The Heartbeat of a Tree
Below is an excerpt of part of a Dear Naturalist column from last summer. It includes updates (at the bottom).
Last spring I asked a good friend, who is a nurse, if I could have her old stethoscope.
“Tired of doing the ranger thing and planning to start a real career?” she asked.
I replied, “Just wanting to do a little experiment.”
My friend rolled her eyes. I suppose I will never get her to understand me, but she has always been good in the acceptance department.
She searched through a box of her nursing school things to find her very first stethoscope, one that she had not used in years. The headset still shined, the tubes a brilliant blue, and the chest piece unnaturally cold.
I have listened to all kinds of noises through the stethoscope since I left my friend’s house that day: my own heartbeat, my dog’s heartbeat, my co-worker’s heartbeats. I’ve listened to grass, fungi and lichen (not too much going on there).
One day, still in the spring, I stuck the chest piece against the smooth bark of the Fraser magnolia tree. From the earpiece came a steady, “Shwish. Shwish. Gurgle. Shwish.”
At first, I thought that it must be picking up sounds from my own heartbeat, but on further examination, realized that the noise was most certainly coming from the tree. What I was hearing that day was the magnolia tree’s circulation.
Yes, my friend, trees have a “heartbeat,” just like you and me. Vascular tissues inside the trunk are moving nutrients and water up and down between the roots and leaves.
In the spring, a deciduous tree’s vascular tissue is “waking up” from winter. For many months the tree has been dormant to help it withstand the cold. The warm temperatures of early spring induce the tree into producing buds. But first it has to get its juices flowing. The tree’s vascular system is made of rings of vertical tubes that exist in the trees’ youngest layers, just under the bark.
Directly under the tree’s outer bark is a layer of phloem. The phloem tubes help to carry sugar and nutrients produced in the leaves to the rest of the tree. But how does water get up to the leaves from the roots? Another layer of vascular tissue, called “xylem.”
Trees need to produce new xylem tissue every year as the old tissue dries out and becomes part of the tree’s central heartwood.
This is why a tree should have a ring for every year that it is alive. As water and sap move up and down the tree, a swishing sound can be heard through a stethoscope.
You can perform this experiment in your own backyard. You just have to be brave enough to ask your own friend for her old stethoscope. Do not be afraid; you risk nothing more than a simple rolling of the eyes.
If you have a question concerning flora and fauna, please email (firstname.lastname@example.org) All of your questions will be answered. One or two will be featured next week. See you on the trails!
Amy Renfranz is a certified naturalist through the Yellowstone Association Institute and a certified environmental educator in the state of North Carolina.