An olive tree as a point cloud from the terrestrial laser scanner. [Image: András Zlinszky]
Researchers from Aarhus University, Denmark, and the Hungarian Academy of Sciences (HAS), Hungary, have used terrestrial laser scanning (TLS)—ground-based lidar—to explore the secret nocturnal lives of trees (Front. Plant Sci., doi: 10.3389/fpls.2017.01814; Plant Signaling Behav., doi: 10.1080/15592324.2018.1439655). The upshot? Trees are surprisingly restless sleepers.
According to the research, the branches of some trees reveal overnight “sleep cycles” including slow movements across a 12-hour period. And trees may also rely on even smaller-scale, repeated overnight branch movement to help “pump” water from root to crown. The team believes that detection of anomalies in these overnight movements could someday help to diagnose stress or disease in crops.
Point clouds
The process by which water is transported from the ground into the leaves of trees, where photosynthesis occurs, is one of the most important life-sustaining processes on Earth. And, as the process is driven by sunlight, it takes place within a 24-hour cycle. It’s been known since the time of the ancient Greeks that the leaves of plants move in response to daily cycles. But it was revealed only in 2016 that the branches of one tree species—the silver birch—also undergo circadian movements.
Aarhus researchers András Zlinszky (also affiliated with HAS, and a coauthor on the 2016 study) and Anders Barfod wanted to take these findings further and explore these hidden nocturnal movements across a range of tree types. To do so, they turned to TLS, the same technique used in the 2016 research.
TLS, like other forms of lidar, uses the time of flight of a stream of laser pulses to measure the distance to a target. The result of such a lidar scan is a 3-D point cloud model—and position information, according to the researchers, accurate to a near-millimeter scale. By comparing changes in height percentiles of laser scanning points in the tree canopy, scientists can measure the exact position of branches and leaves at various measurement times.
Short-term cycles
In September 2016, Zelinsky and Barfod (working with Bence Molnar of the Budapest University of Technology and Economics) set up a controlled experiment in the outdoor nursery of a garden shop near Balatonfüred, Hungary. Using a 905-nm TLS system capable of capturing 7 billion points in 15 minutes, the team performed 18 scans of 22 trees across a 12-hour overnight period, to study their circadian movements. The researchers chose a study period with minimum wind, precipitation and condensation and also “made great effort” to exclude artificial light from the experiment.
The data from this study, published in 2017, showed great variety in branch and leaf movement, which appeared to reflect three patterns: diurnal cycles of circadian movement (“sleep”), non-cyclical unidirectional movement (“drift”), and previously unknown, two-to-six-hour short-term cycles of movement (“oscillation”).
While the team observed some nocturnal movement in all of the trees studied, only seven species demonstrated 12-hour sleep cycles of lowering and lifting their branches—with some trees lowering their branches up to 10 cm at night and raising them again in the morning. The researchers also found, to their surprise, that every one of the trees studied displayed the smaller oscillation pulses of minute periodic movement (as small as a centimeter) during the night.
An arboreal pump?
Why do trees undergo these nighttime movements? In a short follow-up paper in 2018, Zelinsky and Barfod suggest an answer: the short-term cycles in particular, they believe, could relate to a sort of active management of the tree’s turgor pressure (the internal osmotic pressure inside the tree’s cells) to redistribute nutrients and optimize its setup for the next day’s photosynthesis. “The movement has to be connected to variations in water pressure within the plants,” Zelinsky noted in a press release accompanying the recent paper. “And this effectively means that the tree is pumping.”
While it is generally assumed that water transport in trees is a steady process that does not vary in rate based on the time of day, Zelinsky continues, these findings challenge that view. The study’s results suggest that short-term change in water transport is widespread and that short-term plant movements are caused by these changes in water pressure within plant tissue.
“The observation of concerted sub-circadian periodic movement of tree leaves and branches,” according to the study, “definitely suggests some form of synchronized short-term periodicity at the level of the whole tree canopy.” Moving forward, Zelinsky and Barfod believe that more research is necessary to determine the role and mechanism of this process. They suggest that one species in particular, the magnolia tree, is the best subject for further study—it completed three full cycles of vertical movement during one night.
In the future, the team hopes to build on these discoveries and eventually to be able to effectively diagnose crop stress based on overnight movement aberrations. Barfod said that “this would open up for early intervention, which is not only cost efficient but also more environmentally friendly."