"It was a very clear effect and applied to the whole tree," Andras Zlinsky of Hungary's Centre for Ecological Research told New Scientist this week. "No one has observed this effect before at the scale of whole trees, and I was surprised by the extent of the changes."
At the center of the research were two birches, one in Finland and one in Austria. Their canopies were scanned with lasers from sunset to sunrise at regular intervals, each over the course of a single night. The conditions in each location were the same: no rain, little wind and during the solar equinox so that the length of darkness was relatively equal.
"The results obtained from the measurements showed that the crown movement in the Finnish and the branch movement in the Austrian study case presented similar temporal response," the scientists concluded in their study, which was published in Frontiers in Plant Science. "Close to sunrise, the branches were hanging lower than at the time of sunset. ... The movements were observed to happen systematically over a time span of several hours, which ruled out occasional wind effects."
The team offered two hypotheses for their findings: The drooping could be due to a loss of internal water pressure, called turgor pressure, causing the branches and leaf stems to lose their rigidity. Because turgor pressure results from photosynthesis — the use of sunlight to create sugar from carbon dioxide and water, which only happens during the day — the trees would appear to relax with the darkness.
The other possible explanation is that trees really are resting, following the same circadian rhythms of other flora and fauna. In the daylight, leaves and branches are angled upwards to catch more sun, but during the night this high-energy positioning is no longer necessary.
Circadian rhythms in plants have long been studied in the lab, but that's relatively difficult to do when it comes to trees. This makes the use of laser scanners a potentially valuable new tool for research on trees, and understanding the processes and patterns of water use in trees could prove important as scientists study the impact of climate change.
Whether the birches' branches droop because of a loss of water pressure or a sleep-wake cycle "remains to be decided," Zlinsky said. But he and his colleagues have a cautious suggestion.
"The fact that some branches started returning to their daytime position already before sunrise," they wrote, suggests that Betula pendula is "governed by the internal circadian clock of the plant."