No tree has a pump. Water is pulled to the top under tension — and at about 120 metres, that stops working.
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.
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.
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.
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?
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.
Diagram
How water reaches the top of a tree
Three steps, no pump anywhere. The water potential scale on the left runs from near zero at the soil to strongly negative in the leaves.
The same explanation in words
A schematic tree — canopy, trunk and roots — with the water column drawn as a continuous band running from the roots to the leaves, and a single arrow pointing upward along it. Three numbered stages are labelled beside it. First, water evaporates through open stomata in the leaves; nothing anywhere is pumping. Second, the column is pulled: water molecules hold to one another, so the pull at the top is transmitted down an unbroken column in the xylem, and the whole column is therefore under tension — stretched, not pushed. Third, the water is replaced at the roots, where root hairs take up water from soil that sits at a less negative pressure than the leaf. A vertical scale on the left marks water potential, running from near zero at the soil surface to strongly negative at the top of the canopy. The tension is what drives the flow, and it is also what limits it: pull hard enough and the column snaps into vapour.
Water can be lifted to the top of a tree without anything alive doing the lifting.
Established
Specialists would state this without hedging. Multiple independent lines of evidence agree.
Transpiration-driven ascent under negative pressure has been reproduced in a synthetic device made of hydrogel, containing no cells, membranes or metabolism, which sustained flow against gravity at liquid pressures around −1 MPa.
Who this applies to
The physical mechanism, demonstrated in an artificial system and inferred to operate in vascular plants generally.
Studied in
Tracheophyta
You may have heard
“Trees pump water up from their roots.”
There is no pump anywhere in a tree. Water is pulled from above by evaporation, so the whole column is under tension rather than pressure — which is why the system fails suddenly rather than gradually when it fails at all.
Why we rate it this way, and what the caveats are
EstablishedHigh confidence
The cohesion–tension mechanism has survived a century of attempts to displace it, and the synthetic-tree work removed the last plausible objection by showing the effect in a system that cannot be doing anything biological.
How far it can be extended
The mechanism is physical rather than physiological, and the same conditions — evaporation, cohesion, narrow wettable conduits — hold across vascular plants.
Caveats
A living tree is not passive: it opens and closes stomata, and in some species manages tension actively.
The synthetic device operated over centimetres, not the hundred metres of a tall tree.
Still unanswered
How closely the tensions sustained in an artificial system match those a tall tree tolerates in the field.
Reproduced transpiration-driven flow at genuinely negative pressures in a device with no living tissue, and measured the pressure directly rather than inferring it.
The standard synthesis of xylem structure and the ascent of sap, setting out the mechanism and the evidence for it in living trees.
How we know
A tree made of hydrogel, with no living cell in it
Can evaporation alone lift water, or does a tree need to be doing something else?
Two small chambers were carved into a block of hydrogel — a water-loving polymer — and joined by channels a few micrometres wide, all filled with water. One chamber stood in for a leaf and was exposed to unsaturated air so water could evaporate from it. The other stood in for a root and was connected to a water supply. A pressure sensor was built into the liquid so the researchers could read the pressure inside the artificial xylem while the device ran, which is very difficult to do inside a real trunk.
What happened
Water flowed continuously from the root chamber to the leaf chamber and evaporated, with the liquid in the channels sitting at pressures around −1 megapascal — far below zero — while staying liquid. Push the tension harder and the column snapped into vapour, exactly as it does in a plant.
What it shows
Evaporation from a wet surface, water’s cohesion, and channels narrow enough to hold a meniscus are together sufficient to drag water upward against gravity under tension. No pump is required, because the mechanism is not biological.
What it does not show
It does not show that a real tree does nothing else. Living trees regulate stomata, refill some conduits under some conditions, and manage tension actively in ways a gel block cannot. The heights and tensions here are also far below those in a tall tree, so this demonstrates that the physics is sufficient in principle rather than that it accounts for every litre moved in a redwood.
The controls — what makes this evidence rather than a story
The pressure was measured directly rather than inferred, so the negative values are readings rather than calculations.
Humidity around the artificial leaf was varied, and the flow rate tracked it as the mechanism predicts.
The device contains no cells, no membranes and no metabolism, so no biological pumping can be responsible for anything observed.
Cavitation was induced deliberately, showing the failure mode as well as the working mode in the same apparatus.
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?
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.
Trees everywhere — rainforests included — operate close to the tension at which their plumbing fails.
Well supported
Good evidence backs this, though some details remain open.
