Ask a local master gardener: Made to bend in the wind

  • young tree bends in the wind tam_odin

For the Gazette
Published: 3/8/2019 11:56:24 AM

Q: I have a house on a wooded lot and was wondering what enabled the trees to bend and sway in the wind without breaking in last week’s wind storm?  —D.T. Florence

 

A: Watching the towering pines sway mightily outside my office window last week, I, too, was admiring — and very thankful for — their rubberband-like abilities. To help me answer this question, I reached out to Brian Kane, an arborist and arboriculture professor in the department of environmental conservation at the University of Massachusetts Amherst. The below answer is from his helpful response:

The short answer is that, roughly, tree stems and branches can be thought of as beams, analogous to the beams used in the construction of buildings. How much a beam bends (the technical term is “deflects”) depends on the loads to which it is exposed and its geometry — cross-sectional shape and size, and its material properties.

Greater bending and swaying will occur when loads are greater. Wind loads increase with the area exposed to the wind and, more importantly, with the wind speed. Larger diameter stems and branches don’t bend as much because their larger diameter reduces their flexibility. Think of how much harder it is to bend a 2-inch diameter branch compared to a ½-inch diameter branch. Also, live branches and stems tend to have much more elastic wood than typical building materials like lumber, steel and concrete.

It is important to consider that trees generally grow in accordance with the loads they normally experience. Trees will not grow too tall if they are regularly exposed to wind or snow or ice. Usually, winds of 40 or even 50 mph do not cause failure of structurally-sound trees; only when winds start to exceed 50 or 60 mph do we start to see the wide-scale failure of structurally-sound trees.

Most of the trees that fail at moderate wind speeds (up to 40 or 50 mph) are either structurally deficient — having decayed wood or a weak branch union, for example — or are exposed to loads not previously experienced, such as trees that had been growing in a stand (and therefore sheltered somewhat from wind) and are newly exposed to winds after a lot was cleared to build a house.

Coincidentally, I looked at just such a tree for a friend after the wind event. Three large trees had been removed near their house before they bought it; doing so exposed previously sheltered trees to the full force of the wind. The tree that failed had roughly 80 percent of the wood in its cross-section decayed, and so, exposed to new loads and without the necessary wood strength, it failed.

Specifically, regarding swaying, trees have evolved to manage wind loads, not in the same way that engineered structures do. Instead of standing mostly firm against the wind, like a bridge or skyscraper, trees bend and sway. Doing so converts the kinetic energy of the moving wind into elastic potential energy of the wood fibers as they stretch and compress while the tree sways. Bending away from the wind also makes trees more streamlined, reducing the wind load — watch videos of palm trees moving in hurricanes and you can see how much streamlining — the fancy word is “reconfiguration”— occurs.

Hope this helps answer your questions, D.T. Thank you for asking a (Local) Master Gardener! 

Have a gardening dilemma? Please send questions, along with your name/initials and community, to the Western Massachusetts Master Gardener Association at AskAMasterGardener@wmmga.org. One question will be selected and answered per week. wmmga.org


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