Over Pruning and Lateral Spread

Chris Soiset

Live Oaks are often over-pruned, or "lion-tailed" - in Dallas, more often than not. The practice here is so common, that people generally believe that Live Oaks are supposed to have completely vacant interiors, with a very thin, hemispherical "shell" of leaves, like an umbrella with several bent handles. I regularly have tree "services" knock on my door, sure that I'd love a great deal on the pruning my big Texas Live Oak obviously desperately needs.

Often, around here, trees are "mowed" more than they are pruned. The harsh overpruning results in an abundance of epicormic sprouts within two years of pruning, which the owner takes as a sign that the tree needs "pruning" again. I do my best to educate within my neighborhood, but some folks just can't hear it.

My question relates to the response of trees to this kind of overpruning. I've read over and over, in articles by tree "experts" that proper pruning - leaving laterals in place - causes wind and gravity loads to be more evenly distributed along the branch, instead of concentrating the loading at the end, and results in a branch that is less likely to break.

This makes no sense at all, but the claim is extremely common. If you take branch "x" and leave all of the growth on it, and expose it to a 40 mph wind or a 12" snow, it is more likely to break than if you lion tail it, and expose it to the same loads. That is absolutely the case in the short term. It's not as if there is only so much wind or snow load to go around, and if you cut off the intermediate branches, it will all crowd up at the end of the branch. Those are uniform loads, in units of pressure (eg psf in the states, or N/m^2 for all you other weirdos). Reducing the area for those loads to act on reduces the load on the branch, and the net torque or "moment" at the potential breaking point is reduced.

I figure the truth is that over years, perhaps decades, a tree will respond to lion tailing, or "broccoli-topping" by growing out, laterally and vertically, at an unnaturally high rate, and the branches will be thinner, with weaker connections, than they would otherwise have been at that extent of spread, had the tree not been "gasping" for sunlight, and "trying" to spread, as a result of the overpruning. Is this the phenomenon that the writers of the aforementioned articles are attempting to describe?

ETA: I suppose also that leaving the laterals in place results, eventually, in the branch itself being thicker, to accommodate the increased area of vascular tissue required to conduct nutrients back and forth to a greater leafy surface area. Again, though, this does not address the claim that the load being spread more evenly contributes to the structural integrity of the branch.

Eric Frei

The concern with lions tailing is more focused on lever arm length.

Secondary branching in the interior also helps in thickening of the primary branch, a thicker branch is stronger than a slender one.

If you have climbed and cut down a tree you also would know of the dampening affect of branches closer to the trunk. Traditionally when cutting a tree down the climber works his way up, cutting branches from the bottom slowly working his way to the top. When finally at the top with no branches beneath everyone knows how "wobbly" the stem is, to the point that lowering out the top can give quite a whip as demonstrated in this classic video.

http://www.palmtreeservices.com.au/video/yeeha.wmv

Do an experiment with a sapling with some branching. Pull the top over and let it go, watch the whip. Then slowly strip off lower branches and do the same .... you'll see it whip more.

Read the PDF's etc in this post and thread.

How hollow - pull test - stability tests - Wessolly - Brudi - Sterken - Coder

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Chris Soiset

In removal, we have typically removed all branches up the to the extreme end, then we remove the last bunch of growth on the extreme end, and that is when we experience the "bucking horse" if the ground man is not skilled. But branches removed before the extreme end do not experience this effect, because as long as the weight still exists at the extreme, the polar moment of inertia remains relatively high. The damping effect of lower branches is much less, being closer to the axis of rotation.

But, I've definitely experienced the damping effect of remaining branches on trees I've removed, and I can see that as part of the explanation, but only under the extremes of dynamic loading, for very short term loads. With wind loads, in engineering practice, a short term "gust" is considered a 3-second load, and these are in the neighborhood of 30% higher than sustained wind loads. A 3-second load is MUCH longer in duration than than the kind of shock loading experienced during removals, such as when a ground man isn't very good at letting the rope run through the brake.

Snow loads, of course, can last for days. Additionally, there is the fact that wood is much stronger under short term loads. The National Design Specification, used for structural wood design in the US, allows a "Duration Factor" of 2.0 for "impact" loads, and of 1.6 for "Wind/Earthquake" loads. These factors are applied to allowable stress values. So a branch will survive a load that jerks or whips it around, but apply that load for a longer duration, and it will break.

Handy links, though, and I'm reading them now. I'll respond here about how they do or do not directly address my original question.

Chris Soiset

I've read the pdf's, and here are my comments:

In the Moore-Maguire article, they show that trees (presumably we could apply this to individual branches, as well), without branches have higher harmonic frequencies than with. This makes perfect sense from an engineering perspective: basically, a lighter object will wiggle faster than a heavier object. This does not help the claim that a branch with more laterals will be more durable than one without, as the gust frequency that matches the harmonic frequency of one branch would simply be different than another. It hurts the claim in a way: lower frequencies will correspond to longer load durations, and thus the trees with branches in place will not benefit as much from the "shock" resistance of wood.

However, in one of the James presentations, he notes that branches can "detune" a tree (or pole). As anyone who has ever seen and understood the failure of the Tacoma Narrows bridge will understand, this "detuning" can save a structure, because it prevents additive amplitudes. However, this applies primarily to entire trees, not individual branches, because "detuning" is a result of the complexity of interactions of many forces on the single structure being analyzed. A tree will have hundreds of branches to prevent a harmonic frequency, but an individual branch will only have a few.

Also, regarding damping of shock loads, such as you mentioned occur in takedowns when the top is finally removed, look at the James example of the tuned mass dampers in tall buildings. These masses are always near the top, farthest away from the axis of rotation. That is where they will have the greatest effect. So for very short term wind loading on a tree, you could actually decrease the amplitude of sway by biasing the weight toward the end.

All this brings me back to the original question: is the relative fragility of lion-tailed branches actually due to the growth pattern induced by that kind of pruning, and therefore an effect that takes years, perhaps decades, to materialize? I think so, and I think the explanation, as so often stated, that intermediate branches allow a more even load distribution, is bunk. So what you said here, "Secondary branching in the interior also helps in thickening of the primary branch, a thicker branch is stronger than a slender one" makes sense, but not really the rest.

Eric Frei

In this post is a PDF.

Altering climate affecting your pruning?

We do not have any snow or ice loading around these parts. Further north we get cyclones (hurricanes) and yes down south there is snow etc.

I think is fair to say what you have, that immediately there is no issues however in the longer term due to the new growth being on the extremities the issue gets worse (over extended, end weighted branches).

We see failures at the branch union to the trunk or nearby. All of the force and weight taken at that point. Sometimes reduction pruning of long laterals is impossible as there is no inner branches to reduce to, sometimes the reduction might put the branch into shade of the remaining upper canopy ... the branch could die. Sudden Limb Failure or Summer Limb Failure (SLD) often also display symptoms of over extended branches, simply too long with VTA signs of subsidence. I know on some I have seen over the years (eucs) I have scratched my head and wondered when they will fail, and many have as predicted, some in storms and some not. This phenomena brought on discussion about the so called self optimisation .... do trees grow until they bust?

I do think though that it could be measured ..... a branch prior to lions tailing and after on differences, and for wind loading at least. Also species might make a difference.

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