Across 226 species from 81 sites worldwide, the margin between the water potential a species experiences and the water potential at which its xylem loses conductivity is narrow, and the narrowness does not differ systematically between wet and dry biomes.
Who this applies to
Forest tree and shrub species across the world’s major forested biomes.
Studied in
Tracheophyta
You may have heard
“Rainforest trees have plenty of water, so drought is a problem for dry places.”
A tree builds its plumbing for the water it usually receives. A rainforest species is adapted to abundance and has correspondingly little margin, so an unusual dry spell can take it past its limit as readily as a drought takes a desert shrub past its own.
Why we rate it this way, and what the caveats are
Well supportedHigh confidence
A large, geographically broad comparative dataset with a clear and unexpected result. The main uncertainty is methodological, in how vulnerability curves are measured, and affects absolute values more than the pattern.
How far it can be extended
The pattern was found across biomes and phylogenetic groups in a large, deliberately global sample.
Caveats
A narrow margin describes exposure to risk, not a prediction that a particular tree will die.
Most measurements are on branches; roots often fail first.
Still unanswered
Whether species can adjust their safety margins fast enough to track a changing climate.
Shows the structural cost of tolerating more tension, which is why species do not simply build in a larger safety margin.
Whether a tree can clear an air bubble from its plumbing is still being argued.
Contested
Researchers actively disagree, and the disagreement is substantive.
Reports of rapid daily refilling of embolised xylem conduits are substantially confounded by an artefact of destructive sampling: cutting a stem while its xylem is under tension introduces air that was not present in the intact plant.
Who this applies to
Demonstrated as an artefact in temperate broadleaved species; the size of the effect elsewhere is not established.Do not extend this beyond the taxa listed — the popular version over-reaches.
Studied in
Tracheophyta
Why we rate it this way, and what the caveats are
ContestedModerate confidence
That the sampling artefact is real and large is well demonstrated. Whether genuine refilling occurs, and in which species, is actively disputed, with non-destructive imaging producing results on both sides.
How far it can be extended
The artefact depends on conduit length and anatomy, which differ greatly between conifers, diffuse-porous and ring-porous species.
Caveats
Long-term recovery over a season, by growing new conduits, is not in dispute; the argument is about repair within hours.
Where researchers disagree
Studies using imaging methods that never cut the plant have reported refilling in some species, while others using the same approach have not, and the disagreement is partly about how the images are interpreted.
The physical objection has never been answered: refilling a conduit while neighbouring conduits are under tension requires the refilling one to be isolated, and the mechanism for that isolation is not established.
Still unanswered
Whether any species refills embolised conduits while the surrounding xylem remains under tension, and by what mechanism.
How much of the published daily-cycle literature survives re-measurement with relaxed sampling.
Showed that cutting a stem under tension generates embolism, so a large part of the published evidence for daily refilling may measure the method rather than the plant.
Establishes how close trees run to their hydraulic limits, which is what makes the question of repair consequential rather than academic.
How we know
The bubbles that were made by the knife
Do trees repair the air bubbles in their plumbing overnight, or does cutting the sample create them?
Embolism is normally measured by cutting a stem and testing how much water it will still conduct. The researchers varied only the cutting. Some stems were cut while the water inside was still under tension, as it is in a transpiring plant on a sunny afternoon. Others were relaxed first — left to equilibrate so the tension had gone — before the same cut. Others were cut under water. Everything downstream of the cut was identical.
What happened
Stems cut while under tension showed substantial embolism that stems relaxed beforehand did not. A large part of the apparent daily cycle — embolism building through the day, disappearing overnight — tracked how much tension the plant happened to be under when the sample was taken.
What it shows
The standard destructive measurement can manufacture the embolism it reports, and the size of the artefact is large enough to account for much of the published evidence that trees refill conduits rapidly.
What it does not show
It does not show that refilling never happens. Species differ, and imaging methods that never cut the plant have reported genuine refilling in some cases. What collapsed was the confidence in one widely used measurement, not the question itself, and NatureHQ treats rapid repair as unresolved rather than as refuted.
The controls — what makes this evidence rather than a story
The same stems, species and measuring apparatus throughout; only the state of the water at the moment of cutting differed.
Relaxation time was varied to check the effect was not simply a matter of waiting.
Stems supersaturated with dissolved gas were included, to test a second route by which cutting could introduce bubbles.
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.
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?
The water transport that must accompany carbon fixation, and the ceiling it imposes.
