To examine the movement of trees, Zlinszky and his team used laser beams to scan birch trees simultaneously in Austria and Finland. The laser scans allowed the researchers to measure the three-dimensional movement of each tree on a centimeter scale. It also enabled them to conduct the experiment without using a light source, which could affect the results of the study by disrupting the tree’s nighttime behavior. “The experiment is the first of its kind,” said team-member Eetu Puttonen of the Finnish Geospatial Research Institute in Masala. “These studies have only been done before in small plants, but here, it was possible to do it outside in fully grown trees.”
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The team studied two birch trees, collecting 11 hourly scans of the tree in Finland and 77 scans or the tree in Austria at 10-minute intervals. Scans were conducted on a non-windy night and during the equinox to remove external variables that could influence the outcome of the study. When reviewing the results, the researchers discovered that the tree branches drop more at night than during the day, with some tree branches drooping up to 10 centimeters by the end of the night. This type of “sleep” behavior has been reported in plants, but this is the first time this effect has been observed in a whole tree.
Researchers don’t exactly know why the trees droop at night, but one theory suggests the tree may be raising its branches during the day to catch more sunlight and then resting its branches at night when it is dark. Another theory focuses on turgor pressure, which is the internal water pressure inside plant cells. During the overnight hours, the tree is no longer producing energy by photosynthesis and, as a result, loses turgor pressure in its cells. This reduction in pressure then causes the limbs to droop until photosynthesis begins again in the daylight.
The researchers plan to expand their study beyond birches to see if other species also exhibit a similar sleep pattern. The next test subjects will be chestnuts and poplars, both of which have genes associated with circadian rhythms.
