Scientists use laser
scanners to study sleeping trees
Vienna University of Technology
Most living organisms adapt their behavior to the rhythm of day and night. Plants are no exception: flowers open in the morning, some tree leaves close during the night.
Researchers have been studying the
day and night cycle in plants for a long time: Linnaeus observed that flowers
in a dark cellar continued to open and close, and Darwin recorded the overnight
movement of plant leaves and stalks and called it "sleep".
But even to this day, such studies
have only been done with small plants grown in pots, and nobody knew whether
trees sleep as well. Now, a team of researchers from Austria, Finland and
Hungary measured the sleep movement of fully grown trees using a time series of
laser scanning point clouds consisting of millions of points each.
Trees droop their branches at night
"Our results show that the
whole tree droops during night which can be seen as position change in leaves
and branches", says Eetu Puttonen (Finnish Geospatial Research Institute),
"The changes are not too large, only up to 10 cm for trees with a height
of about 5 meters, but they were systematic and well within the accuracy of our
instruments."
To rule out effects of weather and
location, the experiment was done twice with two different trees.
The first tree was surveyed in
Finland and the other in Austria.
Both tests were done close to solar
equinox, under calm conditions with no wind or condensation. The leaves and
branches were shown to droop gradually, with the lowest position reached a
couple of hours before sunrise.
In the morning, the trees returned
to their original position within a few hours. It is not yet clear whether they
were "woken up" by the sun or by their own internal rhythm.
"On molecular level, the
scientific field of chronobiology is well developed, and especially the genetic
background of the daily periodicity of plants has been studied
extensively", explains András Zlinszky (Centre for Ecological Research,
Hungarian Academy of Sciences).
"Plant movement is always
closely connected with the water balance of individual cells, which is affected
by the availability of light through photosynthesis. But changes in the shape
of the plant are difficult to document even for small herbs as classical
photography uses visible light that interferes with the sleep movement."
With a laser scanner, plant
disturbance is minimal. The scanners use infrared light, which is reflected by
the leaves. Individual points on a plant are only illuminated for fractions of
a second. With this laser scanning technique, a full-sized tree can be
automatically mapped within minutes with sub-centimeter resolution.
"We believe that laser scanning
point clouds will allow us to develop a deeper understanding ofplant sleep
patterns and to extend our measurement scope from individual plants to larger
areas, like orchards or forest plots," says Norbert Pfeifer (TU Wien).
"The next step will be
collecting tree point clouds repeatedly and comparing the results to water use
measurements during day and night", says Eetu Puttonen. "This will
give us a better understanding of the trees' daily tree water use and their
influence on the local or regional climate."
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This study was published in an open
access article in the journal Frontiers in Plant Science: Puttonen, E.,
Briese, C., Mandlburger, G., Wieser, M., Pfennigbauer, M., Zlinszky, A.,
Pfeifer N. (2016). "Quantification of Overnight Movement of Birch (Betula
pendula) Branches and Foliage with Short Interval Terrestrial Laser
Scanning". Frontiers in Plant Science, 7:222. doi:
10.3389/fpls.2016.00222