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.
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?
Kelly and Sork, 2002 · Annual Review of Ecology and Systematics
Assembles the evidence for each proposed explanation and identifies weather as cue rather than cause.
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.
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.
They appear to have no fixed lifespan. What they accumulate is difficulty.
The short answer
Why do trees live so long, and can they die of old age?
Nothing found so far sets a maximum age for a tree. They grow from tissue that stays permanently embryonic, they wall damage off instead of healing it, and they can lose most of themselves and carry on. What accumulates is not age but difficulty.
An animal is built once, from a fixed set of organs that wear. A tree is modular: it grows from meristems that never lose the ability to divide, most of its bulk is dead structural tissue rather than working cells, and losing a limb is a routine event rather than an injury to survive. Reviews of the evidence find age-related changes — shoot growth slows, water becomes harder to lift as height increases, injuries accumulate — but no clear sign of an intrinsic programme that ends a tree at a set age. Old trees die because something ordinary finally succeeds against a structure that has become easier to kill. The pattern in the oldest dated individuals fits: they are almost all in cold, dry, poor places, growing so slowly that very little can get at them.
Heartwood, decay resistance, and why a hollow tree is still alive
Trees do not appear to have a built-in lifespan. They become easier to kill rather than reaching an end.
Well supported
Good evidence backs this, though some details remain open.
There is no clear evidence for an intrinsic senescence programme setting a maximum age in trees. Age-related decline in shoot growth, hydraulic conductance and mechanical integrity is documented, and death in old individuals is generally attributable to an external agent acting on an accumulated deficit.
Who this applies to
Long-lived woody plants; the evidence base is dominated by a few well-studied species.
Studied in
Tracheophyta
You may have heard
“Trees can live forever.”
Having no fixed lifespan is not the same as not dying. What accumulates in an old tree is difficulty — greater height to lift water, more weight to hold up, more injuries walled off — until something ordinary finishes it.
Why we rate it this way, and what the caveats are
Well supportedModerate confidence
The absence of a determined lifespan follows from well-established features of plant construction — modularity, persistent meristems, compartmentalisation of damage — but distinguishing intrinsic senescence from accumulated external damage is difficult in principle, and the oldest individuals are rare and hard to sample.
How far it can be extended
Modular growth from persistent meristems is general to plants, but the evidence about ageing comes from a small number of very old individuals.
Caveats
This does not mean trees are immortal. Height, mechanical load and accumulated injury make an old tree progressively easier to kill.
Some species are demonstrably short-lived, and that is a matter of construction rather than of a programme.
Still unanswered
Whether any measurable process in trees constitutes senescence in the strict sense.
How much of the decline in very old trees is hydraulic and how much is a matter of accumulated damage.
Reviews the evidence for and against physiological ageing in trees and finds no clear intrinsic programme, while documenting the constraints that do accumulate.
Reviews maximum ages by dating method and finds the oldest individuals concentrated in slow-growing species on stressful sites, consistent with limits set by circumstance rather than by a clock.
The oldest tree in the world depends entirely on whether you are counting a trunk or a lineage.
Established
Specialists would state this without hedging. Multiple independent lines of evidence agree.
Reliable maximum ages rest on crossdated ring counts of individual stems, of which the oldest verified exceed 4,800 years in Pinus longaeva. Ages attributed to clonal organisms are estimates of lineage age from spread rate or genetic data, and are not comparable measurements.
Who this applies to
Age records for trees, distinguishing the age of a stem from the age of a genetic individual.
Studied in
Pinus longaeva, Populus tremuloides, Picea abies
You may have heard
“The oldest tree on Earth is 80,000 years old.”
That figure refers to an aspen clone — a stand of genetically identical trunks estimated to have been spreading for that long — and it is an estimate, not a count. No individual trunk in it is more than a couple of centuries old. The oldest dated single stem is a bristlecone pine of over 4,800 years.
Why we rate it this way, and what the caveats are
EstablishedHigh confidence
The methods and their error properties are well characterised. What is uncertain is particular ages, not the fact that the two kinds of age are different measurements.
How far it can be extended
The distinction between stem age and lineage age applies wherever a plant can clone itself, which is most of them.
Caveats
Ring counts have real error: rings can be absent in a bad year or doubled, so an extreme age is a careful minimum.
Clone ages are extrapolations from spread rate and carry uncertainty measured in thousands of years.
The oldest known individual of a species is the oldest one somebody has sampled, which is a different thing again.
