Study suggests trees sleep at night

We’ve known for centuries that plants are governed by a circadian rhythm just as much as humans: no less a scientific authority than Charles Darwin himself described the daily cycles of leaf motion as “sleep” all the way back in 1880.

But a study published in the journal Frontiers in Plant Science provides compelling new evidence that trees do in fact have themselves a bit of a snooze after the sun goes down.

The study was not actually designed to test whether or not trees slumber, but instead to measure a novel way of detecting and recording the movements of trees. A team of Finnish and Austrian researchers used terrestrial laser scanning (TLS) to determine the circadian movements of silver birch (Petula Bendula) branches and foliage.

Their study consisted of two geographically separate experiments carried out at the same time of year and under similar outdoor conditions. The movements of 14 birch trees in Finland and 77 in Austria were monitored via individual laser scans taken between sunset and sunrise in order to avoid daytime wind effects.

The TLS method the researchers employed produces accurate 3D point clouds of target objects down to millimeter resolution, providing detailed information about structural and spatial properties. In this case, the researchers found that the trees’ branches relaxed, so to speak, at night — drooping as much as 10 centimeters. Which is to say, it appears trees rest for a few hours at night, starting a couple hours after sunset, before becoming active again just before dawn.

“The temporal development of the movements followed a highly similar pattern in both experiments, with the maximum movements occurring about an hour and a half before (Austria) or around (Finland) sunrise,” the authors of the Frontiers in Plant Science paper write. “The movements were observed to happen systematically over a time span of several hours, which ruled out occasional wind effects.”

The researchers said that there is significant potential for TLS to provide additional information that can be correlated with physiological measurements in order to gain a better understanding of how, when, and why plants sleep.

Perhaps the most important question that has yet to be answered, they said, is whether the branch movements they observed are influenced by light, i.e. if plants’ sleep cycles are dictated by sunset and sunrise, or if they are instead governed by an internal circadian clock.

“The fact that some branches started returning to their daytime position already before sunrise would suggest this latter hypothesis, but the temporal frequency of our investigations is not sufficient for confirming or rejecting it: ideally, measuring several full 24-[hour] cycles of a tree isolated from natural light would be required for this,” they wrote.

As to their study’s intended purpose, the researchers say they have indeed proven the potential of TSL to contribute to our understanding of plants’ circadian rhythms, writing: “The results demonstrate the potential of terrestrial laser scanning measurements in support of chronobiology.”

CITATIONS

• Darwin, C., and Darwin, F. (1880). The Power of Movement in Plants. London, UK: John Murray. doi:10.5962/bhl.title.102319
  
• Puttonen, E., Briese, C., Mandlburger, G., Wieser, M., Pfennigbauer, M., Zlinszky, A., & Pfeifer, N. (2016). Quantification of overnight movement of birch (Betula pendula) branches and foliage with short interval terrestrial laser scanning. Frontiers in plant science, 7. doi: 10.3389/fpls.2016.00222