Scientists, and most schoolchildren, have long thought that water is passively pulled up through the trunks of trees as water evaporates from the leaves.
New research now suggests that trees may have a “heartbeat” that pumps water from their roots. But unlike people, whose hearts beat 70 to 90 times per minutes, trees have pulses that stretch over hours. These slow pulses, though, could play a key role in how plants distribute water and nutrients to its limbs and leaves.
One thing that led researchers to this idea was when they discovered that many trees move their branches up and down at night. In a 2017 study, András Zlinszky of Aarhus University in Denmark and his colleagues scanned 22 species of trees at night with lasers to see if the tree crowns moved.
They carried out the experiment in a greenhouse that had a glass roof, but no walls. During the test, there was no wind, precipitation or condensation. The only light source was the full moon.
All of the trees had some movement in their crown. A couple showed very little movement, while one moved its crown up to 9 centimeters throughout the night. Many of the trees, though, moved their branches one-half to one centimeter in cycles that lasted a few hours.
In a 2018 paper , Zlinszky and his colleague write that these periodic movements of the canopy may represent a previously “unknown pumping mechanism.”
They suggest that if water transport occurs between sections of the trunk, rather than along the whole length, it would only have to overcome the hydrostatic pressure of that one section. This is the increasing pressure that occurs as you move down a column of water.
It’s not clear how trees pump water upwards, but several factors may be involved. The movement of the branches cause a change in the volume of live cells in the xylem tissue which transports water through the trunk. In addition, tiny fluctuations in the diameter of tree stems occur overnight.
Proteins embedded in the membranes of specialized cells that surround the xylem vessels may also play a role in generating the pulses. These proteins, called aquaporins, are known to be involved in regulating water pressure inside roots.
More research is needed to know how these factors relate to the movement of the crown, and whether the pulses affect the entire tree at once or travel in waves. The researchers already have additional experiments in mind that may reshape how we view trees and their “heartbeats.”
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