The Application of Environmental Arboriculture to Tree Pruning in Urban Contexts

Talk by David S. Restrepo on February 12th, 2021 at ArborCon 2021, ISA Michigan, Lansing, MI and beyond...

Subtitles available in English, French and Spanish | Full transcription below...

Welcome to all of you to this prerecorded talk on environmental arboriculture applied to our tree pruning practice in urban contexts.

I would like to give a special thanks to the ISA Michigan chapter for offering me the challenge to prepare this talk. 

Preparing a talk is always a good opportunity to go beyond one’s limits, and this is no exception. I was not expecting to find myself in the position of giving a talk on environmental arboriculture, it happened as a result of a recent article that I published on LinkedIn entitled:

-A practical introduction to environmental arboriculture for arborists: ‘coronet cuts’.

This article gives a brief overview on coronet cuts, basically, why should we implement them into our pruning practice? When? and how? 

The aim of this talk is to complement this article by encouraging and persuading you of three main points:

First of all, I will attempt to encourage and persuade you that artificial fractures, better known as coronet cuts, are a legitimate type of pruning cut. Also referred to as natural fracture pruning cuts, they simulate natural fractures that inevitably occur in all species at one time or another. Coronet cuts are complementary to natural target pruning cuts, which simulate the natural target angles that occur naturally at the base of axes in natural pruning. They belong in the arborist’s pruning tool box just as much as natural target pruning cuts do and we should not look at them as a weird and bizarre technique. It's just a matter of recognizing and acknowledging legitimate circumstances where it's reasonable and appropriate to implement them, in specific situations and exceptional scenarios. 

For instance, when we have to remove dead wood, for security reasons, without affecting the living parts of the tree; or when we leave snags, standing trees, either dead or in irreversible decline, providing a vital source of food and habitat for wildlife. Also, when we are confronted with exceptional scenarios, where extreme measures have to be taken and appropriate cuts exceed the compartmentalization capacity of the species. 

Coronet cuts have already been implemented in the Australian minimal industry standards for environmental Arboriculture and they have been implemented in the UK and Sweden for the last two decades. 

Secondly, I will attempt in this talk to encourage and persuade you to tolerate, allow and promote natural fractures and dead wood in urban contexts, in situations and scenarios where they don't conflict with our management objectives, where they don't pose an imminent risk and it’s reasonable and appropriate to allow them. We as arborists are obsessed with removing dead wood and stubs from broken branches, but a tree lives in mutuality with natural fractures and dead wood, and there's a vast network that depends on it. Stubs from broken axes are exhibited in all trees in a natural environment. Trees also need a fair amount of dead wood to protect themselves. This is a natural condition that we can acknowledge and tolerate when the situation allows for it. In order to profit from the numerous benefits that trees offer to the environment and our quality of life, a reasoned and proportionate level of risk must be accepted. 

We are obsessed with cleaning trees, removing dead wood and making clean cuts every time that we are confronted with fractured stubs. It may satisfy our cultural and social expectations but there's an enormous prize that we are paying for it, that the planet is paying for it. We are basically making a huge effort to impede biodiversity. I will attempt in this talk to make you aware that indeed tree canopies are arguably the most species-rich environment on the planet.

Thirdly, I will attempt in this talk to encourage and persuade you to recognize and acknowledge that we as arborists, we have a privileged position where we can make a difference. We are the guardians and protectors of our arboreal patrimony, we play a crucial role boosting our planet’s biodiversity in urban contexts, by preserving and encouraging the establishment of dendromicrohabitats. It all starts at a micro scale, dendromicrohabitats are at the core of biodiversity. The big change starts at a micro scale, with fungi, bacteria and invertebrates. The macro scale depends on the micro scale.

We can all make a change in our urban environments by simply tolerating and allowing natural conditions in the urban environment. Natural conditions that permit the establishment of dendromicrohabitats, tolerating and allowing natural fractures and dead wood whenever it’s reasonable and appropriate; implementing coronet cuts in specific situations where the integrity of the tree must be preserved and exceptional scenarios where extreme measures have to be taken.

We will cover an overview of dendromicrohabitats and acknowledge their vital role in order to boost biodiversity in urban contexts.

We must be aware that the planet's wilderness and the biodiversity associated with it is in free fall.

It’s inexcusable! There's no other way to put it!

If we look at the latest report of the United Nations Convention on Biological Diversity, the 5th Global Biodiversity outlook report. Almost 200 countries agreed in 2010 in Aichi, Japan, to take immediate measures to be fulfilled by the year 2020, at the latest! Well, we’ve passed the dead line, and still none of the 20 targets set have been met. This report is an alarming call for immediate action. Countries worldwide have failed big time attempting to meet the imperative targets set for 2020 by the United Nations in 2010. 

