Breeze Keeps Trees Apart

Gone with the wind. Even a moderate breeze can tear off branches from close-growing pines.

SNOWBIRD, UTAH--The wind whispering through the pines may sound romantic to human ears, but it's a headache for the trees. According to a study presented here 8 August at the Ecological Society of America's annual meeting, even a mild breeze whips the treetops around with such force that they easily collide and can break. The researchers say this may be one reason why in many tree species, the canopies do not grow close enough to touch neighbors.

The distance between tree canopies has been called "crown shyness." Scientists speculated that if the canopies touched, too little light would filter down to the lower branches. But researchers from the University of Alberta in Edmonton, Canada, thought that wind damage might be an important force keeping trees to themselves.

Mark Rudnicki and his colleagues attached tiltmeters to 10 trees in a natural cluster of close-growing lodgepole pines (Pinus contorta latifolia) in Alberta. These devices recorded how the 15-meter-tall trees swayed whenever the wind speed averaged 5 meters per second for more than 3 minutes. Then the scientists plotted the treetops' trajectories as they were buffeted by the wind, an experiment never done for a whole set of trees.

The treetops, they found, moved as much as 5 meters from side to side. "These trees are really moving--and they're moving fast," Rudnicki says. For some pairs of neighboring trees the crowns' paths sometimes overlapped as much as 24%. The team watched trees collide up to 70 times a minute, often tearing off branch tips. Such damage probably prevents neighboring trees from growing too close for comfort, Rudnicki argues.

The finding is "a real advance," says Charles Canham, a forest ecologist at the Institute for Ecosystem Studies in Millbrook, New York, who has long been puzzled by crown shyness. Tracking 10 trees at a time is a technical feat too, he says. "I was impressed with the sophistication of the study."

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