Tree Crown Reduction Decreases Mass Damping of Wind-Induced Oscillations and Wind Energy Dissipation

Article by David S. Restrepo

Inspired by Tree Myth #98 "Pruning decreases wind load on trees", from Christophe Drénou's latest book: Beyond Tree Myths

L’arbre. Au-delà des idées reçues, Idée reçue #98 « La taille diminue la prise au vent des arbres », Drénou, 2016

Arborists often cut back tree branches and reiterates presuming to reduce wind load, but the effect is the opposite.

Wind load on the tree is subject to the tree's capacity to withstand the load. For many years, Mattheck's analogy of the tree to a sailing boat was left without further research. It assumed that the tree crown behaved in a similar way to a sail with the root system acting as the hull but, relatively recent research has proven this to be false. In particular Ken James' research (Kenneth Ronald James) .

Mattheck's literature fosters extensively the idea that the lesser the sail area, the better the tree can stand the wind. Thus, his omnipresent drawings showing cut back branches, with very little foliage. Mattheck's notoriety has contributed massively to this myth.

(Image: Claus Mattheck, Guide pratique de la methode V.T.A. - Analyse visuelle de l'arbre (Mattheck et Breloer, 1994)

It’s enough to patiently stare at a tree on a windy day and observe how branches sway back and forth independently. Similar to when we momentarily lose our balance we begin to sway our arms back and forth independently.

Studies demonstrate that foliage, twigs and the thin flexible tips of branches dampen oscillations and dissipate wind energy for more than 40 %, while the stem contributes to less than 10 %. (Christophe Drénou, 2016 - Beyond Tree Myths - (L'arbre - au delà des idées reçues)

In the following video, Frank Rinn explains why the stiff parts of branches have a higher wind load than the small flexible parts. 

"Frank Rinn Risk mitigation: Bananas, carrots and tree biomechanics. How to understand trees from their body language and measure"

Excerpt from video starting at 00:26:33:

F. Rinn: "The real wind load that is coming down to the stem base of the tree is approximately 1/10th of that which is usually assumed, because of the damping and the dynamic movement of the crown […] We pruned trees by about 10% and the wind load was increased by 10%. […] When you take away the small flexible parts of the branches, only the stiff (one is there) [parts remains]. The stiff branches have a higher wind load because they cannot move in a dynamic way. So a light reduction of the crown will very likely increase the wind load of the tree."

During the lecture a participant mentions Mattheck's analogy of the tree as a sail and Frank says: "It's TOTALLY wrong!"

He also emphasizes on the stiffness generated by bracing and the increase on wind load as a result. 

Excerpt from video starting at 00:28:48:

F. Rinn: "We measured the same trees with and without dynamic crown cabling and with the crown cabling the wind load was always bigger than without, because it stiffens the crown. So the maximum wind-load at right at the base was always higher with this things in the tree."

It's important to leave dynamic cables loose. They're there to hold the load in case of failure but, they stiffen the crown and increase wind load.

To go further into the question of crown reduction... arborist often face the dilemma of weight reduction.

Here's a lecture by Greg Moore during a Tree Defects Workshop for VTIO (2014). He points out the lack of research concerning weight reduction.

Excerpts from video starting at 00:31:39 :

G. Moore: "Weight reduction aims to reduce excessive weight or load at the end of a long branch. It comes from the application of the simple lever model [...] The lever model is very powerful and very persuasive. Every single one of you in this room remembers the lever model from junior science, it's familiar to you."

G. Moore: "Every engineer loves the idea of something as simple as the lever model: That's a big tree! That's got a big branch! There's a weight on the end! That CAN'T be good! You better do something about it... What will you do? - We'll reduce the weight!"

G. Moore: "So there's an appeal to that, but lets have a look at it: Is it consistent with the basic principles of modern arboriculture? - No, it's not!... Does it stress mature trees? - Yes it does. Is there any proof that it actually achieves it's objectives?... Now, what's its objective? - To stop the branch falling off."

G. Moore: "[…] There's no evidence what so ever! [...] Weight reduction should only be practiced when you don't have any other option and in some instances […] you probably don't have any other option, because your insurers and your managers want some demonstration that you have done "something", "anything". So that if something goes wrong they can then go back and say: - Ha! But, look what we did! and you don't want (someone like) me come along and say: Yes, you did do that but, by doing that you took away a whole lot of the mass that was damping and that other branch fell and that's the one that demolished the house or killed someone. So you've got to think this through... I know that weight reduction is going to be done. I know it's going to be done when there are big branches over foot paths. I've done it my self... or over houses. But I'm not at all convinced that it's actually efficacious, in other words that it achieves the outcome. Something for you to think about it."

"DISADVANTAGES OF WEIGHT REDUCTION by Greg Moore, 2014:

• Creates significant wounds

• Interferes with carbon cycle

• Interferes with water cycle

• Effects capacity for mass damping

• Alters loads and affects physical stress optimization within canopies"

G. Moore: "What all this tells you is, pruning as professionals it's not just about having a chainsaw and chopping a bit here and there… Is it? It's much more than that and in some instances some of you are going to need to call on to other experts, people who actually do have an understanding of some of this physics and some of this biochemistry and chemistry to give you some good advice and once you've got that advice you can then carry on your particular tasks."

