Dear Grant, William and Kate,
When I was a kitten, I liked to scratch tree trunks. I loved sinking my claws into their bark. Now I mostly hug trees—but I still love the way bark feels under my paws.
I asked my friend Henry Adams why trees have that tough outer layer. He’s a tree scientist at Washington State University.
He told me that bark protects trees.
“Bark is like our skin,” Adams said. “It protects trees from all kinds of attackers that might want to get in—like viruses, bacteria, fungi, insects or even non-living things like fire.”
But bark is much more than just that dry, crunchy stuff you touch when you hug your favorite tree. Bark has lots of layers.
The brittle outer layer you can touch is called cork. If bark is like your skin, the cork is like the very outer bit of your epidermis. Both are dead. Both help protect a tree or person from problems—like excessive heat, sharp things or germs that want to get inside.
Cork contains a waxy substance called suberin. It keeps water and other important stuff from leaking out of the tree. It also helps stop germs and insects from getting into the tree. Suberin even resists fire. Trees that grow where there are wildfires might have extra thick cork to help them survive.
Underneath the cork is the cork cambium. This bark layer makes new cells to replace the cork that flakes off. It also keeps the tree from drying out and protects it from physical damage.
Below that, there’s a thin ring of phelloderm. If you scratch a young, springy branch and see a layer of green “skin,” that’s phelloderm. It’s green because it’s full of chloroplasts. Like green leaves, it uses photosynthesis to make sugar from the sun’s light energy.
Tucked under the phelloderm is a thick layer of phloem. That’s the highway a tree uses to move sugar and nutrients up, down and around the tree. It’s super sweet, so some animals want to eat it—like insects or even elk and deer.
Below the phloem is the vascular cambium. Adams told me this is the boundary between the bark and the inside of the tree. The vascular cambium’s job is to make the tree trunk grow bigger around.
One of my favorite things about bark is that it can tell scientists how healthy an ecosystem is. Since trees live for a long time, they store clues that tell us how clean the air was or what kinds of pollution were around.
So, in a way, bark protects the tree it grows on—and helps scientists protect us, too.
I’ll go out on a limb and say that’s pretty tree-mendous.
Sincerely,
Dr. Universe