The Aichi Biodiversity Targets from the strategic plan 2011-2020 can be consulted in the Convention on Biological Diversity site. 

Here we have the mission, as well as the imperative Aichi Targets for the strategic plan for biodiversity 2011–2020.

The mission was to take effective and urgent action to halt the loss of biodiversity by 2020.

Target #1 aimed that: "By [the year] 2020, at the latest, people [would become] aware of the values of biodiversity and the steps to take to conserve and use it sustainably".

This is a fundamental target, that we must reach, but we are way too far from it. There has been an apparent partial interest in protecting and preserving biodiversity in the last few years, but it's not enough, the protection and preservation of the planet's wilderness and the biodiversity associated with it concerns everyone in the planet. 

As David Attenborough, the famous nature documentarist puts it:

“It’s not about saving our planet, it’s about saving our selves. The truth is with or without us, the natural world will rebuild”. “To restore stability to our planet, we must restore its biodiversity”.

So we don't have a choice, we have to act before it's too late and the planet goes into another geological mass extinction era that will wipe us all out of the picture and restart allover again, like it has done many times in the past in its geological history. If we don’t act urgently, right now, at every individual and collective level, we will be wiped out like dinosaurs did, and it won't be a meteorite this time, but a forced mass extinction as a consequence of the impact of our unrestrained activity on earth. 

If you watched the recent shocking documentary by David Attenborough in Netflix, -A life on our planet- You are well aware of what's going on. If you haven't, you must see it!

David Attenborough gives a chilling chronological countdown of the remaining wilderness on the planet. The impact of human activity on earth has affected natural environments beyond recovery. 

When Attenborough started exploring and documenting nature, there was already an alarming loss of biodiversity.

In the 1930s, the estimated remaining wilderness on the planet was already at a devastating 66%.

By the 1950's, it kept on decreasing at a slow pace.  

Even in the 1960's, the decrease was rather steady.

However, by the end of the 1970's, it had dropped almost by half.

By the end of the 1990's the remaining wilderness on earth was at a disconcerting 46%. It’s around this time that global warming became a political issue and we began to hear about it on the news, which was later, more appropriately referred to as, climate change.

A change in direction initiated by the IPCC - The International Panel on Climate Change. The Nobel Prize on 2007 was given to the IPPC along with the US ex-vice-president Al Gore.

The prize mention declared: "for their efforts to build up and disseminate greater knowledge about man-made climate change, and to lay the foundations for the measures that are needed to counteract such change."

The scientists team quantifying the impact of urban forests in removing atmospheric carbon dioxide was led by David Nowak, along with twelve other U.S. Forest Service researchers.

Today, having failed big time the Aichi biodiversity targets set for 2020, we find our selves with a mere 35% remaining wilderness and the biodiversity associated with it on the planet. 

After this massive global failure, we need to act immediately! Every single one of us has to get involved at an individual and collective level.

This is a recent headline on the news website Equal Times, a website that specializes on environmental news among other subjects like human rights:

"After the failure to meet the Aichi Targets, action is needed to ensure that biodiversity does not face another 'lost decade'"

The article starts by emphasizing that "The year 2021 must be the turning point in the fight to protect species and to combat climate change".

Remaining global wilderness may be at a devastating 35%, but the situation with forests around the world is beyond catastrophic.

In 2007 the United Nations Environmental Program declared “New estimates suggest that 98% of the forest may be destroyed by 2022, the lowland forest much sooner”. 

And, indeed, the lowland rain forest is gone right before our eyes, and we have all participated to its demise by our unrestrained consumption of palm oil and soy ever omnipresent in thousands of products.

The contribution to biodiversity by these palm oil mono cultures is almost nil in comparison.

These are patches of rain forest in Indonesia.

And Brazil.

In a recent podcast with Francis Hallé, one of the greatest botanists of all time, pioneer of tree architecture. He estimated that 90% of the forest has already disappeared.

He has seen primary forests disappear from the planet's surface in a single life time, estimating that 

only a mere 5 to 10% of primary forests remain on the planet, holding three quarters of its biodiversity. Primary forests are forests that have never been exploited by human activity.

Hallé's perpetual mission to explore the forest canopy and protect primary forests is remarkable. His explorations are testimony that tree canopies are the most species-rich ecosystems on the planet.

Here we can see Hallé drawing this magnificent Moabi, Baillonella toxisperma, the tallest tree in the African rain forest. We can appreciate that colossal strong, solid and stable main fork.

These are images from "Once upon a forest" a wonderful movie about primary forests.

The amount of light that reaches the floor in the primary forest is nothing but a tiny fraction compared to the light available in the canopy. The diversity of life is much richer. Hallé has been exploring primary forest since the eighties with the famous canopy raft, inviting scientists, botanists and all sorts of specialists. 

