The Moon Through The Trees

The diameter of tree seedlings may fluctuate with the tides, according to a paper in tomorrow's Nature. The changes are barely perceptible--only a few hundredths of a millimeter--but scientists say some influence of the moon might be moving water from living parts of cells to the cell walls and back again. If verified in big trees, the phenomenon could influence how certain high-grade wood is harvested.

This isn't the first time the moon has been found to influence plants. A wide variety of plants grow better if planted just before a full moon, says tree biologist and anatomist Ernst Zürcher of the Swiss Federal Institute of Technology in Zurich. But while measuring the growth of young trees under controlled temperature, humidity, and continuous darkness, Zürcher and his colleagues noticed a peculiar 25-hour cycle in the diameter of stems. The shape of the graph--with two peaks each cycle--reminded them of the regular water fluctuations in bore holes and the flow of springs, which are paced like the tides, but much more subtle.

When they compared their data on tree stem diameters with the local gravimetric curves--which mark the strength of gravity's pull on Earth due to the positions of the moon and sun--they found that the curves seemed to be synchronized. When the moon's pull was weakest, the trees were 5 to 10 micrometers wider than when it was at its strongest. The effect, which does not depend on the stem's orientation, was present even in stem sections cut off from the rest of the tree, as long as the cells remained alive. Zürcher says he suspects changes in the moon's pull may somehow drive the water from the cytoplasm into the cell walls and then back again, causing the subtle swelling and shrinking.

The location of the water could have an important effect on the drying and quality of harvested wood, Zürcher says. Indeed, he claims that loggers have traditionally paid attention to the moon, and some violin makers "consider special moon rhythms" when choosing wood. However, there is no known physiological mechanism that would be this responsive to the subtle tugging of the moon's gravity, says biomechanics expert Karl Niklas of Cornell University in Ithaca, New York. "It looks like there is a real phenomenon that is awaiting a biological explanation," he says. "But it needs to be replicated in a controlled situation."

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