Still unanswered
Whether older individually dated stems exist and simply have not been cored.
Shows that a clonal lineage is not simply exempt from time: somatic mutations accumulate and reproductive function declines with clone age.
How we know
Asking how old a forest of one tree is
Does a clonal tree that keeps growing new stems escape ageing altogether?
Aspen stands are often a single genetic individual spreading by root suckers, so a hillside of trunks can be one organism. Genetic markers were used to map the boundaries of clones and to estimate how long each had been spreading. Pollen was then collected from clones of different estimated ages and tested for viability, giving a measure of reproductive function against the age of the genetic individual rather than of the stem.
What happened
Older, larger clones produced markedly less viable pollen than younger ones. The stems were fine; the lineage was not.
What it shows
A clonal lineage accumulates somatic mutations over its life, and something measurable degrades as a result. Endless stem replacement does not make a genetic individual ageless.
What it does not show
It does not establish the age of any clone precisely — those are estimates from size and spread rate with wide uncertainty — and it measures pollen viability rather than survival or overall fitness. One species is also a thin basis for a general rule about clonal organisms, whose mutation rates and growth forms vary enormously.
The controls — what makes this evidence rather than a story
Clone identity was established genetically rather than assumed from proximity.
Stems sampled were of comparable age across clones, so the comparison is between lineages rather than between old and young trunks.
Several clones per age class, so a single unhealthy stand cannot produce the trend.
The oldest-tree question is really three questions wearing one hat. A crossdated ring count on a living stem gives the age of a trunk, with a known error — the oldest verified are bristlecone pines of over 4,800 years. A clonal colony such as an aspen stand is a lineage that keeps replacing its trunks, and its age is estimated from spread rate rather than counted. And a clone is not exempt from time either: mutations accumulate in the shared genome, and pollen viability measurably declines with the age of the clone.
Words used here
Meristem
A region of permanently dividing, undifferentiated cells. Plants keep them for life, which is most of why a tree has no fixed lifespan.
Compartmentalisation
A tree’s response to damage: it walls the injury off behind chemical barriers rather than healing it. The wound stays inside the tree for good.
There is no such thing as the amount of carbon dioxide a tree absorbs in a year.
Well supported
Good evidence backs this, though some details remain open.
Annual carbon uptake by an individual tree varies by more than an order of magnitude with species, size, age, growing conditions and site, and the widely circulated per-tree figures are means over highly skewed distributions in which a small minority of large trees accounts for most of the uptake.
Who this applies to
Any statement of the form "a tree absorbs X kilograms of carbon dioxide a year", however sourced.
Studied in
Tracheophyta
You may have heard
“A mature tree absorbs about 22 kilograms of carbon dioxide a year.”
Figures of this kind descend from averages over urban tree surveys, where uptake per tree ranged over more than an order of magnitude. Quoted without species, size, age and site, the number is not a simplification of that work — it is a claim the work does not make.
Why we rate it this way, and what the caveats are
Well supportedHigh confidence
Both premises are well established: uptake scales strongly with size, and measured populations of trees are dominated in mass by a small number of large individuals. Together they make a single representative figure impossible in principle rather than merely unavailable.
How far it can be extended
The skew is a property of how tree mass scales with size, which holds wherever trees grow.
Caveats
This is not an argument that trees take up little carbon, or that estimation is impossible. Estimates are made routinely and well — with the species, size, site and method attached.
Population-level figures for a city or a country are meaningful in a way that a per-tree figure is not.
The source of much of the per-tree figure literature, and explicit that its averages sit on a distribution in which a minority of large trees holds most of the carbon.
Shows uptake rising continuously with size, which is the reason a mean across a size distribution describes almost no individual tree.
Almost every gram of a tree came out of the air, and most of it is going back. Those two sentences are the whole of the carbon story at a tree’s scale, and both of them are routinely lost — the first to the intuition that plants eat soil, the second to the idea that a tree "locks away" carbon rather than holding it for a while. How long a while depends on what happens to the wood, which is a question about fungi and fire rather than about trees.
Most of a tree’s roots are in the top half-metre of soil, not deep underground.
Established
Specialists would state this without hedging. Multiple independent lines of evidence agree.
Across 475 published root profiles worldwide, roughly half of root biomass occurred within the uppermost 30 cm and the great majority within the first metre, in woody vegetation as well as herbaceous, with rooting depth tracking water availability and its seasonality rather than plant size.
