Why Trees Are Out to Get You

This video is part of what is potentially the largest collaboration ever on YouTube, along with my friends Mr. Beast and Mark Rober, Destin from Smarter Every Day, and many, many others. We're trying to get 20 million trees planted before the end of this year, and each tree costs $1. So essentially, we're trying to raise 20 million dollars. If you want to help us get to that target, well then, go to teamtrees.org and donate now.

Most of the YouTubers are making original videos for this collaboration, but since I'm obviously currently traveling in Sydney, I don't have time for that. So I am reposting one of my favorite videos of all time, which is about trees and how they bend the laws of physics and do things that engineers and scientists can't yet replicate. So if you haven't seen that, please check it out.

Sometimes the simplest questions have the most amazing answers, like how can trees be so tall? It's a question that doesn't even seem like it needs an answer. Trees just are tall; some of them are over a hundred meters. Why should there be a height limit? I'll tell you why. Trees need to transport water from their roots up until their topmost branches in order to survive, and that is no trivial task. There is a limit to the height that water can be sucked up a tube. It's ten meters. If you suck on a long vertical straw, the water will go no higher than ten meters. At this point, there will be a perfect vacuum at the top of the straw, and the water will start to boil spontaneously.

For a tree to raise water a hundred meters, it would have to create a pressure difference of ten atmospheres. How would trees do that? When I posed this conundrum, a lot of people said the answer is transpiration, and that's when water evaporates from the leaf, pulling up the water molecules behind it. And that's clearly a mechanism a tree can use to create suction, but it doesn't help us overcome this 10-meter limit below, as the pressure can go as if your vacuum, which I imagine is not happening instead of tree leaves.

Right, right, Hank. So you might suspect that a tree does not contain continuous straw-like tubes. The tree effectively has valves in it, so you don't have a column of water this big that you're saying needs to be filled with water. It's actually made up of cells. Although these are good speculations, they don't seem to be correct. Scientists who study trees find that the xylem tubes that transport water do contain a continuous water column.

So how else could the tree transport water from the roots to the leaves? They don't suck; they don't use a vacuum. Okay, so how do they do it? Wheezing like a cow? Like you're squeezing the cow water all the way up? There's little three muscles in there. Yeah, besides being a giant waste of energy, all of the cells that make up the xylem tubes are dead.

What about osmotic pressure? If there is more solute in the roots than in the surrounding soil, water would be pushed up the tree. But some trees live in mangroves where the water is so salty that osmotic pressure actually acts in the other direction. So the tree needs additional pressure to suck water into the tree. Then it must be capillary action. The thinner the tube, the higher the water can climb, but the tubes in a tree are too wide, at 20 to 200 micrometers in diameter. Water should rise less than a meter.

So how do trees do it? Well, one of the assumptions we made is wrong. Below, if the pressure can go, it’s the pure vacuum. Your vacuum... you're back in a gas. This is true when you eliminate all of the gas molecules; the pressure is zero, and you have a perfect vacuum. But in a liquid, you can go lower than zero pressure and actually get negative pressures. In a solid, we would think of this as tension. This means that the molecules are pulling on each other and their surroundings.

As the water evaporates from the pores of the cell wall, they create immense negative pressures of minus 15 atmospheres in an average tree. Think about the air-water interface at the pore; there is one atmosphere of pressure pushing in and negative 15 atmospheres of suction on the other side. So why doesn't the meniscus break? Because the pores are tiny, only two to five nanometers in diameter. At this scale, water's high surface tension ensures the air-water boundary can withstand huge pressures without caving.

As you move down the tree, the pressure increases up to atmospheric at the roots. So you can have a large pressure difference between the top and the bottom of the tree because the pressure at the top is so negative. But hang on; if the pressure at the top is negative 15 atmospheres, shouldn't the water be boiling? Yes, yes, it should, but changing phase from liquid to gas requires activation energy, and that can come in the form of a nucleation site, like a tiny air bubble. That's why it's so important that the xylem tubes contain no air bubbles, and they can do this because, unlike a straw, they've been water-filled from the start.

