A tree has no pump, yet it moves water from its roots to leaves that can sit more than 100 meters up. The water is pulled from above by the leaves rather than pushed from below. That same pulling force also sets a ceiling on how tall any tree can grow.
Leaves Pull Water Up Like Sucking Through a Straw
Leaves constantly lose water vapor to the air through tiny pores, a process called transpiration. Each water molecule that escapes creates suction that tugs on the water in the tree’s pipes below.1Frontiers in Plant Science. Challenges and advances in measuring sap flow in agriculture and agroforestry: A review with focus on nuclear magnetic resonance The energy for the lift comes from the sun and dry air, not from the tree itself.
A straw can only lift water about 10 meters, because it is air pressure pushing the water up from below. Trees go far higher because the water is truly pulled as a cohesive thread.
The thread holds together because water molecules cling to one another through strong attractions called hydrogen bonds.2Tree Physiology. Let’s not forget the critical role of surface tension in xylem water relations A column of water can therefore be stretched under tension without snapping apart, staying liquid.3Tree Physiology. Let’s not forget the critical role of surface tension in xylem water relations The pull from the leaves travels down that unbroken column all the way to water in the soil.4Tree Physiology. Let’s not forget the critical role of surface tension in xylem water relations This account is known as the cohesion-tension theory, and it is the widely accepted explanation of sap rising.5Tree Physiology. Let’s not forget the critical role of surface tension in xylem water relations In tall trees the tension reaches several megapascals, many times the pressure in a car tire, but as suction rather than push.6PubMed Central. Water ascent in trees and lianas: the cohesion-tension theory revisited in the wake of Otto Renner In a big tree the water moves far more slowly than walking pace, and a drop entering at the roots may take hours or days to reach the crown.
Water gets into the pipes by way of the roots. Roots absorb soil water, which crosses several layers of root cells that act as a filter before entering the xylem.7Frontiers in Plant Science. Challenges and advances in measuring sap flow in agriculture and agroforestry: A review with focus on nuclear magnetic resonance The xylem, the tree’s water pipes, is made of dead, hollow cells running from roots to leaves.8Frontiers in Plant Science. Challenges and advances in measuring sap flow in agriculture and agroforestry: A review with focus on nuclear magnetic resonance
Dead cells make good pipes: with no contents in the way, water flows freely, and the tree spends no energy running them. The newest wood near the bark usually carries most of the water, which is why damage just under the bark can starve the crown.
The Tree’s Plumbing From Roots to Leaves
Conifers such as pines carry water in short, single-celled tubes called tracheids.9PubMed. Size and function in conifer tracheids and angiosperm vessels Flowering trees such as oaks and maples mostly use vessels, tubes about ten times longer that are built from many cells joined end to end.10PubMed. Size and function in conifer tracheids and angiosperm vessels Vessels move water faster mainly because they can be much wider, not because they are longer.11PubMed. Size and function in conifer tracheids and angiosperm vessels Wider vessels need extra fibers in the wood to keep it strong, while conifer tracheids double as the wood’s support.12PubMed. Size and function in conifer tracheids and angiosperm vessels The vessel design isn’t better in every way, and some primitive flowering plants went back to tracheid-style wood.13Vascular Transport in Plants. Efficiency Versus Safety Tradeoffs for Water Conduction in Angiosperm Vessels Versus Gymnosperm Tracheids
Water passes from one tube to the next through small openings in the walls called pits.14PubMed Central. Investigating Effects of Bordered Pit Membrane Morphology and Properties on Plant Xylem Hydraulic Functions—A Case Study from 3D Reconstruction and Microflow Modelling of Pit Membranes in Angiosperm Xylem In flowering trees the pits are covered by thin, mesh-like membranes made of layers of cellulose fibers full of tiny pores.15PubMed Central. Investigating Effects of Bordered Pit Membrane Morphology and Properties on Plant Xylem Hydraulic Functions—A Case Study from 3D Reconstruction and Microflow Modelling of Pit Membranes in Angiosperm Xylem Many conifers have a different pit with a central plug that can seal off a neighbor.16Vascular Transport in Plants. Efficiency Versus Safety Tradeoffs for Water Conduction in Angiosperm Vessels Versus Gymnosperm Tracheids Pits add a lot of resistance to flow, so a tree cannot make them as tight as possible without slowing its water supply.17Tree Physiology. Xylem hydraulics: optimal investment
Wide, open pipes carry more water but fail more easily. Narrow pipes with tight valves are safer but slower.
Pits do not only link pipes end to end. They also connect neighboring pipes side by side, so water can go around a blocked pipe through its neighbors. The wood works like a network rather than a set of separate straws, and one failed pipe does not cut off the leaves it was feeding. The same pits that allow rerouting can also let air spread from a blocked pipe into a healthy one.
