Like plants, trees too "sleep" at night, according to a new study which measured the sleep movement of fully grown trees using a time series of laser scanning point clouds.
Most living organisms adapt their behaviour to the rhythm of day and night. Plants are no exception - flowers open in the morning and some tree leaves close during the night.
Researchers have been studying the day and night cycle in plants for a long time and observed that flowers in a dark cellar continued to open and close.
However, to this day, such studies have only been done with small plants grown in pots, and nobody knew whether trees sleep as well.
Researchers measured the sleep movement of fully grown trees using a time series of laser scanning point clouds consisting of millions of points each.
"Our results show that the whole tree droops during night which can be seen as position change in leaves and branches," said Eetu Puttonen from the Finnish Geospatial Research Institute.
"The changes are not too large, only up to 10 centimetres for trees with a height of about 5 metres, but they were systematic and well within the accuracy of our instrument," said Puttonen.
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.
"Plant movement is always closely connected with the water balance of individual cells, which is affected by the availability of light through photosynthesis," said Andras Zlinszky from Hungarian Academy of Sciences.
"But changes in the shape of the plant are difficult to document even for small herbs as classical photography uses visible light that interferes with the sleep movement," said Zlinszky.
With a laser scanner, plant disturbance is minimal. The scanners use infrared light, which is reflected by the leaves.
Individual points on a plant are only illuminated for fractions of a second. With this laser scanning technique, a full-sized tree can be automatically mapped within minutes with sub-centimetre resolution, researchers said.
"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," said Norbert Pfeifer from Vienna University of Technology.
The findings were published in the journal Frontiers in Plant Science.