unselective absorption with no cuticle — is general to lichens, though which species indicate what varies by region and pollutant.

Caveats

  • The classic scale is calibrated for sulphur dioxide; nitrogen compounds now dominate and affect lichens differently, favouring some species.
  • Bark chemistry, humidity and light shift lichen communities independently of air quality.
  • Lichens integrate exposure over years, so they report a history rather than today’s air.

Still unanswered

  • How well do the classic scales transfer to nitrogen-dominated pollution?
  • Can lichen surveys substitute for instrumentation in under-monitored regions?

Last reviewed 2026-08-10

The evidence (2 studies)

A plant controls what crosses its surface with a waxy cuticle and closable pores. A lichen has neither. Water arrives on the whole surface and is absorbed with whatever is dissolved in it, which is efficient in clean air and unsurvivable in dirty air — sulphur dioxide in particular dissolves to acid directly on the thallus.

Because species differ in tolerance, they disappear in a consistent order as pollution rises. That regularity was calibrated in 1970 into a field scale: identify the lichens on a tree and you can estimate the sulphur dioxide the site has been exposed to. Shrubby species go first, leafy next, crusts last; above a threshold the bark carries only green algae.

The scale has partly dated itself. British air improved so much after clean air legislation that sensitive lichens have been returning to cities for decades — and nitrogen compounds, which now dominate, favour a different set of species entirely.

How we know

Reading sulphur dioxide off the bark of a tree

Air quality monitoring in 1970 meant instruments, and there were few of them. Could the organisms already growing on every tree be read as a record of the air instead?

The approach only works if the lichens drop out in a consistent order rather than simply thinning. Epiphytic lichen communities were surveyed on trees along transects running from industrial areas into cleaner countryside in England and Wales, where measured sulphur dioxide concentrations were already available from instrument networks. The species present at each point were recorded and compared against those measurements, and the recurring sequence — which species disappear first, which persist longest — was turned into a numbered scale a surveyor could apply in the field with no equipment.

What happened

Lichen communities changed in a repeatable order along the pollution gradient, regular enough to support a ten-point scale estimating mean sulphur dioxide from the flora present on a tree.

What it shows

That an organism can be a calibrated instrument. The physiological basis is that lichens have no cuticle and no roots and absorb across the whole thallus without selectivity, so they integrate what is in the air over years rather than sampling it for an afternoon — which an instrument switched on last week cannot do.

What it does not show

The scale is specific to sulphur dioxide, and to England and Wales. Applied elsewhere, or to a different pollutant, the species order has to be re-derived. It also gives a long-run average rather than a current reading, and a return of sensitive species lags improvement in the air.

The controls — what makes this evidence rather than a story
  • Bark type and tree species were accounted for, since bark acidity affects which lichens can colonise regardless of air quality.
  • The sequence was checked across multiple independent transects rather than derived from one gradient.
  • The scale was calibrated against instrument-measured sulphur dioxide, not against distance from a city, so proximity effects other than the pollutant were not doing the work.

From Qualitative scale for estimating sulphur dioxide air pollution in England and Wales using epiphytic lichens

Words used here
Cuticle
The waxy waterproof layer covering a plant. Lichens have none, which is the whole reason they work as indicators.
Bioindicator
An organism whose presence or absence measures an environmental condition. Lichens integrate exposure over years.

Making soil out of stone

Lichens can live on bare rock, and slowly turn it into soil

Well supported

Good evidence backs this, though some details remain open.

Lichens weather rock physically, through hyphal penetration along mineral boundaries and the expansion and contraction of the thallus with wetting and drying, and chemically, through secreted oxalic and lichen acids that dissolve minerals and chelate metal ions. Colonisation measurably accelerates surface breakdown and contributes organic matter, initiating soil formation on substrates no plant could root in.

Who this applies to
crustose and foliose lichens on rock surfaces
Studied in
Lecanoromycetes
Why we rate it this way, and what the caveats are
Well supportedModerate confidence

Both mechanisms are directly observed and their contribution to early soil formation is well accepted. Confidence is moderate because rates vary enormously with rock type and climate, and lichen cover can also shelter rock from rain and frost, so the net effect is not always erosive.

