Water reaches the top of a tall tree primarily through tension created by evaporation at the leaves, which pulls an unbroken column of water upward through microscopic tubes in the wood. This process, often called the cohesion-tension mechanism, does not require the tree to expend energy pumping water the way a heart pumps blood. Instead, the physical properties of water itself and the structure of the tree’s internal plumbing do most of the work. The system is elegant but surprisingly fragile, and the ways trees protect and supplement it reveal a lot about why forests look and behave the way they do.
The Pull From Above
The engine driving water movement in a tree sits at the very top, not the bottom. When a leaf opens its stomata, the tiny pores on its surface, water vapor escapes into the air. That evaporation creates a slight negative pressure inside the leaf’s cells, which tugs on the water in the narrow tubes running through the leaf’s veins. Because water molecules cling tightly to one another through hydrogen bonding, that tug is transmitted downward through an unbroken chain of water molecules stretching all the way to the roots. Think of it like sipping through an extremely long straw: you pull from the top, and the liquid follows from the bottom.
This chain works because water has remarkably strong cohesion. Under the right conditions, a thin column of water confined in a narrow tube can sustain enormous tension without snapping apart. The tubes themselves, called xylem conduits, are dead, hollow cells with rigid walls reinforced by a tough polymer called lignin. Their walls are water-attracting, which helps the water cling to the sides and maintain the column. The combination of cohesion between water molecules and adhesion to the tube walls keeps the column intact even when it is under significant pulling force.
Sap flow follows a clear daily rhythm. During the day, when sunlight warms the leaves and the air is drier, transpiration ramps up and water moves rapidly. At night, stomata mostly close, transpiration drops, and flow slows to a trickle. Nighttime flow does not stop entirely, though. Roots continue to absorb some water to replenish what was lost during the day.1PubMed Central. Sap Flow Velocity in Fraxinus pennsylvanica in Response to Water Stress and Microclimatic Variables
The Plumbing Inside the Wood
If you zoom in on a cross-section of a tree trunk, the sapwood is packed with xylem conduits arranged in a pattern that varies by species. Broadleaf trees have vessel elements, which are relatively wide tubes formed by stacked cells whose end walls have dissolved away, creating open pipelines. Conifers rely on tracheids, which are narrower and pass water through tiny pits in their side walls. The width of these conduits matters: wider tubes move water more efficiently but are more vulnerable to failure, while narrower ones are safer but slower.
Between adjacent conduits sit pit membranes, thin, porous walls that serve a critical dual purpose. They let water pass freely from one conduit to the next, but they also act as safety valves. If an air bubble forms in one conduit, the pit membrane can block that air from spreading into neighboring conduits, limiting the damage to a small section of the plumbing.2PubMed Central. Cavitation Resistance in Seedless Vascular Plants: The Structure and Function of Interconduit Pit Membranes In conifers, this system is even more specialized: the pit membranes have a thickened central disc called a torus that can swing against the pit opening and seal it shut under pressure, much like a check valve in a plumbing system.3PubMed. Geometrical and physicochemical considerations of the pit membrane in relation to air seeding: the pit membrane as a capillary valve
This design creates a fundamental trade-off. A tree that builds wide-open conduits for fast water delivery pays for it with greater risk that air will invade the system. A tree that builds narrow, heavily pitted conduits is safer but transports water more slowly. Every species, and even different parts of the same tree, balance this trade-off differently depending on the environment.4PubMed. Bordered pit structure and function determine spatial patterns of air-seeding thresholds in xylem of Douglas-fir (Pseudotsuga menziesii; Pinaceae) trees
When the Water Column Breaks
The cohesion-tension system sounds robust, but it operates under conditions that would make an engineer nervous. The water inside a tree’s xylem is under negative pressure, meaning it is being pulled rather than pushed. Under enough tension, dissolved gases can come out of solution and form a bubble, or air can be pulled in through a pit membrane. When a bubble expands and fills a conduit, that conduit becomes “embolized,” essentially air-locked and useless for water transport. This is called cavitation, and it is the single biggest threat to a tree’s water supply.
