Using a terrestrial laser scanner on windless nights close to the equinox, researchers scanned two birch trees over the course of the night, one in Finland and one in Austria. Researchers scanned the birch in Finland hourly and the one in Austrain about every 10 minutes. The results, published in the journal Frontiers in Plant Science, show that the trees drooped up to nearly four inches during the night.
According to a press release, the leaves and branches slowly relaxed over time, reaching their lowest position about two hours before sunrise. Over the course of the morning, the trees returned to their original positions.
In some ways the study was a test of the laser scanning technology. Using traditional photography, which needs lots of light to produce an image, would have interfered with the trees’ nighttime patterns. But the infrared laser illuminated points on the tree for a fraction of a second. That allowed the entire tree to be mapped in minutes with minimal disturbance.
This technique could allow scientists to delve further into the study of "plant sleep patterns," moving from individual trees to much larger areas, study co-author Norbert Pfeifer explains in the press release.
It’s likely that the droop is caused by a decrease in turgor pressure, a type of internal water pressure that keeps plants upright. “It means branches and leaf stems are less rigid, and more prone to drooping under their own weight,” study co-author András Zlinszky, a biologist at the Hungarian Academy of Sciences, tells Coghlan. When photosynthesis stops at night, turgor pressure is reduced, likely causing the branches to “relax.”
It’s possible the droop is also related to circadian rhythms, which are encoded in almost every creature on earth, Brian Resnick at Vox points out. The researchers tell Coglan that they hope to repeat the experiment on other tree species, and are particularly interested in chestnuts and poplars, two trees in which genes associated with circadian rhythms have been found.
“Perhaps the most important open question is whether the observed branch movements take place under the influence of light from sunset and sunrise, or if they are independent from light and governed by the internal circadian clock of the plant,” according to the study. Some branches started to return to their daytime position before sunrise, hinting that perhaps the plants do follow an internal clock. But only more study of the tree's daily patterns will help determine if this is the case.
“There have been some studies on circadian rhythms in trees, mostly studying gene expression, but this latest research is a beautiful way to watch it happen in individual trees,” biologist C. Robertson McClung of Dartmouth College, who is not involved in the study, tells Coghlan. “It shows things are happening in the real world.”
The study might have practical applications as well. Study author Eetu Puttonen says knowing the daily cycle of how water moves through trees could help both the timber and rubber industries, both of which rely on water content of the trees.