The next time you are out on your evening stroll, be mindful of your sleeping neighbors — the trees. That’s right, the hundreds of thousands of towering ancient plant species that occupy 31% of Earth’s land surface follow a circadian rhythm, similar to humans, animals, and small plants. Such findings were discovered by a curious team of scientists from Austria, Hungary, and Finland who wanted to know if trees followed a day/night cycle.
In order to accomplish this study, researchers assembled laser scanners and focused on two silver birch trees, one in Finland and the other in Austria, at night to monitor any physical changes. They did so during a calm and dry night in September, close to the solar equinox when daylight and nighttime are just about equal, allowing for the most accurate information. Additionally, the scanners used infrared light to enhance individual sections of the trees, providing supplemental detail of the trees within a matter of minutes.
Among their discoveries, they noticed that the tips of the branches drooped upwards of 4 inches towards the end of the night. Eetu Puttonen from the Finnish Geospatial Research Institute remarked “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.” The drooping behavior shown by the trees is believed to be caused by a decrease in the tree’s internal water pressure, known as turgor pressure. With the lack of sunlight, trees are unable to go through photosynthesis and therefore must save energy by relaxing their branches.
The research showed that the branches were at their lowest point a few hours before sunrise. Returning to their normal position angled towards the sun in the early morning hours, it is firmly believed that these trees function on their own internal circadian rhythm. This type of study has never been done before on entire trees, making this discovery a fascinating one.