How Trees Lift Water

The tallest tree known is a coast redwood a hundred and sixteen metres high. Every day it lifts water from the soil to its top needles, with no heart and no pump. How?

That system is now in the news. In a study reported this autumn, scientists at UC Davis found that photosynthesis in young redwoods' needles, the way they make food from sunlight, roughly halved between thirty and forty degrees Celsius.

In heat waves, the trees shut the tiny pores on their needles to save water, and that has a cost. To see why, start with a puzzle about pumps.

Try lifting water with a suction pump. Pumping takes the air out of the pipe, and the weight of the atmosphere, pressing on the water outside, pushes it up the pipe.

But the air can only push so hard. Even with a perfect vacuum in the pipe, water stops rising at about ten point four metres. Above that, the pump just sucks on empty space.

As the pipe keeps growing, the water stays put. A redwood lifts water more than ten times higher than this, with no pump at all. Something other than suction must be at work.

The answer starts in the leaves. Their surfaces are dotted with tiny pores called stomata. They open to let in carbon dioxide, which the leaf needs to make sugar.

But every open pore also lets water vapour out. Losing water this way is called transpiration, and it adds up. A large oak can give off about forty thousand gallons, some hundred and fifty thousand litres, of water in a year.

Close the pores and the water stays in, but carbon dioxide cannot get in either. Every plant has to balance food against water, all day long.

Water molecules are sticky. Each one forms weak links, called hydrogen bonds, with its neighbours, so water holds together as a continuous thread. That is cohesion.

Water also clings to the walls of the narrow tubes it travels through. That is adhesion. Together they let a thin column of water behave a little like a rope.

When a molecule evaporates from a leaf, it tugs the next one up behind it, and that one tugs the next, all the way down to the roots. Watch the thread move as the top evaporates.

So the water in a tree is not pushed up from below. It is pulled from the top, and it is under tension, a pressure below zero, like a stretched rubber band.

Botanists track it with a quantity called water potential, measured in megapascals; one megapascal is about ten times the push of the air. In one textbook example, it is about minus zero point two in the roots, minus zero point six in the stem, and minus one point five in a leaf at the top.

Dry air is far lower still, around minus one hundred: not its pressure, but how hard it draws water in. Water always moves toward the lower value, so it flows from soil to root to leaf to sky, driven by the sun's energy, not the tree's.

This idea is called the cohesion-tension theory. Two Irish scientists, the botanist Henry Dixon and the physicist John Joly, published it in 1895.

In one experiment, they sealed a leafy maple branch inside a thick glass tube, with its cut end reaching down into water. The branch kept drawing water up against three times normal air pressure. So the leaves pulled harder than three atmospheres could push back, far beyond what suction can do.

They argued that the tree spends no effort of its own lifting water. Botanists were divided then, but cohesion-tension is now the textbook explanation.

The water travels through wood, in a tissue called xylem. Its pipes are made of cells that die when they mature, leaving hollow tubes with walls stiffened by lignin, the tough substance that makes wood hard.

Stiff walls matter, because the pull would squeeze soft pipes flat, like a straw when you suck too hard. In conifers such as redwoods, the pipes are narrow cells called tracheids, joined through tiny openings called pits.

Watch water climbing from cell to cell. In a tall tree, these threads of water stretch more than a hundred metres, and must stay unbroken.

Pull too hard on a water thread, though, and it can snap. A tiny bubble of gas appears, the water springs away from it, and the pipe fills with air and vapour. This is cavitation.

The bubble that blocks the pipe is called an embolism, and that pipe can no longer carry water. The harder the leaves pull, the more likely this becomes, which is one reason very tall trees live close to their limit.

A trunk holds a great many pipes side by side, so losing a few does not stop the flow. Watch a thread break, and the water carry on through the others.

Every extra metre of height adds to the pull the top leaves need, because the water must be held up against gravity and dragged through narrow pipes.

In 2004, a team led by George Koch climbed the tallest redwoods in California. Near the top, the needles were smaller and more crowded, living in near-constant drought even though the soil below was moist.

From those measurements, they calculated a maximum height for coast redwoods of about one hundred and twenty-two to one hundred and thirty metres. Watch the tree grow, and its top dry out.

The tallest tree known today is a coast redwood named Hyperion, in Redwood National Park in California. It was found in 2006 and measured at one hundred and sixteen metres in 2019.

That is still below the predicted limit. Lifting water that high takes a pull of about two megapascals, roughly twenty times the push of the atmosphere on a suction pump.

Redwoods have help. Their tops reach into the coastal fog, where they can take up water straight from the air, and the fog keeps the forest cool and shaded.

Now back to the news. Todd Dawson of UC Berkeley, who was not part of the study, says coast redwoods are adapted to a narrow band of temperatures, roughly eight to twenty-seven degrees, and that fog has declined by thirty to forty percent.

In the study's heat waves, the young trees in Mendocino County kept their stomata closed to save water. That also kept carbon dioxide out, and without evaporation to cool them, some needles ran up to ten degrees hotter than the air.

In the lab, photosynthesis fell even when branches stood in water with their pores open, so heat itself slows the needles. They recovered after the heat passed, but where lasting harm begins is not yet known.

So a tree is not a pump: the sun pulls water up unbroken threads, from the leaves all the way down to the roots.

But it has limits: threads that can snap, a height where the top goes dry, and pores that must close when water runs short.

Hotter summers and thinner fog press on all of them at once, and no one yet knows how the oldest giants, like Hyperion, will cope.