How do trees sleep?
Most living organisms adapt their behavior 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.
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Image credit: Eetu Puttonen