A tree has no heart
A large tree can lift water many stories above the ground every day, yet it has no heart or muscle to push it. The system works because of a simple physical fact: water evaporating from leaves pulls more water behind it, much as liquid rises through a straw when you sip. Understanding this explains a lot about why trees are sensitive to dry air and dry soil.
The plumbing: xylem
Water moves through a tissue called xylem. In the wood of a tree, xylem is made of long, narrow, dead cells joined end to end. Because the cells are dead and hollow when they are working, water can flow through them with little resistance, like fluid through a bundle of very thin pipes.
The two main tree groups have slightly different plumbing. Conifers have cells called tracheids, which are narrow and connect to their neighbors through tiny valve-like pits. Many broadleaf trees also have wider tubes called vessels, which allow faster flow but are often more vulnerable to blockage. These differences are part of why softwoods and hardwoods look different under a microscope, a distinction behind the trade terms hardwood softwood.
Only the outer, younger wood, the sapwood, is typically used to carry water. The older inner wood often no longer conducts.
Step 1: water enters through the roots
Fine roots take up water from the soil, often with the help of fungal partners (see mycorrhizal fungi). Water enters because the root tissues have a lower water potential than the surrounding soil; in plain terms, water moves toward places where it is held less loosely. When the soil is moist, this is easy. As it dries, water clings to soil particles more tightly, and roots must work harder to extract it.
Water moves across the root and into the xylem, then joins the continuous column running up the trunk.
Step 2: evaporation at the leaves
Leaves are covered by a waxy coating that limits water loss, but they have thousands of tiny pores called stomata. The stomata open to let carbon dioxide in for photosynthesis, and while they are open, water vapor escapes. This loss of water from leaves is called transpiration.
As water evaporates from the moist surfaces inside the leaf, it creates a pull. Water in the leaf’s xylem is drawn toward the evaporating surfaces to replace what was lost.
Step 3: cohesion makes the pull travel
The pull reaches the roots because water molecules stick to each other, a property called cohesion, and also stick to the walls of the narrow xylem tubes, called adhesion. Because the tubes are so thin and the water forms an unbroken chain, tension at the top is transmitted down the whole column.
This is often called the cohesion-tension theory. It is the generally accepted explanation for how water rises in tall trees. The water in the xylem is under tension, which means it is being pulled rather than pushed, and that is unusual for a liquid. It works only because the water is pure enough and the tubes narrow enough that the chain rarely breaks.
Some trees may also use root pressure at certain times, such as in spring before the leaves open, to push sap upward. This helps explain why sap flows from cut maple stems in early spring. But root pressure is generally too weak to supply a whole tree in summer, so the pull from evaporation is the main driver.
How much water is involved
A large tree can move a great deal of water on a warm day. Exact amounts depend on species, size, weather and soil moisture, so it is best to avoid fixed figures. As a general idea, the great majority of the water a tree takes up is lost through the leaves rather than kept; only a small fraction is used in growth and chemical reactions. This is one reason forests have a visible effect on the global water cycle and on local rainfall patterns. In humid regions, the water released by trees contributes to moisture recycling over the forest.
Opening and closing the pores
The tree controls water loss mainly through stomata. Guard cells on either side of each pore swell or shrink to open or close it. In hot, dry or windy conditions, or when the soil is dry, trees tend to close stomata to save water. That protects the water column, but it also stops carbon dioxide from entering, so the tree makes less sugar. This is a trade-off that sits at the heart of how trees respond to dry weather, and it ties directly to photosynthesis in forest trees.
When the chain breaks
Because the water is under tension, it can fail. If an air bubble forms inside a xylem tube, a process called cavitation or embolism, the tube can no longer carry water. A tree that loses too many tubes cannot supply its leaves.
Several things increase the risk:
- Drought. As soil dries, tension in the xylem rises, making bubbles more likely. This is a major mechanism behind drought stress and drought-related tree death.
- Freezing. Water that freezes in xylem can form bubbles when it thaws. Narrow tubes, as in conifers, tend to be more resistant to freeze damage.
- Damage. Wounds, disease or insect attack that cut through the sapwood can interrupt flow.
Trees have ways of coping. They can grow new xylem each year, close stomata early, drop leaves, or shed fine roots. Different species vary in how tolerant they are, and this affects which trees can survive in dry places and which are most vulnerable if droughts become more frequent.
Height limits
The same physics suggests why there may be an upper limit to tree height. The taller the tree, the longer the water column, and the more tension is needed to lift water to the top leaves, which raises the risk of cavitation and makes it harder to keep leaves supplied. Researchers have suggested that this is one factor limiting the maximum height of trees, along with others such as the effort of moving sugars and the effects of wind. The exact limit is still debated.
Why it matters beyond the tree
The movement of water through trees connects the soil to the atmosphere. Forests that draw water from deep or wide root zones and release it from leaves influence humidity, cloud formation and runoff in a catchment. Catchment water flow and the wider water cycle all begin with this narrow column of water rising inside a trunk.