Have you ever driven across Montana and noticed that the farther west you go, the bigger the trees get? In fact, if you kept on driving all the way to Seattle, you’d notice that the trees there are even bigger than those in western Montana.
Why do trees get bigger as you travel west? More water is available as you travel toward the coast. But how does water cause trees to grow larger? Well, water is a resource and everything gets larger if it has ample resources. But what keeps the trees in your yard from reaching the moon if you water every day? Is there an ultimate physical limit to tree height on earth? In order to answer these questions, we’ll need to consider “sticky” water, tree plumbing, and gravity.
All water is “sticky” water under the right conditions. “Sticky” water is not sap: sticky water is the very same liquid water that comes out the end of your faucet.
Everyone who’s helped a friend who licked a frosty metal pole, and especially the person who licked the pole, knows that water can be very sticky when it’s cold.
Warm, liquid water may not seem very sticky, but to convince yourself you should try an experiment. Put a dab of water on the end of a pointed object, such as your finger, get out a magnifying glass, and hang your finger upside down. You’ll see that a drop of water stick to the end of your finger. Liquid water, defying gravity at room temperature! Trees know, and now you know too, that the force between individual liquid water molecules can be stronger than gravity if you have molecules in a small space.
Tree rely on very thin pipes in their trunks, branches, and leaves, special plumbing that allows them to take advantage of sticky water. The pipes are much thinner and work differently than the plumbing in your house. Unless you live below a water tank, your house relies on water pushed through the pipes by a pump. Regardless of where you live, the mechanism is the same: water is pushed to where you want it to go.
Trees don’t have pumps. Instead of pushing water around, evaporation from the tree’s leaves pulls water up through the tree’s trunk. Imagine that you are standing on the ground and a friend is up in a tree fort dangling a rope on the ground. You grab the rope and pull, but your friend pulls harder and eventually the rope slips through your hand and goes up to the tree fort. The rope is like a very thin chain of water molecules stuck together: your friend represents the atmosphere evaporating water from little holes in the tree leaves, and you represent gravity trying and failing to pull the water down from the top of the tree and back into the soil. In order to make this system work, the tree needs very thin tubes, just a few molecules in diameter, because if the tubes are too wide gravity will win the tug-of-war.
All of this happens with little effort on the part of the tree, other than the initial building of the thin tubes. The thin tubes can be wider closer to the ground, but must taper up the tree as the water column grows in height in order to hold the sticky water against gravity. Smaller tubes have a significant drawback, however, because the narrower the tube gets, the less water can move through it.
There must be a height at which the tubes are so narrow that sufficient water cannot fit through to quench thirsty leaves. This height is slightly more than the tallest tree on earth, a redwood in coastal California that is 370 feet tall. This height is specific to the gravity of our planet. If gravity were stronger, such as on the massive Ewok planet from Star Wars, trees would be shorter!
Now that we understand tree height, we can explain why there are short, tapered trees in eastern Montana, tall, straight tree in Washington, and really tall, straight trees in California. Eastern Montana has dry and in some cases saline or alkaline soils. These characteristics meant that a tree must have especially small, tapered tubes to extract water from soil against the pull of gravity. In contrast, trees in Washington and California, locations with high annual rainfall and wet atmospheres, can much more easily move water against the downward pull of gravity.
But as we’ve seen, even in optimal condition, trees can only reach a certain height. Trees effortlessly defy gravity by building a clever plumbing system, and it is gravity and the nature of water confined in small spaces that determine the ultimate height of trees.
"Field Notes" is produced by the Montana Natural History Center.