Wind damage to trees shows up as either root ball rotation, structural collapse due to harmonic resonance – Huh?!, or unchecked horizontal reaching.
Trees that grow from seedlings produce root and fiber structure in response to actual site conditions. Trees raised in nurseries and planted on a site are more susceptible to root ball rotation than trees grown from seed on site. The larger the planted tree the more dramatic the effect and the longer it takes to overcome it. The smaller the tree planted on a site, the better chance it will have of surviving extreme wind events throughout the course of its life.
Trees with healthy tissue don’t usually break simply because a strong wind pushes on them. Breakage usually occurs as a result of a perfect confluence of a handful of variables: Gust strength, frequency and duration, and the spring resistance provided by the tree structure. Picture a Pine stalk silhouetted against the sky. A gust will set it in motion, like an initial push on a kid in a swing.
The first push on a kid in a swing will produce one range of motion. Another push of the same force applied when the kid returns will expand the range of motion. So a constant force applied to the swing at just the right frequency and duration will produce a broader range of motion – that is called harmonic resonance. Harmonic resonance is what snaps the trunks and branches of trees in a windstorm.
If you’ve experienced the collapse of healthy pine tree trunks due to wind, you’ve experienced an extremely unlikely event, especially if the trees grew from seed on the site and the surrounding canopy has not been substantially modified recently. Your trees are probably as wind resistant as they possibly can be.
In General, the idea of thinning a crown to let the wind pass through comes from a misunderstanding of a technology that is millions of years old. The volume of a tree canopy develops in order to dampen harmonic resonance. Thinning it will often make a tree more susceptible to catastrophic collapse, not less.
On the other hand, trees other than conifers that grow in the open will reach horizontally until they collapse under their own weight, especially the lower canopy. Horizontally splayed trees are more susceptible to wind damage. A crown reduction that redistributes the volume of the crown closer to the trees center of gravity, without stripping out the interior, can produce aerodynamic symmetry and balance, which increases a trees resistance to extreme weather events. That means snow loads as well as wind.
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I didn’t know that deciduous trees in the open would respond to thinning and a crown reduction.
Indeed a very nice post. I do lot of netsurfing for finding the key information on trees and forests. I just came across your blog and has subscribed with a wish that you will be posting good posts like this over the coming days. Thank You.
The piece on wind damage is what I know about wind, trees and physics from a lifetime of working and studying. Harmonic resonance is a topic from high school physics. I’ve been a Certified Arborist for 9 years. Maintaining the certification requires pursuing continuing education units every year that come from seminars and dedicated articles in ISA trade journals. You can search the ISA website for articles from industry trade journals on tree topics that you’d like to know more about.