Skip to content
NatureHQ

higher taxon

Trees

A tree is a growth form, not a family — being tall and woody has evolved many times over. Its central problem is water: a tree has no pump, and pulls water up under tension until, at around 120 metres, the physics runs out.

The first surprising thing about trees is that they are not a group. Being a tree is a solution — grow tall, stay up, get the light — and unrelated lineages have arrived at it repeatedly, which is why palms, tree ferns and oaks are not close relatives in any useful sense. The second is how the water gets to the top. There is no pump anywhere in a tree. Evaporation from leaves pulls on a continuous column of water held together by its own cohesion, so the water in a trunk is under tension — stretched, not pushed — the entire way up. That single fact explains most of what trees do and cannot do: why they die in drought, why wood is built the way it is in different climates, and why the tallest tree ever measured sits just under the height at which the column would break. Set against that mechanical story is a second one about what trees do to each other, which is where popular coverage runs furthest ahead of the evidence, and where NatureHQ is deliberately careful.

Developed record · 68% complete · reviewed 2026-08-10

What this page covers

Not a taxonomic group. "Tree" is a growth form — a tall, self-supporting, woody perennial — and it has evolved independently many times across unrelated plant lineages.

Quick facts

What it is
A growth form, evolved independently many times
Water transport
Pulled up under tension by evaporation — no pump
Height limit
Around 122–130 m, set by hydraulics
Mast years
Synchronised heavy seed crops, cued by weather

A tree is a strategy, not a family

There is no group of organisms called trees. A tree is a woody perennial that holds itself up and grows tall, and that combination has been arrived at independently by conifers, flowering plants, tree ferns and others. An oak is more closely related to a strawberry than to a pine.

Even wood is not one thing. Palms have no true wood and no growth rings; they thicken differently and cannot heal damage the way an oak does. Bamboo is a grass. Asking what makes a tree a tree is asking about a shape, and the interesting question underneath is what that shape costs.

Growth rings exist because a temperate tree lays down wide, thin-walled vessels in spring and narrow, dense wood later in the season. A tree in an aseasonal tropical climate may have no usable rings at all.

Words used here
Cambium
A thin cylinder of dividing cells just under the bark. It makes wood inward and bark outward, and it is the only part of a trunk that is growing.
Perennial
A plant that lives for more than two years, as opposed to an annual.

How water gets to the top

The central problem of being tall, and the reason there is a ceiling.

A tree has no pump. Water is pulled up, stretched, by evaporation from the leaves

Established

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

Water ascends xylem under negative pressure generated by evaporation at leaf surfaces, held as a continuous column by the cohesion of water and its adhesion to vessel walls. The column is under tension throughout, and can cavitate — breaking into gas — under drought or freezing, blocking that conduit.

Who this applies to
vascular plants, trees above all
Studied in
Tracheophyta
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

The central mechanism of plant water transport, supported by a century of measurement and by the observation that the predicted height limit matches the tallest trees measured.

How far it can be extended

Cohesion–tension is the accepted mechanism of sap ascent across vascular plants, supported by direct measurement of xylem tension in many taxa.

Caveats

  • Root pressure contributes in some species and conditions, but cannot account for height.
  • How trees repair embolised conduits is actively debated.
  • Measuring xylem tension directly is technically contested at the extremes.

Still unanswered

  • How do trees refill embolised vessels, if they do?
  • How close to the cavitation threshold do trees routinely operate?

Last reviewed 2026-08-10

The evidence (2 studies)

  • Supports · primary

    Xylem Structure and the Ascent of Sap

    Tyree and Zimmermann, 2002 · Springer

    The standard synthesis of xylem structure and the cohesion–tension mechanism.

  • Supports · primary

    The limits to tree height

    Koch et al., 2004 · Nature

    Measured water potential up the tallest trees on Earth, showing the mechanism approaching its limit.

Water evaporating from a leaf pulls on the water behind it. Because water molecules stick to one another strongly, that pull transmits all the way down a continuous column in the xylem to the roots. Nothing is pushing. The whole column is under tension, which is why a cut trunk can hiss and why the mechanism fails catastrophically rather than gradually: if the column breaks, an air bubble forms and that conduit is finished.

Trees stop growing at around 120–130 m because water cannot be pulled higher

Well supported

Good evidence backs this, though some details remain open.

Measurements up five coast redwoods above 110 m showed progressively more negative leaf water potential and declining photosynthesis with height, as the tension required to lift water approaches the cavitation threshold. Extrapolation places the maximum achievable height at roughly 122–130 m.

