The Shaken Trees

Environmental horticulturists have long been stumped by the strange behavior of trees in groups. Why do trees on the perimeter of a forest have stocky, tapering trunks, while those in the interior are tall and slender and are easily toppled by wind after the tough outer trees have been felled either by nature or man? P. Landreth Neel and Richard Harris, environmental horticulturists working at the University of California at Davis, have come up with the most convincing answer so far. Their theory: perimeter trees that are fully exposed to the wind and are shaken by it--or any tree shaken by any means--will be stronger but shorter at maturity.

Writing in the journal Science, Neel and Harris explain the simple procedure that they used to test their hunch. At a local nursery, they bought eight matching pairs of young sweetgum trees. They potted the sweet gums in four-gallon cans in their greenhouse and stopped in every morning to give one member of each pair a brisk 30-second shaking. After 27 days of this routine, the shaken trees had grown only one-fifth as much as those left in peace, had put out fewer lateral branches and developed stouter, tougher trunks. Trees, conclude the authors, have evolved a supersensitive response to shaking, whether by a breeze or by hand. Thus, in any forest or glade, nature has ensured that the relatively delicate, fast-growing trees of the interior are guarded by a sturdy defense against the winds that buffet the forest fringe.

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