The Wind and the Willows

While it is well known that wind causes trees to sway, University of Connecticut researcher Mark Rudnicki says that the movement of trees also affects the wind. “Wind gusts have a structure that moves like a wave,” said Rudnicki. “You can see those waves moving through a field of grass, which is called honami in Japanese. In a forest, the tree motion influences that structure. The size of the gust is proportional to the height of the vegetation, so in a forest, the structure is tall and the wave size scales up.”

Previous research on how wind moves through the forest canopy treated the forest as if it were stationary, but Rudnicki proposed that researchers examine the interaction and feedback between the wind and the trees.

So in 2010 he installed tiltmeters on 150 trees that make up the canopy in the Howland Forest in Maine. Tiltmeters, which resemble old-fashioned percolator coffee pots, use air bubbles to measure the tilt of a structure, much like a carpenter’s level. Ten times per second Rudniki’s tiltmeters record the wind speed and how far the trees bend. He will collect this data for the next two years to ensure that it includes a wide variety of atmospheric conditions.

A preliminary analysis of the first year’s data found that the “tree sway frequency” increased 28 percent in the winter, which he said is because frozen trees are stiffer and vibrate quickly whereas warm trees sway slowly and deeply. However, when snow accumulates on the canopy, the wintertime sway frequency is just 10 percent greater than in summer because the weight of the snow slows the sway speed.

Two 100-foot-tall meteorological towers in Howland Forest have been equipped with three-dimensional sonic anemometers. Unlike traditional cup anemometers, which spin beneath many weathervanes and measure wind speed, the sonic anemometer transmits sound waves between opposing transducers. Because the speed of sound is constant at a given frequency, variations in the sound waves’ travel across the gap indicate the exact motion of the wind. By feeding that data and the tiltmeter data into a computer model, Rudnicki can see how the sway of the trees influences the wind.

He notes, however, that there are many factors that influence wind traveling through a forest, especially the spacing of the trees. “When you have widely spaced trees, there is more turbulence because the wind gets down between the trees and more force is being applied to each tree,” Rudnicki said. “But if you have a tight forest canopy, the wind does not penetrate as much, so there is less turbulence.”

He said that his data could help the forest industry reduce the risk of tree failures by identifying how much a given forest should be thinned.

 
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