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 at night.
Researchers have been studying the day-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 nighttime movement of leaves and stems and called it "the dream." .
But even today, these kinds of studies have only been done with small plants grown in pots, and no one knew if trees sleep like this. Now a team of researchers from Austria, Finland and Hungary have measured the sleep motion of adult trees using a time series of laser scan point clouds, consisting of millions of points each.
"Our results show that the whole tree tilts at night, which can be seen as the change in position of the leaves and branches," says Eetu Puttonen, from the Finnish Geospatial Research Institute.
"The changes are not too great, only up to 10 centimeters in trees with a height of about 5 meters, but they are systematic and measurable within the accuracy of our instruments", adds the scientist.
To rule out effects of climate and location, the experiment was performed twice with two different trees. The first tree was examined in Finland and the other in Austria. Both tests were conducted near the solar equinox, under calm conditions with no wind or condensation.
Leaves and branches fall slowly, with the lowest setting a couple of hours before sunrise. In the morning, the trees returned to their original position within a few hours. It is not clear yet whether they were "awakened" by the sun or by their own internal rhythm.
"At the molecular level, the scientific field of chronobiology is well developed, and especially the genetic background of the daily periodicity of plants has been extensively studied," explains András Zlinszky from the Center for Ecological Research, Hungarian Academy of Sciences.
“Plant movement is often closely related to the water balance of individual cells, which is affected by the availability of light through photosynthesis. But the changes in the shape of the plant are difficult to document even for small herbs, since classical photography uses visible light that interferes with sleep movement, ”he adds.
With a laser scanner, plant disturbance is minimal. The scanners use infrared light, which is reflected off the sheets. Individual points on a floor only light up for fractions of a second. With this laser scanning technique, a full-size tree can be scanned automatically in minutes with sub-centimeter resolution.