GardenDrumWhy removing lower tree branches creates instability

GardenDrum Angus Stewart Angophora1

Did you know that pruning off the lower branches on trees can cause them to blow over when semi-mature? Commercial tree growers who underprune tree canopies to create lollypop-shaped trees now know that it’s not just straight wind load that can cause these trees to topple.

By measuring the tiny, imperceptible movements of a mature tree’s main trunk when subjected to load, such as people pulling on attached guy ropes, experts such as Dr. Ken James at the University of Melbourne and Rowan Reid at Bambra Agroforestry Farm have found a distinct difference in the movements of trees with, and without, substantial lower branches.

In an excellent blog post on Agroforestry.org.au, (click and scroll down – or read his other very interesting posts too), Reid likens it to the way a building engineer uses large water tanks on the top of a tall building to dampen the effect of building sway in high winds. The weight of the water provides an inertia that stops the building from moving too far in the direction of the wind. By the time the water finally starts to move, the building is already swaying back, so now the movement of the water dampens movement in the opposite direction.

Similarly, any large branches low down on a tall tree sway independently of the main trunk and dampen the effect of wind on the main trunk. If they have been removed by pruning, then the main trunk is subject to an unmitigated wind load that can blow over the whole tree when it is semi-mature. Reid notes that he has fewer tree losses where the trees are old enough to have developed large branches above his pruning height.

So the next time a local tree lopper says you should crown-lift your 2-3 year old tree, think about its exposure to wind.

Click here to listen to a podcast of Dr. Ken James discussing these intricacies of tree biomechanics – Statics v Dynamics

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