The Ranger’s Stethoscope

By Amy Renfranz (

Article Published: Sep. 13, 2012 | Modified: Sep. 13, 2012
The Ranger’s Stethoscope

Trees, like most other living things, make sounds. You just have to listen closely.
Photo courtesy Amy Renfranz

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. 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.

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, the vascular tissue is “waking up” from winter and very active.

What I was hearing that day was the magnolia tree’s circulation.

Lately, the trees’ heartbeats are much harder to detect, which plays into my answers for this week’s questions.

Why do leaves turn colors and fall off of the trees in autumn? — Ben, age 6

The spectacular fall show that the trees put on for locals and tourists, alike, actually starts the day after the summer solstice in June. Beginning that day, sunlight hours shorten and temperatures steadily drop. Winter is on its way.

Deciduous trees are not made for winter. Their leaves are fragile, and if covered with snow, the trees’ branches would break. With shorter days, the leaves would not be able to provide the tree with enough energy to make their existence worthwhile. The tree is better off shedding the leaves and going into a dormant state for the coldest months.

During the growing season, the chlorophyll that makes the leaves green is continually being produced and broken down. As night length increases in the autumn, chlorophyll production slows down and, eventually, all the chlorophyll is dissolved.

What begins to happen after the chlorophyll has been dissolved is that the other color pigments, which have always been present, but outdone by the chlorophyll, start to emerge.

Certain colors are characteristic of particular species. Beech, poplars, Quaking Aspen and hickories usually turn a brilliant yellow. Sourwood, red maple and dogwoods turn red. Oak leaves turn russet. Of course, there are variations from tree to tree.

Typically, leaves change their color based on elevation, but this, too, varies from species to species. After changing, the leaves fall and decay, returning their nutrients to the earth. The tree goes into a dormant state. At this point hearing the “heartbeat” is impossible.

Are all those wives’ tales true about rainfall having an effect on the fall color? —Budding naturalist, age 65

Surprisingly, they are. Temperature and moisture rates during the growing season and autumn are strongly related to the development of the fall tree color.

A good growing season for fall color will be one that is warm and wet. Drought in the summer can cause the trees to go dormant early, and the leaves to fall before they reach ultimate color.

Successions of early fall days that are warm and sunny with nights that are cool and crisp, but not freezing, do seem to bring out the most brilliant displays of color.

Heavy wind and rain can cause the leaves to fall before they have fully burst into color. Early frost can also have this effect.

Will there be good color this year? I think so, but it is hard to predict. One thing is certain though: Every autumn is unique. Just like every tree is. They follow the rhythms of nature with the beating of their “hearts.”

If you have a question concerning flora and fauna, please email ( All of your questions will be answered. One will be featured next week. See you on the trails.

Amy Renfranz is an interpretive park guide on the Blue Ridge Parkway. She is a certified naturalist through the Yellowstone Institute and a certified environmental educator in the state of North Carolina. Her comments are made independently and do not reflect the views of the National Park Service.

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