Tuesday, February 23, 2016

Blowing in the Wind

"When there is a storm, and you stand in front of a tree—if you look at its branches, you swear it will fall. But if you look at the trunk, you will see its stability."

This quote from 2015's film The Revenant puts into words an impression that I've gotten more than once, walking through the woods with my eyes turned skyward. Trees are astoundingly flexible, and it seems that as they get bigger, their propensity to bend in the breeze only grows. Trees rely on the pores in their leaves to capture the carbon that gets converted into physical structure, to eliminate the oxygen they produce by this process, and even to pull nutrients hundreds of feet up their trunks, so it makes sense that they'd want to catch as much wind as possible in their leaves, but the effect is still unnerving as trunks groan and creak around you and multiple-ton structures sway drunkenly above your head in a light breeze.

Trees of all ages, sizes, and species tend to break
at about the same windspeed—42 meters per second.

The quote above could use appending, however, according to a paper recently accepted for publication in Physical Review E. The study, by some of the same scientists responsible for last year's in-depth analysis of popcorn dynamics, reveals that virtually all trees—regardless of shape or size—start to suffer serious structural damage and break at the same wind speed: 42 meters per second. Translating to more familiar imperial units, that's something like 93 miles per hour, the high end of a category I hurricane.

A number of factors appear to contribute to the uniformity of the break-point, but the most obvious is the relationship between the sturdiness of the tree's trunk and the extent of its branches. At first, you might think that a towering oak would stand up to gusts better than a slender palm, but this isn't necessarily the case—a tree's canopy acts almost like a permeable parachute. The larger that "parachute" is, the easier it is for the wind to bend and—in some cases—break it.

Another factor contributing to instability in larger trees is the presence of structural defects—knots. In any material, small imperfections often end up being the "point of failure" when that material is subjected to too much stress. From a materials science perspective, a tree trunk is a lot like a bundle of fibers, almost analogous to the braided steel cables that support suspension bridges. Disrupting this structure with small irregularities—such as knots in the grain of the wood—reduces its overall integrity. Since older, taller trees tend to have larger knots, their ability to withstand strong winds also decreases with time.

It comes as something of a surprise, then, that all these variables tend to balance out when they're added up, leading virtually all trees to break at a windspeed of 42 m/s. This tells us something about the frequency and intensity of storms in Earth's recent history, and what we can expect to see in the next hundred or so years—over which time period the frequency of extreme storms is projected to double.

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