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TREES, PLEASE: Chaotic Trees Set Sail

Canary island pines are frequently over-thinnned.

By Tim Clancy.

I have worked for a few tree "care" companies. One thing I noticed is that as fall and winter approach, the business of needlessly pruning trees picks up. Presumably the reason for this is to make the trees safer for the inevitable storms that bring strong winds. The theory goes something like this: My trees are very full and it looks like wind will have a difficult time blowing through them. It's almost like a ship's sail. So I better call a tree "care" company and have the trees thinned out so the wind can blow through and not break any branches or the whole tree.

This line of thinking used to be the norm for those of us interested in preserving trees through proper care. It also makes sense to us when we compare the situation to a ship's sail. For many years, thinning was thought to be a positive procedure for tree pruning. This of course led to very much work for tree "care" contractors. Many of whom did quite well.

Luckily, some people pondered conventional wisdom and did some research. It turns out that leaves and branches, besides being vital to the tree to manufacture food, also serve another purpose. They dampen winds and help dissipate the energy from the wind.

When you think about it, it makes sense. Why would a tree evolve over millions of years only to partially or completely fail due to a wind event? It just doesn't make sense. So what is happening when a tree is under a load from wind? The wind acts upon the leaves. The leaves move in a chaotic fashion as they absorb energy from the wind. This energy is then transferred to the branch to which the leaf is attached. From the branch the energy is transferred to the trunk and then the ground. The next time it's windy, step outside and look at the leaf movement on a tree. Take a few minutes and really watch what is happening. You will see that chaos ensues. This is a beautiful design.

This magnolia tree has not been thinned.

The leaves and branches do not move in unison, but in a random manner. This creates a mass damping effect and helps the tree survive the load. This concept is so impressive that the mechanics of how trees employ this technique are being studied by various disciplines for possible application in the design of man-made structures. The whole process is very complex and not fully understood. What is understood is that thinning to minimize the "sail" effect is not a good strategy.

Routine thinning was and still is performed on many trees. The evidence against its efficacy continues to mount and routine thinning is slowly (too slowly) going out of fashion. Remember pruning (unless it's for aesthetics) is a treatment and should be prescribed once a valid diagnosis is made. As with any treatment, dosage is important. The less the better.

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