Here we can appreciate his canopy office overlooking the rain forest.

This is a shocking fact regarding the canopy raft.

In the whole history of canopy explorations directed by Francis Hallé, the amount of money given by sponsors for the various projects, has been roughly less than the cost of the construction of a mile of highway. A tiny amount in comparison to what has been allocated to exploring biodiversity in the oceans, which doesn't even come close to the colossal amount of life diversity present in the canopies of tropical rain forests. By now many species that we don't even know existed in the planet must have gone extinct.

Nowadays, with the evolution of accrobranching the canopy raft has been replaced by hanging bridges.

And platforms

In 2013, the forest canopies were estimated to hold about 40% of terrestrial biodiversity.

In 2020 it's estimated to be about 50%.

"The forest canopy is arguably the most species-rich environment on the planet" declared Terry Erwin, who was one of the first entomologists to give statistics about biodiversity in the canopy. 

Using rather unorthodox techniques, like fogging trees in the rain forest and proceeding to classify and count the hundreds and hundreds of fallen insects on the ground. 

Francis Hallé is pleading for a primary forest in Europe. This is a project that emanates hope and lots of sponsors have already manifested them selves. 

In the podcast which I mentioned a while ago, Hallé says that: for once that money is not needed, sponsors of all sorts are ready to inject money in the project! 

A primary forest implies doing the maximum to do nothing, allowing nature to do what it does best when is not disrupted by human activity. For the moment borders between France and Belgium are likely to be candidates.

A primary forest is an extremely complex ecosystem. This is a drawing from Hallé in French Guyana.

The only patch of primary forest remaining in Europe is in Poland. This is the forest of Białowieża, which unfortunately is being exploited today.

I realize that this has been a rather long update on the biodiversity crisis, but there’s no short cut to expose the colossal loss of biodiversity that we are facing today and the big failure to combat and compensate our destructive impact on the planet. Hopefully, I have made my point across convincing you that tree canopies excel hosting biodiversity and encouraging and persuading you to acknowledge that, indeed, we are in a position where we can contribute to restore the stability to our planet by contributing to restore its biodiversity.

Like pointed out by David Attenborough in his documentary -A Life On Our Planet-:

"To restore stability to our planet we must restore its biodiversity, the very thing that we've removed. It's the only way out of this crisis that we have created. We must re-wild the world."

The contribution to re-wilding the world that we can put in place immediately is simple. It consists in tolerating and allowing natural conditions in our arboreal patrimony, to permit the establishment of dendromicrohabitats, as long as it does not compromise a reasoned and proportionate risk management.

We should be strictly removing the dead wood from trees that compromises our safety and leaving a fair amount for the tree and its environment. 

Like I mentioned earlier, in order to profit from the numerous benefits that trees offer to the environment and our quality of life, a reasoned and proportionate level of risk must be accepted. The "zero" risk notion is an illusion. There's no such thing as life without risk. Otherwise, something as mundane as crossing the street would have to be banned in order to have "zero" risks. The risks of getting hit by a scooter, a bicycle, a motorcycle, a car or a bus are frequently present on the street. 

We must understand that accidents implying trees are rare events, and as such, the exceptional media coverage and the exaggerated fear generated pushes us to take disproportionate measures. Mesures that compromise the integrity and the longevity of trees.

Whether it be living infrastructure or inert infrastructure, accidents happen!

Accidents implying inert infrastructure sometimes serve to further improve our safety, but in general no extra measures are taken. For example, when a massive chain reaction traffic accident occurs on a highway, the next day the traffic remains the same, as if nothing had happened, without prohibiting users from using the highway. 

This type of accident in some cases can lead to an improvement in traffic signs, without any other major change. What happens is that this degree of tolerance is proportional and conditioned by the level of benefits and services provided by the highway, which are superior to the inconveniences and nuisances associated with the highway. The same happens with trees! If the ecological, economic, aesthetic and social benefits that trees provide are superior to the inconveniences and nuisances, our tolerance is way, way, way superior.

On the highway there are traffic signs to avoid car accidents, as well as signs indicating pedestrians not to venture onto the road, warning them of the danger. Underneath the canopy of our urban trees, accidents may happen, branches may fall, and yet it's extremely rare to see relevant signs warning the public.

Our tree campaigns should consider sensitizing and educating the public regarding the cohabitation with urban trees in a responsible and reasoned way. To be able to profit and enjoy the benefits that trees provide, and make the most out of it, we should make sure that the implementation of urban trees goes hand in hand with relevant signs in the city, warning the public of the potential risk during days of heavy rain and strong winds, since common sense is not common at all. 

