THE FOREST IS quiet and humid. It feels as though it must always have been like this, as though change is rare here. The forest floor between the tree trunks is carpeted with moss and ferns, and the only sounds are the songs of unseen birds and the deep, unnerving creaking as the trees shift their weight. Up above, blue skies are visible between the spindly green branches, and down at my feet there is water everywhere: streams, patches of damp soil, exploratory rivulets on their way down the valley. Every so often as I walk, my subconscious kicks me into alertness, because there’s a looming patch of darkness in the forest, something that doesn’t fit. But it isn’t a predator. It’s a tree: one of the real giants, a thousand-year-old colossus lurking among the youngsters, stamping its status on the forest with its shadow.
The coastal redwood, Sequoia sempervirens, used to cover vast swathes of this part of northern California. These days, those huge forests have been reduced to a few small pockets, and I’m visiting one of the most well-known, the Redwood National Park in Humboldt County. These giants are striking because each tree trunk is completely straight and vertical, reaching only for the sky. The tallest known tree on the planet is here, and it’s a staggering 380 feet high.## On my hike, I frequently pass trees with trunk diameters of 6 feet or more. Possibly most astonishing of all is that just behind the deep ridges and wrinkles in the bark, these trees are still growing new rings. They’re alive. The tiny evergreen leaves 300 feet above me are capturing the Sun’s energy, storing it up, making the stuff from which new tree is built.
But life demands water, and the water is down here, where I am. So all around me in the forest, water is flowing upward. And this flow has never been interrupted, not once since each tree sprouted from its seed. Some of these trees have been here since the Roman Empire fell. They were sitting in the California fog when gunpowder was invented, when the Domesday Book was written, when Genghis Khan was rampaging across Asia, when Robert Hooke published Micrographia, and when the Japanese bombed Pearl Harbor. And not once, in all that time, has the water stopped flowing. The reason we can be sure of this is that the whole mechanism relies on the flow never stopping. There is no way to restart it. But this is very clever plumbing, and the fabulous piece of living architecture that keeps it all going only works because it’s just a few nanometers across.
The water travels in the xylem, a system of tiny cellulose pipes that reach through the tree, stretching from the roots to the leaves. This is mostly what “wood” is, although the innermost wood stops helping with the plumbing as the tree gets bigger. Capillary action, the mechanism that made my towel absorbent, is only strong enough to suck water upward for a few yards in the tree’s plumbing. That’s no use for a tall tree. The tree roots can also generate their own pressure to push water up the pipes, but that too is only enough to push the water a few more yards upward. Most of the work isn’t being done by pushing. The water is being pulled. The same system operates in all trees, but the redwoods are the kings of it.
I sit on a fallen tree trunk, just next to one of the giants, and look up. Three hundred feet above my head, tiny leaves flutter in the breeze. To photosynthesize, they need sunlight, carbon dioxide, and water. The carbon dioxide comes from the air, and it enters the tree through tiny pockets on the underside of each leaf, the stomata. Part of the inner wall of each of those pockets is a network of cellulose fibers, and in between the fibers are water-filled channels. These are the top of the water pipes, after those pipes have branched and branched again, reducing in size each time until they reach the stomata. Here, where the water pipes finally touch the air, each one is approximately 10 nanometers across.*** The water molecules stick firmly to the cellulose sides of each channel, and the water surface curves down into a nanoscale bowl-shape in between. Sunlight heats up the leaf and the air inside it, and sometimes it gives enough energy to one of those surface water molecules to pull it away from the mob below it. That evaporated water molecule drifts out of the leaf into the air. But now the nanoscale bowl is out of shape—it’s too deep. Surface tension is pulling it inward, pulling the water molecules closer together to reduce the surface area. There are lots of new molecules that could fill the gap, but they are all farther back in the channel. So the water in the channel is pulled forward to replace the lost molecule. And then the water farther back in the channel has to shuffle along to replace that, and so on down the tree. Because the channel is so tiny, the surface tension can exert an enormous pull on all the water below it, enough (when you include the contribution of a million other leaves) to pull the entire column of water up the tree. It’s a staggering thought. Gravity is pulling the entire tree’s worth of water downward, but the combination of many tiny forces is winning the battle.??? And it’s not just a battle against gravity; the upward forces are also defeating the friction from the tube walls as water is squeezed through the tiny channels.
Pushing up from the forest floor around me are the real babies—trees that are just a year old. Their water columns are just beginning to take shape. As the new tree grows, the pipe system stretches but never breaks, and so the top of the water column is always wetting the inside of the stomata. Water is just pulled up toward the air as it keeps growing. The tree can’t refill the pipe if it empties, but it can keep the pipe full as it grows. However tall the tree gets, this water column must never be broken. The reason why the tallest redwoods are near the coast is that the coastal fog helps their leaves stay moist.??? Less water needs to reach the top from the roots, so the system can be slower and the trees taller.
This process of water evaporating from the leaves of trees is called transpiration, and it’s happening whenever you look at a tree in the sunlight. These sleepy giant redwoods are actually massive water conduits, sucking it from the forest floor, rerouting some of it for photosynthesis, and then letting the rest escape into the sky. It’s the same for every tree. Trees are a vital part of the Earth’s ecosystems, and they wouldn’t be able to climb up into the sky unless they could take water with them. And the beauty of it is that they don’t need an engine or an active pump to do that. They just shrink the problem, solve it using the rules of the small, and then repeat that process so many million times that it becomes the physics of giants.