Unlike other plants and animals, how trees sleep remain a mystery until now. According to a team of researchers from Austria, Finland, and Hungary, the whole tree droops at night when it sleeps.

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

To study a tree’s overnight movements, the researchers, whose study has been published on Feb. 29 in the journal Frontiers, studied two different Birch trees, one in Finland and the other in Austria. The trees were analysed under calm conditions, without the presence of condensation or wind to rule out the effects of weather and location.

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

Using a laser scanner that used infrared light, the team found that the trees’ leaves and branches drooped gradually. The lowest position was observed just two hours before sunrise. The trees then turned to their original position in a few hours in the morning. The researchers assert that they cannot tell if the trees were awoken by their own internal rhythm or by the sun.

“Plant movement is always closely connected with the water balance of individual cells, which is affected by the availability of light through photosynthesis,” says András Zlinszky from the Centre for Ecological Research at the Hungarian Academy of Science. “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.”

The researchers will investigate more about the plants’ sleep patterns. They plan to include plants in orchards, forest plots or in other larger areas.

“The next step will be collecting tree point clouds repeatedly and comparing the results to water use measurements during day and night,” adds Puttonen. “This will give us a better understanding of the trees’ daily tree water use and their influence on the local or regional climate.”