This way, water remains in the metastable liquid state when it really should be boiling. It's just like supercooled water remains liquid even though it should be ice. So you could say that the water in a tree is super sucked because it remains liquid at such negative pressures. And why are trees moving all this water up the tree? I want you to make a guess. Say it out loud.

For photosynthesis? Actually, no. Less than 1% of the water is used in photosynthetic reactions. Any other ideas? Okay, what about growth? Well, five percent of the water is used to make new cells. So what happens to the other 95 percent of the water? It just evaporates. For each molecule of carbon dioxide that tree takes in, it loses hundreds of molecules of water.

Whoa! Can you believe how amazing this is? Trees create huge negative pressures of tens of atmospheres by evaporating water through nanoscale pores, sucking water up a hundred meters in a state where it should be boiling but can't because the perfect xylem tubes contain no air bubbles, just so that most of it can evaporate in the process of absorbing a couple molecules of carbon dioxide. I will never look at trees the same way again.

Trees are some of the biggest organisms on the planet, but where do they get that matter to grow? Which nutrients? Is it the grain status or any... Yeah, goodness! Said of the soil. I suppose comes out of the soil. Yeah, goodness, goodness! Why isn't there a big hole around the tree where it's taken out all the soil? It's just gradually let the soil has time to recover.

Now I think it's intuitive to believe that the tree gets most of its mass from the soil because you can see those roots digging into the soil and they must be taking something out of there. I mean, a tree looks like dirt and it feels solid like dirt, but it's not. In the early 1600s, a scientist named Johann Baptiste van Helmont tried to figure out where the mass of a tree was coming from. So he got a pot of soil and very carefully measured the amount of soil in there. Then he planted the tree and took care of it for five years, making sure that no soil left or was added to his pot.

At the end of this experiment, he laid the tree to find that it was 72 kilograms, but the mass of soil had only decreased by about 60 grams. This was pretty strong evidence that the mass of the tree does not come from the soil. I've never thought about that. Actually, because they don't really eat anything. Don't eat me? No, no, they don't eat anything. Water? It's always absorbed; that's all they eat.

Yeah, they don't eat anything else. No, that's all they eat. Well, presumably from the water and the nutrients from the soil. Is there anything else that you need besides the soil and the water? This will be late, isn't it? To make other than the original seed for that particular trail... the seed and the soil and the water, and that makes this big tree. Of course, Johann Baptista van Helmont did conclude that the tree was made entirely of water.

Now, while that's not correct, at least he was on the right track, realizing that the matter of a tree doesn't come out of the soil. The Sun? Energy? Yeah, the Sun. Energy? Are they converting energy into mass? Or do you know what I mean? Yeah, like that. There wasn't stuff, and then there was. Like, where did that stuff come from? My question is, where do they get that mass to grow? Bigit from the rain and the Sun, presumably.

Like sunlight... and the sunshine? The sunshine does it! Does the sunshine add mass to the tree? Well, yes, it wouldn't— they wouldn't grow without it! I don't know if it adds mass, but they wouldn't grow without it. Of course, the Sun's energy is needed for the tree to build the matter into its branches and leaves. But the Sun itself, the energy, is not matter.

Oxygen! Didn't the trees need the... Oxygen indeed! The earth and... I guess oxygen? The oxygen, of course, the oxygen. Are there any ingredients that we're missing? Come, doc. So it's carbon dioxide. Carbon dioxide. So would it surprise you to find out that 95 percent of a tree is actually coming from carbon dioxide? Trees are largely made up of air.

So, as it turns out, trees are mostly made out of air, out of the carbon dioxide that they take in. And what's interesting is that we breathe out carbon dioxide and water. That's how we lose mass. But it's the exact same substances that trees breathe in to gain mass. So if you can imagine a closed system where it was just you and a tree, you would breathe out that carbon dioxide and water; the tree would take it in. So you would get smaller while the tree is getting bigger, and in a sense, you're becoming the tree.