Trees don't seem to do all that much. Occasionally their branches might sway in the breeze and many of them drop leaves on a regular basis. But it seems there's a lot more going on with trees that we thought.
Researchers have found that, at night, many trees periodically move their branches up and down slightly. This suggests that perhaps the trees are pumping water upwards slowly, hinting that the trees have some semblance of a pulse.
"We’ve discovered that most trees have regular periodic changes in shape, synchronized across the whole plant and shorter than a day-night cycle, which imply periodic changes in water pressure," András Zlinszky of Aarhus University in the Netherlands told New Scientist.
For a 2017 study, Zlinszky and his colleague Anders Barfod used high-resolution terrestrial laser scanning, a technique often used in civil engineering to measure buildings. They surveyed 22 trees representing different species over a 12-hour period during a windless night to see if their canopies changed.
In several of the trees, branches moved by about a centimeter up or down. Some moved as much as 1.5 centimeters.
Looking for a heartbeat
After studying the nocturnal tree activity, the researchers came up with a theory about what the movement means. They believe the motion is an indication that trees are pumping water up from their roots. It is, in essence, a type of "heartbeat."
Zlinszky and Barfod explain their theory in their newest study in the journal Plant Signaling and Behavior.
"In classical plant physiology, most transport processes are explained as constant flows with negligible fluctuation in time, especially at the level of the entire plant, or on timescales shorter than a day," Zlinszky told New Scientist. "No fluctuations with periods shorter than 24 hours are assumed or explained by current models."
But the researchers aren't sure how a tree successfully manages to pump water from its roots upwards to the rest of its body. They suggest maybe the trunk gently squeezes the water, pushing it upwards through the xylem, a system of tissue in the trunk whose main job is to transport water and nutrients from roots to shoots and leaves.
In 2016, Zlinszky and his team released a study showing that birch trees "go to sleep" at night.
The researchers believe the dropping effect of birch branches before dawn is caused by a decrease in the tree's internal water pressure. With no photosynthesis at night to drive the conversion of sunlight into simple sugars, trees likely conserve energy by relaxing branches that would otherwise be angled towards the sun.
These birch movements are circadian, following the day-night cycle. However, researchers don't believe the newly discovered movements are similar because they typically follow much shorter time periods.