Almost all organisms are ruled by the day/night cycle of Earth. In humans, and many other mammals and birds, this is dominated by periods of wakefulness and sleep. But what about trees? They too work on the cycle of the Sun, and are known to have genes that are switched on and off in what is known as the circadian rhythm. Well, it turns out that trees might in fact spend some of their night times “sleeping" too.
“Our results show that the whole tree droops during night which can be seen as position change in leaves and branches,” explains Eetu Puttonen from the Finnish Geospatial Research Institute, and co-author of the study published in Frontiers in Plant Science. They found that the trees droop their branches by as much 10 centimeters (4 inches) in the hours leading up to dawn. The effect was seen across the whole tree, and even in plants measured in different countries thousands of kilometers apart.
The team measured the plants using laser scanners, focusing them on two birch trees, one in Austria and one in Finland, and building up complete 3D images of them. By using the lasers, they were able to track changes in the positions of the tree’s leaves and branches down to the centimeter, while also allowing them to work at night without lights to make sure that didn’t disturb the plants' day/night cycle by accident. They found that both trees measured in different countries drooped over the course of the night, allowing them to rule out the effect of weather and location.
The birch trees were found to droop their branches at night (left), and straighten up again during the day (right). Vienna University of Technology, TU Vienna
What the researchers are yet to figure out is whether the drooping of the leaves and branches is either a passive process, or an active one. The drooping of the branches could be caused due to a drop in internal water pressure. During the day, the trees are photosynthesizing, which increases the “turgor” pressure within the tissues of the trees. As the Sun sets and the photosynthesizing stops, the turgor pressure may be dropping, causing the leaves and branches to droop.
But, it could be that the trees are genuinely “resting” their branches. During the day, trees will tilt their leaves to make the most of the light availability, and to prevent leaves from the same tree casting shade on other leaves. This action costs energy, and so there is a chance that at night, when there is no Sun and the leaves don’t need to tilt, the trees could simply be trying to save energy.
While research has been carried out on the circadian rhythm of other plants, particularly in the laboratory, these experiments are the first to look at how trees behave during the night in the field. The scientists hope next to look at whether or not other species of tree also show this “sleeping” behavior, which they suspect they do. They also think that research should look into whether or not the movements observed were influenced by the light/dark, or whether it was independent of this and governed instead by the internal circadian clock.