Trees were observed overnight and their branches become droopy, which the scientists making the measurements compared to a sleep/awake pattern just like that seen in humans.
The study was carried by Hungarian scientists, from the Centre for Ecological Research in Tihany, Hungary, who observed branches of birch trees drooping by as much as 10 centimetres at the tips towards the end of the night. They used laser beams to scan trees in Austria and Finland between sunset and sunrise. From the time it takes beams to bounce back from branches and leaves, they could measure the movements of each tree, in three dimensions and at resolutions of centimetres.
“It was a very clear effect, and applied to the whole tree,” said one of the scientists, András Zlinszky to the New Scientist “No one has observed this effect before at the scale of whole trees, and I was surprised by the extent of the changes.”
Scanning was done on calm nights to avoid wind effects, and at the solar equinox in both countries to ensure the length of night were roughly the same. “We’re certain it’s not caused by some other effect, added Norbert Pfeifer of the Technical University of Vienna in Austria, another team member.
Studies had only been done before in small plants, so this is the first time the phenomenon was seen in fully grown trees. The drooping effect is probably caused by loss of internal water pressure within plant cells, a phenomenon called turgor pressure. “It means branches and leaf stems are less rigid, and more prone to drooping under their own weight,” says Zlinszky. Turgor pressure, in turn, is influenced by photosynthesis, the process by which plants use sunlight to create sugar from carbon dioxide and water. Photosynthesis stops in the dark, so this in itself may explain why the branches droop, explained Zlinszky.
The trees may also be “resting” their branches. During the day, branches and leaves are angled higher, allowing leaves to catch more sunlight, but this is energy-intensive and serves no purpose at night, when there’s no light.
So is the drooping deliberate, dictated by an active sleep-night cycle, or passive, dictated by differences in the availability of water and light? “This remains to be decided,” says Zlinszky.
Hmm, I think the drooping at night time is dictated by differences in light availability, but I still like the idea of saying the trees are asleep when their branches are droopy at night!
Link to original article in the New Scientist.