As it Turns Out, Trees go to Sleep Too


Scientists from Austria, Finland and Hungary are using laser scanners to study the day-night rhythm of trees. And yes, they go night-night just like us! They even slump over like they have had a hard day at the photosynthesis office.

amazing-736884_960_720We always knew that most living organisms adapt their behaviour to the rhythm of day and night. Plants are no exception: flowers open in the morning, some tree leaves close during the night.

Researchers have been studying the day and night cycle in plants for a long time: Linnaeus observed that flowers in a dark cellar continued to open and close, and Darwin recorded the overnight movement of plant leaves and stalks and called it “sleep.” But even to this day, such studies have only been done with small plants grown in pots, and nobody knew whether trees sleep as well.

Now, a team of researchers from Austria, Finland and Hungary measured the sleep movement of fully grown trees using a time series of laser scanning point clouds consisting of millions of points each.

Trees Droop their Branches at Night

“Our results show that the whole tree droops during night which can be seen as position change in leaves and branches”, says Eetu Puttonen (Finnish Geospatial Research Institute), “The changes are not too large, only up to 10 cm for trees with a height of about 5 meters, but they were systematic and well within the accuracy of our instruments.”

To rule out effects of weather and location, the experiment was done twice with two different trees. The first tree was surveyed in Finland and the other in Austria. Both tests were done close to solar equinox, under calm conditions with no wind or condensation. The leaves and branches were shown to droop gradually, with the lowest position reached a couple of hours before sunrise. In the morning, the trees returned to their original position within a few hours. It is not yet clear whether they were “woken up” by the sun or by their own internal rhythm.

MATLAB Handle Graphics

Trees have their own day-and-night-rhythm. Credit: Eetu Puttonen

It has always been difficult to study plants at night. This is because photography needs light, and light disturbs the sleep cycle of the plant. But with a laser scanner, plant disturbance is minimal. The scanners use infrared light. With this laser scanning technique, a full-sized tree can be automatically mapped within minutes.

“We believe that laser scanning point clouds will allow us to develop a deeper understanding of plant sleep patterns and to extend our measurement scope from individual plants to larger areas, like orchards or forest plots,” says Norbert Pfeifer of TU Wien.

So, trees need their beauty sleep, and soon we will know if forests as a whole do as well. It just goes to show, all of nature’s creatures are pretty much the same. And if you wake us up before we are ready, then all of us, including the trees, will be damn cross. Watch out for falling branches at 4am, moon hikers.

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