How far it can be extended

Both weathering mechanisms have been documented across many lichen taxa and rock types, with rates varying by orders of magnitude.

Caveats

  • Rates differ by orders of magnitude between rock types and climates.
  • Lichen cover sometimes protects rock from rain and frost, so the net effect can be protective.
  • Primary succession on rock operates over decades to centuries, not seasons.

Still unanswered

  • How much of global silicate weathering do lichens account for?
  • When does lichen cover protect rock rather than erode it?

Last reviewed 2026-08-10

The evidence (2 studies)

Lichens are the standard example of primary succession because they can occupy a surface that offers nothing at all. A bare rock face has no soil for a root, and a lichen does not need one — so it arrives first, and in living there begins to make the thing the next colonist will need.

It works two ways at once. Hyphae push into cracks and along mineral boundaries, and the thallus swells when wet and shrinks when dry, prising fragments loose. Secreted acids dissolve minerals chemically. Add trapped dust and the organic matter of dead lichen and, over decades to centuries, there is enough substrate for a moss, then a small plant.

The honest qualification is that lichen cover sometimes protects rock instead — sheltering it from rain and frost — so the net effect depends on climate, rock type and species. It is not simply an erosive force.

Related

  • Fungi

    The kingdom the fungal partner belongs to

  • Photosynthesis

    What the partner is doing

  • Trees

    The commonest thing to find a lichen on

Words used here
Primary succession
Colonisation of a surface that has never held life — bare rock, new lava, retreating glacier. Lichens usually go first.

How lichens spread, and how slowly

A lichen has a problem no single organism has: to establish somewhere new, both partners must arrive together. It is solved either by cheating — dispersing fragments containing both — or by the fungus dispersing spores alone and having to find a compatible partner on landing.

  • Soredia: powdery bundles of algal cells wrapped in hyphae, which rub off and blow away carrying both partners.
  • Isidia: small outgrowths that break off, doing the same job with a sturdier structure.
  • Fungal spores: dispersed alone from cup-shaped fruiting bodies, and must encounter a suitable photobiont to form a lichen.
  • Fragmentation: a piece knocked off by weather or an animal, which simply carries on.

Growth is very slow — commonly a fraction of a millimetre a year for crustose species. That is what makes lichenometry possible: measure the largest lichen on a rock surface, know the growth rate for that species and climate, and estimate how long the surface has been exposed. It has been used to date glacial retreat and rockfalls, and it carries real uncertainty.

Some Arctic crustose lichens are estimated at several thousand years old, which would make them among the oldest living things on Earth.

Words used here
Soredium
A dispersal packet of algal cells bound in fungal threads — the whole partnership in a speck of powder.
Lichenometry
Dating a rock surface from the size of the lichens on it. Approximate, and sometimes the only method available.

What is still unsettled

  • How many partners does a lichen actually have?

    Why it matters: The two-partner definition survived 150 years before a third was found in 2016. Bacteria are routinely present and their role is unclear, so the current count may not be final either.

  • How well do classic pollution scales transfer to nitrogen-dominated air?

    Why it matters: The calibrations were built for sulphur dioxide, which has largely gone. Nitrogen compounds favour a different flora, so the same survey now means something different.

  • How old can a lichen get?

    Why it matters: Estimates of several thousand years rest on extrapolated growth rates rather than measurement, which is the same weakness as every claim about very old organisms.

Claims about this, checked

Things people have heard, and what the evidence actually supports.

The research behind this page

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

You may also find this interesting

What this page is still missing

NatureHQ publishes its own gaps. This record is at 57% completeness against what we would call a finished subject.

  • no research from the last few years is attached — check for newer work
  • more experiments could be explained in plain English
  • Lichen identification is heavily searched and deliberately not attempted — it needs chemistry and microscopy.
  • Traditional uses, dyes and reindeer forage are absent.
  • Lichen responses to nitrogen pollution specifically deserve their own treatment.

Last reviewed 2026-08-10 · 4 claims · 0 search questions answered on this page