Who this applies to
coast redwoods, the tallest trees known
Studied in
Sequoia sempervirens
Why we rate it this way, and what the caveats are
Well supportedHigh confidence

Direct measurement at the top of the tallest trees on Earth, with a predicted ceiling that sits just above the tallest individual ever recorded — an agreement that would be a remarkable coincidence if the mechanism were wrong.

How far it can be extended

The hydraulic constraint follows from physics common to all vascular plants, though the height at which it binds differs by species and climate.

Caveats

  • One species at the extreme of its range; other tall species may meet different limits first.
  • Mechanical stability and nutrient supply also constrain height and are not separated here.
  • Five individuals, because there are not many trees this tall.

Still unanswered

  • Do mechanical limits bind before hydraulic ones in broadleaved trees?
  • How will the ceiling shift under drier climates?

Last reviewed 2026-08-10

The evidence (2 studies)

How we know

Climbing the tallest trees to find out why they stopped

Why do trees stop getting taller, and what sets the ceiling?

The question could not be answered from the ground. Researchers climbed five coast redwoods over 110 metres tall — among the tallest living things — and sampled leaves and measured water potential at intervals from the base to the very top. If water transport is the constraint, conditions at the top should be measurably worse than lower down, in a specific and predictable way.

What happened

Leaf water potential became progressively more negative with height and photosynthesis declined, exactly as the tension required to lift water approaches the point at which the water column breaks. Extrapolating the trend put the maximum achievable height at roughly 122–130 metres.

What it shows

Tree height is limited by the physics of pulling water upward. The predicted ceiling sits just above the tallest tree ever measured — an agreement that would be a startling coincidence if the mechanism were wrong.

What it does not show

It covers one species at the extreme of its range. Other tall trees may hit mechanical or nutrient limits before hydraulic ones, and this design cannot separate those constraints. Five individuals is also very few — though there are not many 110-metre trees to choose from.

The controls — what makes this evidence rather than a story
  • Sampling at intervals up the same trunk means each tree is its own control: the comparison is height, not species or site.
  • Leaf structure and photosynthetic rate were measured alongside water potential, so the physiological consequence could be seen rather than assumed.
  • Five separate individuals were climbed, guarding against one unusual tree.

From The limits to tree height

The trade-off in wood anatomy follows directly. Wide vessels move a lot of water and cavitate easily; narrow ones are slow and safe. Ring-porous trees like oak gamble on wide early-season vessels; conifers use narrow tracheids and survive freezing better. Wood structure is a risk decision about water.

Words used here
Xylem
The tissue carrying water up from the roots. Once mature, its cells are dead — it is plumbing, not living tissue.
Cavitation
A water column under tension snapping and forming a bubble, blocking that conduit. The failure mode that limits tree height.
Transpiration
Water evaporating from leaves. It is the engine that lifts water, and the reason a large tree can move hundreds of litres on a summer day.

Feeding a trunk from leaves

The mechanism is canonical; what a tree adds is scale and a gradient.

A tree is built mostly out of air — the carbon in wood came from carbon dioxide

Established

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

The dry mass of a plant is predominantly carbon, hydrogen and oxygen. Carbon enters exclusively as atmospheric carbon dioxide fixed in photosynthesis; hydrogen and oxygen derive from water. Soil supplies mineral nutrients required in comparatively small quantities, not the bulk of the material.

Who this applies to
plants generally, trees most strikingly
Studied in
Plantae

You may have heard

“Trees get their mass from the soil”

Van Helmont tested this in the 1640s by growing a willow in weighed soil for five years. The tree gained 74 kg; the soil lost about 60 g. He concluded the mass came from water, which was half right — most of the dry mass is carbon, and it came out of the air.

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

Established by the carbon-fixation work and by elemental analysis of biomass. Van Helmont’s seventeenth-century willow experiment showed the soil loses almost no mass, though he drew the wrong conclusion from it.

How far it can be extended

Elemental composition of plant biomass is general, and the carbon source is established by isotope tracing.

Caveats

  • Soil nutrients are essential; the claim concerns bulk mass, not necessity.
  • A large fraction of a living tree’s mass is water, which does come through the roots.
  • Mineral nutrient limitation constrains growth even though it contributes little mass.

Still unanswered

  • How will rising atmospheric carbon dioxide alter growth where nutrients limit it?

Last reviewed 2026-08-10

The evidence (2 studies)

How photosynthesis works is explained once, on its own page. What a tree contributes is a problem of scale: a canopy is not one leaf but a stack of them competing with each other for the same light, and the leaves at the bottom are living in the shade cast by the leaves at the top.