Drought is the most common trigger. As soil dries out, the tree has to pull harder to extract water from the ground, which increases the tension in the xylem. At some threshold, conduits start failing. Different species hit that threshold at different soil moisture levels, which is a big part of why some trees survive dry spells that kill others.
Freezing is the other major trigger. When xylem sap freezes, dissolved gases are forced out of solution and form bubbles within the ice. When the ice thaws and tension returns, those bubbles can expand and block the conduit. Wider conduits are more vulnerable because they contain more dissolved gas and form larger bubbles during freezing.5PubMed Central. Analysis of freeze-thaw embolism in conifers. The interaction between cavitation pressure and tracheid size This is one reason conifers, which rely on narrow tracheids, dominate cold climates: their smaller conduits are far less likely to embolize after a freeze-thaw cycle. Broadleaf trees that keep their wide vessels through winter often lose significant hydraulic capacity each spring and must grow new wood to replace it.
How Trees Fix Air-Locked Pipes
For a long time, researchers assumed embolized conduits were permanently lost and only replaced by new growth. It turns out the picture is more interesting. Living cells adjacent to embolized conduits can actively push water back into the gas-filled space, dissolving the bubble and restoring function. This refilling process has been observed even while the surrounding xylem is still under tension, which initially puzzled scientists because it seemed to violate basic physics: how can you push water into a space that neighboring water is being pulled away from?6PubMed. Refilling embolized xylem conduits: is it a matter of phloem unloading?
The current thinking is that the sugar-transporting tissue (phloem), which runs alongside the xylem, plays a role. Sugars may be unloaded from the phloem into cells near the embolized conduit, creating an osmotic gradient that draws water locally into the damaged area. The pit membranes help by isolating the embolized conduit from the tension in neighboring functional conduits. It is an active, energy-consuming repair process, and it does not work in every situation, but it gives trees a way to recover from moderate drought or freeze-thaw damage without waiting to grow entirely new wood.
A Boost From the Roots
The pull from the leaves is the dominant force moving water upward, but roots can also generate a modest push. When transpiration is low, such as at night or during cool, humid weather, some species accumulate dissolved minerals in their root xylem. This creates an osmotic gradient that draws water in from the soil, building positive pressure in the root system. The result is sometimes visible as guttation, those little droplets that appear on leaf edges in the early morning, or as sap flowing freely from a cut stem.
Root pressure is not strong enough to push water to the top of a tall tree on its own. It typically generates pressures on the order of a fraction of a megapascal, nowhere near the several megapascals of tension that transpiration creates in a mature canopy tree. But it serves important supplementary roles. In spring, before leaves have fully emerged, root pressure can refill xylem conduits that embolized over the winter. Some grasses and crop plants rely on root pressure more heavily than trees do.7PubMed Central. Drought-Induced Root Pressure in Sorghum bicolor Researchers have even found that root pressure can be triggered by drought in some species, suggesting it may be a stress response rather than just a passive overflow.
Is Cohesion-Tension the Whole Story?
The cohesion-tension theory has been the textbook explanation since the late 1800s, and it accounts for most of what we observe. But it has faced challenges over the decades, particularly from researchers using less invasive measurement techniques that avoid the artifacts caused by cutting into pressurized xylem. Some experiments have produced results that do not perfectly fit the predictions of a purely tension-driven system, leading to proposals that multiple forces, including osmotic gradients in living xylem cells and electrical effects at cell membranes, contribute to water ascent alongside the tension mechanism.8PubMed Central. Water ascent in trees and lianas: the cohesion-tension theory revisited in the wake of Otto Renner
The debate is not settled, and the majority of plant physiologists still consider cohesion-tension the primary driver. But the idea that it works in concert with other mechanisms, a “multi-force” approach, has gained traction. For practical purposes, the tension generated by transpiration is clearly the main event; the question is whether smaller, complementary forces help keep the system running smoothly, especially during stress or in very tall trees where the water column is under extreme tension.
Why Trees Cannot Grow Infinitely Tall
If the pull from transpiration can lift water against gravity, you might wonder what limits how tall a tree can grow. Gravity imposes a baseline cost: for every ten meters of height, the tree must overcome roughly 0.1 megapascals of gravitational pressure just to hold the water column in place. On top of that, friction as water moves through narrow conduits adds resistance that increases with height. The taller the tree, the harder it has to pull to move the same amount of water.
