Ever wondered why trees get so tall — or why they don’t get much taller? Me, too! People who have met me know I think about transpirational pull more than most folks. This topic has fascinated me without pause since I first heard about it, so I want to do my best to explain it in plain terms for this article.
A few years back, researchers at OSU determined that gravity and other forces would eventually overcome the water column in trees, and that this limit on the upward reach of water would limit growth to 350 – 400 feet; tabs kept on the world’s tallest trees show that so far, they conform to this range. Why is that?
Like most things in life, it all starts with water. As many of us are probably aware already, there is very strong cohesion and adhesion between water molecules. Water molecules are also arranged at an unusual angle, with a spread of about 105 degrees between its hydrogen atoms. These properties allow water to form strong chains, which have flexible “kinks” like most unsaturated fats. Double bonding in such fats create a zig-zag fatty acid chain which doesn’t allow for good stacking; this is why oils are liquid at room temperature. Similarly, water doesn’t exactly form a straight chain, which is important to certain adaptations in tree tissues. More on this later — just remember… olive oil.
This is what the water conducting tissue of a tree looks like. The big tubes are phloem, which helps move sugars around in a tree; we don’t care about them today. It’s those little tubes that concern us, the xylem. You may know that they move water upward in a tree. Phloem down, xylem up! Now, this photo is taken in cross-section, so it’s easy to imagine that xylem is just a bundle of long straight tubes that run the entire length of a tree.
But this isn’t the case, and it’s a good thing! If water had to move straight up, gravity would limit tree height more severely than it does. As you can see in these pictures, xylem fibers called tracheids are more like uneven stacks and bundles of tapered tubes, and the ends overlap a bit. The second picture shows more clearly that the tubes have caps and that there are pits in the side wall so water moves back and forth between them, creating a zigging, zagging, branching water column instead of one that goes straight up.
So, scientists used to think they had figured out the mechanism for moving water up a tree. If you’ve ever been in a chemistry class, you may have seen the thistle-tube experiment, wherein a column of water defies gravity due to osmosis. This is positive osmotic pressure — a force that essentially pushes the water upward; this exists in varying degrees in the roots of most all plants, so scientists once thought this pressure just pushed the water right to the tops of trees. But then came some pesky discoveries showing that very tall trees can have almost zero root pressure!
Turns out the major cause of upward water movement in trees is negative osmotic pressure: a pull from above instead of a push from below. And the thing doing the pulling is the air surrounding trees.
Basic rules of diffusion tell us that things will follow a gradient from high to low concentration; this works in the atmosphere, too, since air has a lot more empty space to hold water vapor than a liquid like tree sap does (this is called water potential). The atmosphere will pull the water up and out of plants, right up until about 95% relative humidity. You may rightly be wondering how the air manages to pull water out of the top of a tree.
Well, it’s all part of photosynthesis. Most of us are probably familiar with this basic plant concept: H2O and CO2 in, oxygen and sugar out. Transpiration is the whole process that moves these substances through the entire plant, both on a cellular level and from root to leaf. Here you can see that gradient I was talking about, with higher water concentration at the roots, and lower at the crown. H2O enters through the roots and CO2 enters through leaves, then inside the chloroplasts of a cell, they’re broken apart with the help of light. The plant takes the C and H bits and jams them together to make sugars. And it pretty much tosses the oxygen out like an old glove.
It’s a pretty complicated process that I may explain more in a separate post sometime, but the important part for this discussion is that in the end, oxygen is expelled through the leaves.
Plants have openings are called stomates where gas exchange happens. The plant can open and close them based on things like heat, humidity and availability of water. The thing is, when the plant lets out o2, a little bit of water escapes as well! If it’s hot out, many plants are willing to halt photosynthesis and transpiration just to hang onto that precious water. (Which is why you shouldn’t water your garden in the heat of the day!) This tiny little water leak is where the atmosphere grabs hold and the air starts pulling the water upward through evapotranspiration. Because water molecules hang onto each other so tightly, every time one pops out of a stomate, it pulls another one right up behind it.
This is a pretty simplified version of events that ignores important factors like the capillary action involved with cohesion of water molecules to the sides of these tiny little tracheids. I have loads more cool stuff I could tell you about water, so maybe I’ll save this part for another post, too.
But there’s one hitch. Thinking back again to the thistle tube experiment, you’ll notice that osmosis pushes water up, but at a certain point, it falls just a bit. This is because gravity eventually overcomes the positive pressure pushing the water up; even when talk about the negative pressure pulling the water up in a plant, tension and gravity will still break a column that rises too high.
But plants have an important physiological adaptation in those little overlapping xylem tubes. The fact that these xylem tubes send a water column branching helps plants to cheat gravity and tension a little to keep moving a water column upward after it would have been broken. And remember that part about olive oil? Water is also uniquely fit for twisting around under such conditions without losing its cohesion.
For you mechanical- or physics-minded people, think of it like this: The more pulleys or leverage points you put into a system, the less force you need to move something. Or, in another way of thinking, adding this extra leverage will help you move the same thing further using the same force. We’re talking about the water again; in a similar way, these tubes provide a little extra leverage and tension relief every time they direct the water column the least bit to the side instead of straight up. So that is the major way that trees grow taller than gravity would otherwise allow water to go. Or for the bigger-better crowd, that’s why they won’t just explode upward into the clouds forever, even with unlimited sun and soil.
So, who wants to come along for additional posts on this topic? You might as well get comfortable, because we’re going. I’ll update this post and the others as I expand the explanation!