You can see the pipes yourself. On a cut oak or ash stump you can often spot pinprick holes, which are vessels. Pine wood looks smoother and more even because its tubes are narrow. A celery stalk standing in colored water shows the same plumbing, and the dye climbs the stalk because the leaves pull it up just as in trees.
When Air Breaks the Water Column
Water stretched under tension is in a fragile state that can suddenly give way.18PubMed Central. Water ascent in trees and lianas: the cohesion-tension theory revisited in the wake of Otto Renner Air can be pulled in at the pits between pipes.19Tree Physiology. Xylem hydraulics: optimal investment The pores in pit membranes can act as bubble starters that let air spread into a pipe.20PubMed Central. Investigating Effects of Bordered Pit Membrane Morphology and Properties on Plant Xylem Hydraulic Functions—A Case Study from 3D Reconstruction and Microflow Modelling of Pit Membranes in Angiosperm Xylem A pipe filled with air can no longer carry water.21PubMed Central. Investigating Effects of Bordered Pit Membrane Morphology and Properties on Plant Xylem Hydraulic Functions—A Case Study from 3D Reconstruction and Microflow Modelling of Pit Membranes in Angiosperm Xylem
The tree must send water around the blockage through neighboring pipes, which then carry a heavier load. The extra load raises the suction in those neighbors and makes them more likely to fail in turn, so damage can snowball during a long drought. A tree can survive losing some pipes. The real danger comes when the share of blocked pipes becomes very large.
Drought makes the suction stronger and blockages more common.22New Phytologist. Functional xylem characteristics associated with drought-induced embolism in angiosperms Widespread air blockages are a major reason drought cuts tree growth and kills trees.23New Phytologist. Functional xylem characteristics associated with drought-induced embolism in angiosperms Some trees may repair blocked pipes by moving sugars into them so water seeps back in.24PubMed. Hydraulic safety margins and embolism reversal in stems and leaves: why are conifers and angiosperms so different? Those sugars likely come from starch stored in living cells next to the pipes.25PubMed. Hydraulic safety margins and embolism reversal in stems and leaves: why are conifers and angiosperms so different?
In a yard tree, leaves at the top or the tips of branches brown or wilt first in a hot dry spell. Whole branches may die back if their pipes stay blocked, and the damage often shows weeks after the drought itself. Dieback after drought is a sign of plumbing failure, not necessarily disease.
Blocked pipes can starve leaves even when the roots sit in moist soil. Watering helps prevent damage, but it cannot instantly reopen blocked pipes.
Why Trees Can’t Grow Forever
The suction in a tree’s water column grows with height because of gravity and the longer path water must travel.26PubMed Central. Maximum height in a conifer is associated with conflicting requirements for xylem design The tops of tall trees therefore face more water stress, even when the soil is moist.27PubMed. The limits to tree height Leaves near the top grow smaller and slower, which limits further height growth.28PubMed. The limits to tree height Top leaves also lose some of the internal water pressure that cells need to expand, especially in late spring as buds swell.29Plant, Cell, and Environment. Does turgor limit growth in tall trees?
Even with no water flowing, a column of water in a tall tree must be held up against its own weight, so the water at the top is under more tension than the water at the bottom. On top of that, flowing water loses pressure to friction, so it needs even more suction at the top.
In Douglas-fir, water flow across pits is predicted to drop to nearly zero at about 109 meters in the trunk.30PubMed Central. Maximum height in a conifer is associated with conflicting requirements for xylem design That fits record heights of 100 to 127 meters for the species.31PubMed Central. Maximum height in a conifer is associated with conflicting requirements for xylem design Height is limited partly by the clash between moving water fast and keeping the column from breaking.32PubMed Central. Maximum height in a conifer is associated with conflicting requirements for xylem design
Redwoods still win the height contest because the tallest trees live where fog and mild weather ease water stress. Moist air means leaves lose less water and suction stays lower, which gives them extra room before the column fails.
How Trees Control the Pull
Leaf pores let in carbon dioxide and let out water. In drought, closing them is the first response trees make.33PubMed Central. The Sequence of Drought‐Driven Stomatal Closure, Stem Xylem Embolism, Dehydration, and Aquaporin Gene Expression Differs Among Species The pores never fully seal, even in badly dried leaves.34PubMed Central. The Sequence of Drought‐Driven Stomatal Closure, Stem Xylem Embolism, Dehydration, and Aquaporin Gene Expression Differs Among Species Species differ in how early they close, with beech closing sooner than pine, olive or juniper.35PubMed Central. The Sequence of Drought‐Driven Stomatal Closure, Stem Xylem Embolism, Dehydration, and Aquaporin Gene Expression Differs Among Species
Closing the leaf pores protects the pipes but stops the leaf from taking in the carbon dioxide it needs to make sugar. A tree that keeps its pores shut for a long time lives off stored sugars and may grow little that year.
