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, and we’d decided to kick off our trip with a walk in the Armstrong Redwoods Natural Reserve. 

If you haven’t visited, I highly recommend this alternative to Muir Woods. You can feel the magic of these mighty trees as soon as you step into this forested area.

It’s hard not to feel awed as you stand in the dappled sunlight, craning your neck to see the tops of these natural giants.

I couldn’t help but ask the question: How do these trees get water from the ground all the way up to their leaves?

Luckily, I’ve been reading a book on everyday physics, “Storm in a Tea Cup: The Physics of Everyday Life” by Helen Czerski, which helped me answer that question. This, along with further research to ensure I understood the concept and captured the latest science, helped me gain an understanding of the incredibly feat conducted by these trees.

Caveat: I am not a scientist and am keeping this as simple as possible for this blog post I also discovered during my further research that there is some controversy about the theory of how water reaches the branches. This post outlines how most scientists think it’s done.

How do trees get water to their branches (and leaves)? 

1. Water loves water - There’s a polar attraction between water molecules. Negative and positive charges attract each other, so hydrogen (slightly positive) and oxygen (slightly negative) want to stick together. What to know: water molecules want to be near other water molecules.

2. Smaller can be better - We tend to think of gravity as the ultimate force. After all, it’s why we humans aren’t able to fly. However, we experience gravity in this way because of our size. When we think about something that as much, much, much smaller than us (teeny tiny with less mass), other forces can have a bigger impact. For example: surface tension may have more of a pull on something microscopic than gravity.

3. Water also loves other things besides water - Though I love the fact that water molecules want to stick together for the most part, it is important to note that h2o can also feel an attraction to other things.

In her book, physicist Czerski uses the example of a towel. She explains that towels are made up of a bunch of tiny fibers which are attracted to water. At a small enough scale (see point 2), they can pull water up surface water molecules (defying gravity).

This, however, is when water remembers its first love: water. The molecules that have attached to the towel’s fibers will then attract more water from the spill below.

What does a towel have to do with trees?

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

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. | Source: California Academy of Sciences. “Stomata Printing: Microscope Investigation.”

This then leads to water evaporating from the stomata (transpiration). This evaporation or loss of water is stressful for the water remaining in the leaves. Remember, water wants to be near water. So, to make up for this, surface tension/cohesion causes the water to pull more to join it in the leaves. 

Because these stomata are so microscopic, the tension is greater than the force of gravity.

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). 

I have also started reading “Braiding Sweetgrass” by Robin Wall Kimmerer and am fascinated by her knowledge of trees utilizing their biologies during various seasons.

But until I can finish that book, and have time to rewatch Ant-Man, 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.