  • Sun leaves at the canopy top are small, thick and tolerant of high light. Shade leaves lower down are broad, thin and efficient at low light — the same tree building two different organs.
  • Stomata must open to admit carbon dioxide, and water escapes while they are open. In a tall tree that water has to be pulled a hundred metres, which is why hydraulics and photosynthesis constrain each other.
  • A deciduous tree dismantles its chlorophyll and recovers the nitrogen before dropping a leaf. Autumn colour is disassembly, not decay.
  • An evergreen keeps cheaper, longer-lived leaves and photosynthesises whenever conditions allow — a different bet, favoured where the growing season is short or nutrients are scarce.

The mechanism itself

Words used here
Canopy
The layered upper surface of leaves. Light falls off sharply through it, so leaves at different depths face different problems.

Why acorn crops swing so wildly

Huge acorn years are a strategy to swamp the animals that eat seeds, not just a good summer

Well supported

Good evidence backs this, though some details remain open.

Many perennial plants produce seed crops that are highly variable between years and strongly synchronised across populations. The pattern is better explained by predator satiation and, in wind-pollinated species, pollination efficiency, than by resource availability. Weather acts as a synchronising cue rather than as the proximate cause.

Who this applies to
masting perennial plants, oaks and beeches above all
Studied in
Quercus, Fagus, Nothofagus

You may have heard

“It was a good year, so the oaks made lots of acorns”

Backwards. Good weather is the shared signal trees use to fruit at the same time, not the reason they can. The point of a mast year is to produce more seed than every squirrel, jay and weevil in the district can possibly eat — which only works if all the trees do it at once, and only works if there are lean years in between to keep those populations down.

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

The pattern is unambiguous and the resource-matching explanation is ruled out by the degree of variability. Confidence is held at moderate because predator satiation and pollination efficiency are difficult to separate in wind-pollinated trees, and both are probably operating.

How far it can be extended

Masting is documented across many unrelated genera on several continents with the same signature of synchrony and variability.

Caveats

  • Which explanation dominates varies by species and system.
  • Long-term seed-crop data outside the temperate northern hemisphere are scarce.
  • Masting has consequences far beyond the tree — rodent, bird and tick populations track it.

Still unanswered

  • How is synchrony maintained across hundreds of kilometres?
  • What physiological switch translates a weather cue into a flowering decision?

Last reviewed 2026-08-10

The evidence (1 study)

The consequences run well beyond the tree. A mast year feeds a rodent population that then crashes; the animals that eat rodents follow, and in some regions tick numbers and the diseases they carry track the acorn crop two years earlier. A single reproductive decision by oaks propagates through an entire food web.

What eats and moves the seed

Words used here
Masting
Synchronised, highly variable seed production across a population. Common in oaks and beeches.
Predator satiation
Producing so much seed at once that seed-eaters cannot consume it all, so some survives.

Do trees talk to each other?

Three separate things get merged under this heading, and separating them is most of the work. Trees do release volatile compounds when damaged, and neighbours exposed to them defend themselves better — that is demonstrated in the field. Trees are connected by fungal networks, and carbon does move between them. And trees are said to recognise their offspring and send them resources, which is where the evidence thins sharply.

NatureHQ treats the first as established, the second as real but routinely overstated — a systematic review found positive citation bias running well ahead of the underlying data — and the third as not demonstrated. The word "talk" implies a sender addressing a receiver for the sender’s benefit, and the best current explanation of airborne signalling is that a plant is signalling to its own distant leaves and neighbours are overhearing.

The full treatment

What is still unsettled

  • Can trees repair embolised vessels, and how?

    Why it matters: If a cavitated conduit is permanently lost, drought damage accumulates irreversibly over a tree’s life. The evidence for repair is contested.

  • How is masting synchronised across hundreds of kilometres?

    Why it matters: Weather is the cue, but the precision of the synchrony is greater than shared weather alone easily explains.

  • Do mechanical limits bind before hydraulic ones in broadleaved trees?

    Why it matters: The height ceiling was measured in redwoods; whether the same constraint sets the limit for an oak is not established.

The research behind this page

14 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 68% completeness against what we would call a finished subject.

  • no research from the last few years is attached — check for newer work
  • no popular claim about this subject has been checked yet
  • Photosynthesis is referred to constantly and never explained in its own right.
  • Leaf loss, dormancy and autumn colour are heavily searched and absent.
  • Forest ecology, deforestation and carbon storage are major topics not covered here.

Last reviewed 2026-08-10 · 12 claims · 40 search questions answered on this page