How Does Water Go From a Tree's Roots to Its Branches & Leaves?

I inhaled deeply, taking in that earthy aromatic scent that can only be found in a forest. A friend and I had escaped the city for a day trip to Guerneville, California, where we’d decided to kick off our trip with a walk in the Armstrong Redwoods Natural Reserve. 

If you haven’t been, it’s a lovely alternative to Muir Woods. As soon as you arrive, you feel the magic of these mighty trees. They stretch wide and reach (seemingly) endlessly toward the sky. Standing on the wide accessible dirt path, I craned my neck back to try to catch a glimpse of the underside of the canopy. Rays of sunlight broke through dappling my surroundings below. 

I thought about what I’d read in “The Overstory” by Richard Powers (or perhaps it was something I learned when I stopped to talk to a scientist working in Muir Woods). The Sequoia sempervirens only live along southern Oregon to central California because they need the fog to stay hydrated. It’s frightening to think about the effect of decreased fog, especially during summer, on these trees.

But that’s for another post. 

Still, this brought another question to mind: how are these trees (and any trees) able to keep their upper branches hydrated enough to grow that high?

Luckily, the book I’ve been reading, “Storm in a Tea Cup: The Physics of Everyday Life” by Helen Czerski, answered that question for me. I’m a big fan of Czerski’s writing as she is skilled at explaining things I’ve always thought would be complicated in easy-to-understand language. I’ll try to recreate that same experience here. 

Please note, I am not a scientist and am oversimplifying. In my additional research on this process, I discovered that there is some controversy about exactly how trees get their water that far up into their branches, but the standard answer seems to be what I explain below. 

How does water get to the top branches and leaves of trees? 

First: (This is my favorite lesson from Chemistry 101) Water is obsessed with water. Like, if water was a person, it’d be arrested for stalking other water. 

Second: Size does matter. Okay, Czerski doesn’t exactly say that in her book. Instead, she explains how gravity can lose out to other forces (like surface tension/cohesion - in this instance) when an object has less mass. Teeny tiny things tend to have less mass (though size - volume - doesn’t always equate to mass). 

Third: Water can also be attracted to things other than water. Gasp! I know… how scandalous. But don’t worry water’s true love is still water. 

In the book, Czerski begins by describing how a towel works. Towels are made up of a bunch of tiny fibers that are attracted to water. At such a small scale, the attraction to these fibers will cause water molecules to lift into the towel.

Then, remembering their true loves, pull more water to them. This continues until “exactly balanced by the upward pull of the surface tension.” 

The same thing happens in trees as water crawls up fiber tubes in the trees called xylem. 

However, with the trees so very tall in the sky, there is another process (or mechanism) at play here. 

Water is held in microscopic pockets called Stomata on the outer layer of trees’ leaves. These holes are essential for photosynthesis, opening during the day to allow for the intake of gases. California Academy of Sciences. “Stomata Printing: Microscope Investigation.”

This then leads to water evaporating from the stomata (transpiration) which, we now know, is stressful for water which loves itself. Surface tension/cohesion causes the water to pull more up to join it in the leaves. 

Because these stomata are so microscopic, the tension is able to be greater than that of the force of gravity which is pulling the water toward the earth (which has very great mass indeed). 

This world of water and small-scale forces at work has left me curious about more like hydrotropism (plus, I now want to take another read/watch of Ant-Man because I’m pretty sure that comic references some of what she talked about). 

But until then, I think I’ll simply go for another walk in the woods.

Additional resources that I enjoyed reading and think you might too:

• https://bio.libretexts.org/Bookshelves/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.05%3A_Transport/4.5.01%3A_Water_Transport/4.5.1.03%3A_Cohesion-Tension_Theory#:~:text=According%20to%20the%20cohesion%2Dtension%20theory%2C%20transpiration%20is%20the%20main,pulled%20up%20by%20this%20tension.

• https://www.scientificamerican.com/article/how-do-large-trees-such-a/

• https://www.nature.com/scitable/knowledge/library/water-uptake-and-transport-in-vascular-plants-103016037/#:~:text=After%20traveling%20from%20the%20roots,from%20that%20in%20the%20stem.