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Wind Loading in Trees

on Saturday, 23 March 2013.

By Mark Mortensen, ISA Certified Arborist, RM-1002

Two other common requests are to thin a tree out excessively to minimizing the likelihood of broken limbs from high winds, or to raise the canopy on spruce or pine trees.

Again, like the snow loading in trees, if this request is fulfilled and the tree is excessively thinned or raised the tree may actually become more vulnerable to storm damage. Let me explain why a denser tree can better survive and "weather the storm" without storm damage, limb failure or being "wind-thrown."

Sometimes in an urban environment it is necessary to raise the canopy of trees. Cities in the Denver metro area have codes for what is considered the "public right of way" and minimum clearances are enforced. They vary a little bit from city to city but the basic criteria is to allow both vehicular and pedestrian traffic to use the streets and sidewalks unimpeded and allow adequate vision for intersections, signage and traffic signals. Also we must realize on private property we have to live among our trees be able to walk around and protect our homes as well. There is a lot of credence in the popular suggestion of planting the right tree in the right place.

Those issues aside let's address the request, "I would like to raise the skirt on this spruce. I would like to see a little trunk." If the lower branches are not actually interfering with anything the tree is actually benefitting from the lower branches and for several reasons. They are actually there by design to help the tree survive. It is best to preserve the lower branches.

To stay on track with the wind loading aspect, the lower branches will deflect the wind around the tree. If they are removed, then the wind can get in under tree resulting in tree failure. A good analogy would be like walking in high wind with an umbrella. Most spruce tree branches are strong enough to not actually invert and fold up but actually take the tree right out of the ground and heave over. This is what is called a wind thrown tree. I have seen catastrophic results from wind thrown trees.

In regards to over thinning to prevent wind damage or limbs breaking I will never forget the speaker at one of our ISA conferences in Denver, when he was addressing this very subject. He asked the question, "How many of you thin the trees to prevent wind damage?" The majority of those in attendance raised their hands. He went on to explain the fallacy of this approach and why a denser tree is better able to resist limb failure from high winds.

For purposes of illustration, let's say we have a 50 MPH wind gust. As we look at typical limb failures, the damage is on the outside of the tree and the effected branch breaks usually from the top side of its connection. It is very rare that the branch on the windward side breaks.

Think about a high wind hitting a tree, a more densely tree will deflect the wind around the tree and not incur damage. Now think about a garden hose nozzle constricting water forces and pressure. Overly thinned trees branches will act like that hose nozzle. A wind hitting the tree at 50 MPH on one side may actually be coming out the other side at 150 MPH greatly increasing the chances of branch failure.