As trees go, the willow looks tired. Not so much weeping as it is sleeping—as if it might need a nap, like an old man slouched over, its burdens too great to continue to lift its limbs. Not so the pine, the birch, the palm, even the oak—these trees always seem to stand upright, solid, well-rested. But don’t let their posture fool you—even the birch needs to rest its branches after a long day standing in the hot sun.
This is what Eetu Puttonen from the Finnish Geospatial Research Institute in Masala, Finland, and his team of researchers recently discovered. As described in the journal Frontiers in Plant Science, Puttonen and his team used terrestrial laser scanning on a pair of silver birch trees—one in Finland and one in Austria—to measure the movements of their branches and foliage. While it has been known for a couple hundred years that plants experience daily leaf motion, this level of observation has never before been possible. Additionally, Puttonen’s study, unlike others, focused on just one day, rather than a season. What they confirmed and quantified in detail was that as the sun set, so did the trees; and as the sun began to rise, so did the trees. In everyday parlance, the trees fell asleep at night and awoke the next morning.
Do trees really need to rest? To sleep? To some degree this still needs to be determined. Puttonen explains that water balance within a tree or plant photoperiodism—a plant’s response to light or dark periods—could be the driving force behind the nightly slumber patterns. Yet there is increasing evidence, observational and genetic, that plants follow a circadian rhythm, much like animals. Circadian rhythm in plants is not simply an ...
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