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Trees may seem sedate but it turns out they are more active than we thought. Many trees move their branches up and down during the night. The findings hint that the trees are actively pumping water upwards in stages, and that trees have a slow version of a 鈥減ulse鈥�.

鈥淲e鈥檝e discovered that most trees have regular periodic changes in shape, synchronised across the whole plant and shorter than a day-night cycle, which imply periodic changes in water pressure,鈥� says of Aarhus University in the Netherlands.

In a study published in October 2017, Zlinszky and his colleague used a form of laser-scanning normally deployed to monitor tall buildings. They scanned 22 species of tree for one night each in windless, lightless conditions to see if the trees鈥� canopies changed shape.

In seven species, branches moved up or down by about a centimetre. These see-saw oscillations in branches were most pronounced in magnolia trees, averaging up to 1.5 centimetres. The cycles repeated every 3 to 4 hours.

We got a heartbeat

Now the pair have an idea for what the movements could represent. They think they might be evidence that trees have a 鈥渉eartbeat鈥�, and that they are actively pumping water up from their roots in pulses that last hours.

Previously it was thought that trees don鈥檛 do that, but simply wait for the evaporation of water from leaves to passively 鈥減ull鈥� the water up.

鈥淚n 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,鈥� says Zlinszky. 鈥淣o fluctuations with periods shorter than 24 hours are assumed or explained by current models.鈥�

Moving the water up in stages might save energy, says Zlinszky. 鈥淚f water is pumped between sections, only the hydrostatic pressure of an individual section would need to be overcome by transport, not the full pressure resulting from the height of the tree.鈥�

A small squeeze

It鈥檚 not clear how a tree might pump water upwards. Zlinszky and Barfod suggest that the trunk might gently squeeze the water upwards, pushing it up through the xylem: a column of dead cells through which most water moves up the trunk.

They say this idea is supported by previous studies by other teams, which found that .

鈥淲e suggest a so-far-unknown pumping mechanism,鈥� says Barfod. It may be that living cells in the xylem can change size, creating a squeezing action.

They also think that water-transport proteins called aquaporins in the membranes of the cells may play a key role in this process. Aquaporins are known to trigger rapid changes in water flow.

In 2016, Zlinszky and his colleagues showed that birch trees 鈥渟leep鈥� at night, resting their branches by drooping them as much as 10 centimetres. These movements were 鈥渃ircadian鈥�, meaning they reflected the day-night cycle: the branches returned to their normal position by morning. However, the newly discovered movements operate on shorter timescales that cannot be circadian.