At some point, a tree’s upper leaves cannot get water fast enough to keep their stomata open and photosynthesize efficiently. A review of dozens of studies on the hydraulic limitation hypothesis found that taller trees often have lower stomatal conductance, reduced photosynthesis, and decreased hydraulic conductance compared to shorter trees of the same species.9PubMed. The hydraulic limitation hypothesis revisited Height growth slows as a tree gets taller, and the maximum height a species can reach tends to be lower on resource-poor sites. The tallest trees on Earth, coast redwoods topping 100 meters, appear to be pushing close to the physical ceiling for a water-transport system based on tension and cohesion.
How Redwoods Cheat the System
Coast redwoods have a trick that helps them cope with the enormous hydraulic challenge of being the world’s tallest trees: they drink through their leaves. In the foggy coastal forests of California, moisture from fog condenses on leaf surfaces and is absorbed directly into the foliage. Research on redwoods found that xylem sap flow actually reversed direction during heavy fog events, with water moving from the crown downward at rates peaking at roughly five to seven percent of maximum transpiration.10Plant, Cell & Environment. The contribution of fog to the water relations of Sequoia sempervirens (D. Don): foliar uptake and prevention of dehydration Isotopic analysis confirmed that up to six percent of a leaf’s water content could be traced to a prior night’s fog.
This foliar uptake is not unique to redwoods. A study of ten species in the redwood forest found that eight demonstrated the capacity for leaf-level water absorption.11PubMed Central. Foliar water uptake: a common water acquisition strategy for plants of the redwood forest The effect goes beyond simply adding a little water: fog interception increases leaf water status above what soil water alone can explain and temporarily decouples the leaves from whatever drought stress the roots may be experiencing.12PubMed. Fog interception by Sequoia sempervirens (D. Don) crowns decouples physiology from soil water deficit For trees that are 100 meters tall and struggling to pull water from the roots, being able to absorb moisture directly at the canopy is a significant advantage.
The Trunk as a Water Tank
Trees do not rely solely on a continuous real-time stream of water from soil to canopy. The trunk itself stores a meaningful amount of water in its wood, and trees draw on this reservoir daily. In a study of five temperate broadleaf species, trees roughly 25 to 28 meters tall withdrew between 5 and 12 kilograms of stored water per day from their stems, accounting for 10 to 22 percent of their daily transpiration.13Oxford Academic. Stem water storage in five coexisting temperate broad-leaved tree species: significance, temporal dynamics and dependence on tree functional traits Ring-porous species like ash contributed much less from storage, only about half a kilogram to two kilograms per day, likely because their wood structure does not hold as much accessible water.
This buffer matters most during peak afternoon demand, when transpiration spikes and root uptake cannot keep up. The tree borrows from its own trunk to bridge the gap. You can observe the result with sensitive instruments: the trunk actually shrinks slightly in diameter during the day as water is pulled out of storage, then swells again overnight as it refills. When soil dries out severely, these internal stores are eventually exhausted, leaving the tree fully dependent on the increasingly difficult task of extracting water from dry soil.
The Xylem-Phloem Partnership
Water transport through the xylem does not happen in isolation. Running alongside the xylem in the inner bark is the phloem, a living tissue that carries sugars from the leaves, where they are made by photosynthesis, down to roots and growing tissues. The two systems are hydraulically coupled: they share water across their boundary, and the performance of one affects the other.
Phloem transport works on a different principle. Sugars are loaded into phloem tubes at the leaves, which draws water in by osmosis and creates pressure. Sugars are unloaded at the roots and other destinations, water exits, and the pressure gradient drives flow. Modeling work has shown that the maximum rate of sugar transport in the phloem depends on xylem water status. When xylem tension is high, as during drought, less water is available for the phloem, slowing sugar delivery. The two systems are linked so tightly that investing more structural capacity in the xylem reduces the need for investment in the phloem, and vice versa.14PubMed. Linking phloem function to structure: analysis with a coupled xylem-phloem transport model This coupling means that drought does not just threaten a tree’s water supply; it simultaneously impairs the tree’s ability to distribute the food it makes.
