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"Sleeping Like a Log? What Trees Do—and Don't Do at Night"[ Show/Hide Article ]

Trees—they're just like us. They sleep, they drink—and they even have a pulse. What the latest research can also tell us about whether they're stressed out.

Trees by YangTS

We humans are born with a natural clock—an internal timekeeper that makes us sleepy at night and ready to wake up in the morning. Maybe a little more ready after coffee, but our circadian rhythms (as they’re known) are a 24-hour cycle synced to sunrise and sunset.

And people aren’t the only ones with these master clocks. All living beings from algae and bacteria to insects and mammals have these sleep-wake cycles. You might have noticed, for example, that plants close their leaves and flowers at night. Darwin noticed, too. He studied plant movement, and one of the last books he wrote (in 1880) was called “The Power of Movement in Plants.” He defined and named many different types of plant movements and one was a sleep movement, which he defined as movement of leaves, having a 12-hour cycle, and taking place during the night.

oleaeuro_webFast forward about 138 years and Darwin probably would have been pretty wowed by what Andras Zlinszky of Aarhus University discovered. He and his colleague Anders Barfod recently published a paper in the journal Plant Signaling & Behavior on what happens to trees when the sun goes down.

They observed that trees have a so-called “sleep” movement, at least some trees do. In the birch trees that they studied the branches moved down up to 8 centimeters during the night and went back to the starting position by early morning. Darwin didn’t know that the branches moved because he did not have the precision equipment to measure changes like modern scientists do.

Why do trees lower their branches? Zlinszky says there are two possible explanations: One would be that it’s a passive thing. It’s just a product of the water pressure dropping in the tree as the transport mechanism loses power with the evaporation stopping during the night. During the day as plants photosynthesize, water evaporates from the leaves and the tree sends water up from the roots to replenish it. At night, the sun goes down and that activity slows. And because water gives plants their shape, a lack of water in the branches and leaves causes them to slightly droop.

That’s one possibility. The other explanation Zlinszky says, might be what we would do if it got chilly after dark. He notes that it’s been proven that leaves that are horizontal lose more energy toward an open sky at night than leaves that are vertical. Also, if a tree crown or canopy has a slightly smaller volume—like somebody who is pulls themselves together when they’re cold—then they lose less heat.

Zlinszky says not all trees lower their branches, but one thing they learned from their study that all trees have in common? They have a pulse.

He explains that the pulse moves water upward by contracting the stem and reducing the volume of the trunk. Bu it’s definitely very slow—around one “heartbeat” every two hours at the shortest or even one every four or five hours at the longest.

So how does the tree “know” to do all of this? How does it decide to raise its branches in the morning? It goes back to the circadian rhythm. Zlinszky says his study answers quite clearly that in the trees where he found sleeping movement they were waking up and going back to their starting position well before first change in the ambient light. “The trees actually know the sun is going to come up and prepare for it.”

Not only did the trees know the sun was coming up—they set their alarm. Zlinszky and his team observed that the trees—like clockwork—started returning their branches to the starting position around four or five in the morning in anticipation of the sunrise.

So how would this new knowledge be useful other than revealing the secret lives of trees? One use might be as a potential tool for fruit growers or foresters. Zlinszky and his team noticed that unhealthy trees or ones in a drying situation moved their branches up without returning to the starting position. He said that action could indicate that something is wrong with a tree before its leaves start turning yellow—a possible early warning sign about tree health in places like orchards.

Whether branches are sending distress signals or simply marking time, we now know that trees aren’t just quietly growing taller—they’ve got rhythm.  💧

Published 24 May 2018 | © H2O Media, Ltd. | Permalink | I Got Rhythm!, George Van Eps & Howard Alden | Tree Photo: YangTS, Creative Commons



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