Who this applies to
Vegetation worldwide, analysed by growth form; trees follow the same pattern.
Studied in
Tracheophyta
You may have heard
“A tree’s roots go as deep as the tree is tall.”
Excavations find the opposite shape: a broad shallow plate, because roots need oxygen as well as water and both are near the surface. Depth records exist — some roots reach beyond 60 metres — but they are exceptions, not the structure.
Why we rate it this way, and what the caveats are
EstablishedHigh confidence
A large synthesis of direct excavation, which is the only method that sees where roots actually are. The main weakness is sampling effort per site rather than the direction of the result.
How far it can be extended
The dataset spans every vegetated continent and all major growth forms, and the pattern holds within each.
Caveats
Deep roots exist and can be decisive in drought, even where they are a tiny fraction of the system.
Excavation destroys what it measures, so per-site samples are small and very large trees are under-represented.
Still unanswered
How much water a tree actually draws from its deepest roots, as opposed to how deep they reach.
The current synthesis of root function, and an explicit account of how narrow the evidence base under any general statement about roots still is — few species, mostly herbaceous, frequently in pots.
The picture most people carry — roots as an underground reflection of the branches — is wrong in both dimensions, and correcting it changes what a wood is. Root systems are broad and shallow and spread well past the canopy edge, so the trees in a wood share their soil thoroughly. That overlap is the precondition for everything below ground: fungal networks joining one tree to another, root grafts between individuals, and competition for the same water.
A mechanism proposed in the 1890s, disbelieved for decades because it seemed physically impossible, and eventually demonstrated in a block of gel.
1727
First observation
Hales measures the pull and finds it enormous
Stephen Hales attached manometers to cut stems and measured the suction a transpiring plant exerts. The forces were far larger than anything root pressure could account for, which established that whatever lifts water is not pushing it from below — and left nobody with an explanation for two centuries.
1895
Landmark experiment
Dixon and Joly propose cohesion and tension
Water molecules hold to one another strongly enough, they argued, that evaporation at the leaves could drag an unbroken column up the whole height of a tree, with the column under tension the entire way. It was the right answer and it was widely disbelieved, because water at negative pressure ought to boil.
1965
Replication
The pressure chamber makes tension measurable in the field
Scholander’s pressure chamber let anyone put a number on the tension in a shoot, in the field, in minutes. Measurements from conifers on mountainsides matched what cohesion–tension predicted, and the mechanism went from contested to standard — though still by inference rather than by direct observation.
2004
Landmark experiment
Climbing the tallest trees to find the ceiling
Measurements taken at successive heights up the tallest living redwoods found leaves near the top working under tensions close to the point at which the water column breaks, putting the theoretical maximum height at roughly 122 to 130 metres — just above the tallest tree ever recorded.
A tree built from hydrogel does it with no biology at all
An artificial tree — two chambers in a block of hydrogel joined by micrometre channels — sustained flow against gravity at pressures around −1 MPa, with evaporation at one end and no living cell anywhere in it. The pressure was read directly rather than inferred.
Changes how the 1895 result reads
The objection to Dixon and Joly was always that liquid water cannot be held at large negative pressure. This settles it by doing exactly that in a device with no biology to appeal to: the physics is sufficient on its own, which is what the original proposal claimed.
Half the evidence for overnight repair turns out to be the knife
Cutting a stem while its xylem is still under tension introduces air that the intact plant did not have. Stems relaxed before cutting showed far less embolism, and much of the published daily cycle of embolism and overnight refilling tracked how the sample was taken rather than what the tree was doing.
Changes how the 2004 result reads
It does not touch the height limit, which stands. What it unsettles is the comfortable assumption that a tree can undo cavitation overnight — and therefore how permanent the damage from a drought actually is.
A sealed vessel that pressurises a cut shoot until sap appears at the cut surface. The pressure needed is a measure of the tension the water was under in the intact plant.
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, and part of it turned out to be an artefact of how stems were cut.
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.
How complete this page is, and what it is still missing
NatureHQ publishes its own gaps. This page is at 93% completeness against what we would call a finished subject, and was last reviewed on 2026-08-30. It carries 29 claims and answers 49 mapped search questions.
1 high-priority search intent(s) not yet covered
Leaf loss, dormancy and autumn colour are heavily searched and still only mentioned in passing.
Bark — its structure, its variety and what lives on it — is absent, and it is the part of a tree people actually touch.
Reaction wood, and how a leaning tree rights itself, is a good story and is not here.