Stomata as Traffic Controllers
The entire system ultimately hinges on the behavior of stomata. These microscopic pores on the leaf surface open to let carbon dioxide in for photosynthesis, but they simultaneously let water vapor out. A tree faces a constant dilemma: open the stomata wide to grow faster, or close them to conserve water and protect the xylem from dangerous levels of tension.
Research across a variety of tree species has found that stomatal regulation is more closely tied to the tree’s hydraulic safety than to its photosynthetic capacity.15Agricultural and Forest Meteorology. Tree-level stomatal regulation is more closely related to xylem hydraulic traits than to leaf photosynthetic traits across diverse tree species In other words, trees tend to close their stomata not when they have enough carbon dioxide, but when the water tension in their xylem gets dangerously close to the point where cavitation would start. Some species are conservative, keeping a wide safety margin and closing stomata well before any real risk. Others operate closer to the edge, squeezing out more growth but risking xylem damage during sudden dry spells.
When the System Fails Catastrophically
Severe drought can overwhelm all of these protective and backup mechanisms. When soil water runs out, the tree cannot avoid dangerous tension in the xylem no matter how tightly it closes its stomata, because even closed stomata leak a small amount of water through the leaf cuticle. Xylem tension climbs, embolism spreads through more and more conduits, and eventually the tree can no longer deliver enough water to keep its cells alive.
A large European study following the extreme drought of 2018 found that a third of monitored trees died. Mortality was linked to how narrow a species’ hydraulic safety margin was, meaning the gap between the tension it normally operates under and the tension at which widespread cavitation begins.16PubMed. Mutually inclusive mechanisms of drought-induced tree mortality The study also found that hydraulic failure did not act alone; it was interrelated with carbon starvation and shifts in sugar chemistry, reinforcing the idea that the xylem and phloem systems are deeply interdependent. A tree that cannot move water also cannot move sugars, and a tree that cannot move sugars cannot fuel the energy-demanding processes that might repair its xylem.
How Scientists Measure Sap Flow
Much of what we know about water movement in trees comes from sap flow measurements, and the tools for doing this are less straightforward than you might expect. The most widely used methods involve inserting heated probes into the sapwood and measuring how the heat dissipates or is carried along with the flowing sap. These techniques are commercially available and relatively affordable, which is why they dominate field research. But they come with drawbacks: drilling probes into a trunk wounds the tree and can create artifacts, and misalignment of probes introduces measurement errors.17PubMed Central. Challenges and advances in measuring sap flow in agriculture and agroforestry: A review with focus on nuclear magnetic resonance
Nuclear magnetic resonance imaging offers a non-invasive alternative. The same basic technology behind medical MRI scanners can visualize water movement inside a living stem without cutting into it. Portable NMR systems have been used on small trees and crop plants in the field, revealing flow patterns and embolism dynamics that probe-based methods can miss. The technology is still expensive and limited in the trunk diameter it can accommodate, but it represents a real step forward for understanding how water moves through intact, undisturbed trees.
Four Hundred Million Years of Plumbing
The water-conducting system in modern trees did not appear all at once. The fossil record shows a stepwise evolution of transporting tissues in land plants. The earliest land plants, over 450 million years ago, likely moved water through simple cell-to-cell diffusion. Specialized water-conducting cells called tracheids, reinforced with degradation-resistant wall material, appeared by roughly 425 million years ago in the late Silurian period. By about 410 to 400 million years ago, the structural diversity of tracheid wall thickenings had already reached a level of variety that has not been surpassed since.18PubMed Central. Deep origin and gradual evolution of transporting tissues: Perspectives from across the land plants
The evolution of lignified, hollow, dead-at-maturity conducting cells was one of the key innovations that allowed plants to grow tall and colonize dry land. Without a system to move water efficiently from soil to aerial tissues, plants would be limited to the height that capillary action and simple diffusion could reach, a few centimeters at most. The tension-driven xylem system that evolved in the Silurian and Devonian periods is essentially the same system that today lifts water to the crowns of hundred-meter redwoods. It was, in evolutionary terms, a spectacularly successful piece of engineering.