Hot, dry air raises the pull on the water column.36PubMed Central. Increasing temperature and vapour pressure deficit lead to hydraulic damages in the absence of soil drought Heat waves can damage the water pipes even when the soil is wet,37PubMed Central. Increasing temperature and vapour pressure deficit lead to hydraulic damages in the absence of soil drought because some trees, such as beech and some oaks, do not close their pores enough.38PubMed Central. Increasing temperature and vapour pressure deficit lead to hydraulic damages in the absence of soil drought Dry air also speeds the drain on a tree’s water and stored sugars, though higher carbon dioxide can soften the effect.39Nature Reviews Earth & Environment. Mechanisms of woody-plant mortality under rising drought, CO2 and vapour pressure deficit
During a heat wave, deep watering at the root zone before and during the heat keeps water available. It cannot fully offset very dry air pulling on the leaves. Mulch keeps roots cooler and moist, easing the load.
Backup Systems, Root Pressure and Stored Water
Many plants build up pressure in their roots overnight, seen as sap oozing from cut stems.40International Journal of Plant Sciences. Survey of Root Pressure in Tropical Vines and Woody Species It is strongest at night and around sunrise.41International Journal of Plant Sciences. Survey of Root Pressure in Tropical Vines and Woody Species But even the strongest measured push could only lift water about 15 meters with no resistance.42International Journal of Plant Sciences. Survey of Root Pressure in Tropical Vines and Woody Species Root pressure helps refill pipes, but it cannot explain water reaching the top of tall trees.
Drops at the edges of some leaves on a damp early morning are also root pressure at work. They form when the soil is moist and the air is humid enough that the plant is not losing water through its leaves.
The trunk also works as a water tank. Large trees draw 6 to 28 percent of their daily water from the trunk itself, depending on species.43PubMed. Water storage dynamics in the main stem of subtropical tree species differing in wood density, growth rate and life history traits In a dry season stored water supplied up to 45 percent of daily use in one study area.44PubMed. The importance of tree internal water storage under drought conditions Without that store, leaves would dry past the point where pipes fail.45PubMed. The importance of tree internal water storage under drought conditions Trees with more storage keep their leaves better watered at midday.46PubMed. Biophysical properties and functional significance of stem water storage tissues in Neotropical savanna trees
Overnight the trunk refills as roots keep drawing water. Morning sun starts the pull and the tree taps its store first. By afternoon the store runs low and the leaves feel the strain.
Some trees also drink through their leaves. Most dominant plants of redwood forests can absorb water this way.47PubMed Central. Foliar water uptake: a common water acquisition strategy for plants of the redwood forest Coast redwoods take up fog directly through their foliage.48Plant, Cell & Environment. The contribution of fog to the water relations of Sequoia sempervirens (D. Don): foliar uptake and prevention of dehydration In one cloud-forest tree, fog supplied up to 42 percent of the water in its leaves.49PubMed. Foliar uptake of fog water and transport belowground alleviates drought effects in the cloud forest tree species, Drimys brasiliensis (Winteraceae)
How Maple Sap Runs in Late Winter
Maple sap flows without leaves, so the usual leaf pull is not the driver. During freezing nights, water is drawn into air-filled wood fibers and bubbles compress.50PubMed Central. Maple Sap Uptake, Exudation, and Pressure Changes Correlated with Freezing Exotherms and Thawing Endotherms After a thaw, gravity and those squeezed bubbles push sap down and out of the tree.51PubMed Central. Maple Sap Uptake, Exudation, and Pressure Changes Correlated with Freezing Exotherms and Thawing Endotherms The leading explanation combines freezing and thawing with water drawn by dissolved sugars, and it is still debated.52PubMed Central. Multiscale model of a freeze–thaw process for tree sap exudation
Tappers watch the weather. The best runs follow nights below freezing and days above freezing, while a long warm spell or steady cold stops the flow. Once buds open, the sap turns bitter and the season ends.
The sap is sweet because it contains sugar made by the tree the previous summer and stored in the wood over winter. The sugar content is low, so it takes many liters of sap to boil down into a small amount of syrup.
Diseases That Clog a Tree’s Pipes
Some fungi and bacteria invade the water pipes and disrupt them.53Forest Pathology. Vascular Wilt Diseases Less water then reaches the top of the tree, and in hot, dry spells the leaves wilt and die.54Forest Pathology. Vascular Wilt Diseases Dutch elm disease and oak wilt are well-known examples.
Disease wilting often hits one branch or side first, not the whole crown evenly. Dark streaks under the bark of a wilting branch suggest a clogged pipe. Watering does not revive diseased branches, and an arborist can test a sample to confirm.
Dutch elm disease is spread by bark beetles that carry fungal spores from one tree to another as they feed. Oak wilt spreads both by beetles that visit fresh wounds and through roots that have grown together between neighboring oaks. So where you prune can matter: cutting oaks during the beetles’ active season can leave fresh wounds that invite the fungus.