"WEIGHT REDUCTION by Greg Moore, 2014

• Weight reduction aims to reduce excessive weight or load at the end of a long branch

• It is a simple lever model

• There may be a lack of branch taper and a low foliage to branch ratio

• Does not appear consistent with the basic principles of modern pruning practices

• Stresses mature trees

• No proof that it achieves its objectives

• Is an intrusive practice that should be discourages under most circumstances

• Should only be considered when there has been significant interference with a mature specimen"

Some of you may off course believe that the lack of research that Moore talks about is fake news and you may point out to Gilman's research, where his research exposed trees to four twin turbocharged diesel engines:

(From Ed. Gilman's Youtube channel)

Tree not pruned blowing in the wind:

vs

Reduced tree blowing in the wind:

Gilman concludes in his video :

E. Gilman: "Notice how much less the main trunk is moving left to right in 75 to 90 mph wind compared to the tree that was not pruned shown in the accompanying video."

That increased sway and flexibility on the non pruned tree is exactly what the tree is supposed to do in order to dampen oscillations and dissipate wind energy in an optimal way.

Does this kind of research suffice the professional community to conclude that arborists can do better than nature's design by cutting back branches and reiterates? 

While you think about it, consider the fact that even though the non pruned tree swayed more, it did not fail during the test.

Persuaded to reduce wind load, arborists must understand and recognize tree response before pruning. Trees do not form reaction wood unless they are mechanically stimulated. Reaction wood growth is indeed induced by mechanical stimuli. Mechanical triggers stimulate cambial activity modifying wood quality. (Fournier et al., 2015)

There are many factors that act as mechanical stimuli to tree response but, wind-induced thigmomorphogenesis certainly plays a major roll. A phenomenon easily appreciated in flagged trees whose shapes are molded by intense dominant winds.

"Thigmomorphogenesis tends to reduce the strain and stress the stem experiences, making it less likely to fail under mechanical loading." (Badel et al., 2015)

Plant response to mechanically induced flexing, whether by wind, ice, snow or animals was defined as thigmomorphogenesis by Jaffe in 1973. (Badel et al., 2015)

Arborists may get intimidated and impressed by sophisticated mathematical equations derived from archaic theories of optimal design and constant safety, originated by Metzger in the XIX century and resurrected by Mattheck in the early 90s but, these mathematical equations rarely consider tree response. Recent research in biomechanics demonstrate that trees do not form reaction wood unless they are mechanically stimulated. (Fournier et al., 2015)

The influence of dominant and regular winds on tree response must be considered. As the flexible dynamic parts of the tree dampen oscillations and dissipate wind energy to a much larger scale than the stiff parts, they trigger the tree to respond to the stimuli by growing reaction wood tissue in order to consolidate its structure. So, even though wind events of exceptional magnitude may still uproot trees, dominant and regular winds reinforce their structure making them more solid and stable in order to withstand stronger winds. (Drénou, 2009, Fournier et al., 2015)

In no way does this mean that trees may not fail. Periodical aerial inspections must be performed but, reaction wood as a response to wind-induced thigmomorphogenesis has to be considered before making major decisions. Also theories that lack research concerning tree development must be put aside. Like the omnipresent Harris-Gilman school that insists on giving apical dominance to all trees, not having a clue about the different types of fork. Main forks are mechanically solid and stable and each fork element explores its own space, on its side, without competing with each other.

For further reading on tree development related to pruning you may consult "Tree Architecture vs Tree Structure, a Practical Approach Related to Pruning by David S. Restrepo" and "Main fork vs accidental fork explained (Drénou) and the so called “structural pruning” and all forks regarded as defects (Harris) by Restrepo".

Despite the fact that we have made giant steps over the last 40 years in tree pruning, starting with the publication of the CODIT by Shigo and Marxx in 1977, it's imperative for us as arborist to establish the foundation of a sustainable future. Where systematic pruning will henceforth become an archaic practice.

There are many other things that we can do for trees as arborists but, we have to come to envision an avant-guard sustainable tree heritage management, where:

"We have do the MAXIMUM to do NOTHING, nothing that will interfere, alter or modify the tree and its environment"

- Gerard Passola (on the management of remarkable trees) 

It may sound simple but, it's something extremely complex to achieve on urban trees. This is an arboricultural principle that may be applied in the management of remarkable trees but, we can nonetheless contemplate applying it to urban trees in a no so distant future. Keeping in mind that in arboriculture:

"It's often wiser to do nothing than to intervine. The disparity between actions to avoid, or even to proscribe, and those to recommend is immense."

- Christophe Drénou (Beyond Tree Myths, 2016 - L'arbre. Au delà des idées reçus)

Instead of pruning trees systematically, we can move on to periodical aerial inspections and intervene if there are legitimate structural defects that may compromise the tree's longevity, people's safety or the integrity of urban infrastructure. But Sooner or later we have to come to realize that systematic tree pruning is nothing but the result of our excessive anthropomorphism. Our hair grows and we cut it. In a similar way, tree branches grow and we cut them. Except that there is a colossal difference. While our hair grows at its base, trees grow in their extremities and that... TOTALLY changes all the rules!

References:

• Badel et al., 2015. Acclimation of mechanical and hydraulic functions in trees - impact of the thigmomorphogenetic process

• Drénou, Christophe, 2016. L'arbre. Au delà des idées reçus

• Drénou, Christophe, 2009. Face aux arbres

• Fournier et al., 2015. Pas de vent, pas de bois - L’apport de la biomécanique des arbres pour comprendre la croissance puis la vulnérabilité aux vents forts des peuplements forestiers 

David S. Restrepo, Montreal, QC, 2018.07.03

v.3.2.2019.06.23

Title updated on 2022.07.08 "Tree Crown Reduction Decreases Mass Damping of Wind-Induced Oscillations and Wind Energy Dissipation". Original title: Tree Crown Reduction = Wind Load Increase.