There's a quote from Alex Shigo that appears in his book "A New Tree Biology" from 1986 “Common sense is the rarest ingredient in the world today.” That gives us an idea of the lack of common sense that the public has regarding trees.

Signs warning the public can be appreciated in Australia and Singapore indicating: "warning! limbs may fall", "beware of falling branches" or... yet...

"trees and branches may fall, take care!".

When we have high level alerts  with strong winds very few injuries or deaths happen because people take them seriously and stay safe without venturing themselves outside as much, but lower alerts are seldom taken seriously and then accidents happen much frequently than with high level alerts. Implementing coherent alerts and signs in our arboreal patrimony may facilitate our task of contributing to restore stability to our planet by restoring its biodiversity. 

Environmental arboriculture also referred to as conservation arboriculture, seeking to maintain old trees in the human landscape, has been the expertise of the English school for the last 20 years or so, flourishing since the launch of the Ancient Tree Forum charity in the UK, which was formed by a panel of multi-disciplinary ecologists, experts and specialists, gathered to protect and preserve one of the most ancient arboreal patrimonies in the world, visualizing the tree as an ecosystem beyond its physical presence, acknowledging an immense biodiversity network linked to the tree and its environment, very much like the relationship between a mushroom and mycelium; a mushroom is the fruiting body of a vast network of mycelium's hyphae filaments. In an analogous way, the tree is an organism linked to an immense network of interactions in its ecosystem, in which it acts as an orchestral conductor. 

Environmental arboriculture excels in the management of dead wood, considering the ecological implications of the complex interrelationships between trees and fungi. A wide range of invertebrates depend on fungi, either directly as a primary food or praying upon other invertebrates which feed on fungi.

Without taking this into account and solely focusing on safety, conventional arboricultural management may be unintentionally detrimental to endangered wildlife.

Obviously, we have to take care of our safety, but it has to be in a reasoned and proportionate way taking into account the environment.

Environmental arboriculture has been largely influenced by the interpretation of the works of mycologists, exploring the complex interrelationships between trees and fungi and its ecological implications, challenging the narrow view notion that decaying dead wood is harmful. As a result, a new perspective has flourished, permitting to understand trees as far more complex organisms, where decay is not purely perceived as the outcome of conflict between the "aggressive" forces of fungi and the "defensive" responses of trees. In many cases, the interrelationships between trees and fungi is beneficial, not only for the environment, but as a survival strategy for the tree.

For instance, in the case of hollow trees, being hollow is a survival strategy to cope with lateral dynamic forces from wind load. 

Skyscrapers are built with tubular structures because they are way more resistant to lateral loads. 

Whether it be to cope with wind load or earthquakes, tubular structures have permitted to reach unimaginable heights.

In many occasions after heavy winds, many solid trees are uprooted while hollow trees remain standing.  

Being hollow for a tree is a natural condition that may open the door to potential immortality, the decomposed wood, makes an ideal environment where delayed reiterates may develop independent root systems. 

A tree in this condition has the potential to clone itself indefinitely. Only biotic or abiotic factors may kill the tree, senescence is not a concern any more. 

Hollow trees are suppressed from our landscapes way too easy without considering that for many trees being hollow is simply a natural condition not a defect. It's normal for an old tree to end up hollow, a tree is a colossal mass of dead tissue, the living part is a tiny fraction in comparison. Without counting the leaves, the living part is basically the cambial zone and the new growth, all the rest is dead tissue. The tree is a gigantic mass of dead tissue surrounded by a tiny fraction of living tissue. There are fungi spores everywhere in a tree and the tree lives in symbiosis and mutuality with many fungi, and even though some may be detrimental for the tree it is often due to a lack of fungi and a lack of balance in the environment.

Regarding harmful insects, insects may be categorized as primary, colonizing living trees, and secondary, colonizing declining trees. These insects feed on living tissue, once the tree is dead, they leave the tree to saproxilic insects that feed on dead wood and are incapable of feeding on living tissue. A tree that has been dead for more than a year does not represent any danger for its neighboring trees as far as insect infestation is concerned. In the case of harmful fungi is another story, they may be transmitted in several ways, by root contact, flying spores, insects, etc.

Precious knowledge on the management and conservation of ancient and veteran trees has come out of the Ancient Tree Forum charity.

Although the terms 'ancient' and 'veteran' have been used as a synonyms in the past, currently there is a determinant distinction: A veteran tree is a tree that has some of the features and qualities of an ancient tree, but is not necessarily old. 

Environmental arboriculture draws much of its inspiration from primary forests, seeking to allow and simulate natural conditions in urban contexts. It aims to do the maximum to do nothing. Nothing that may modify, alter or disrupt the tree and its environment, and it achieves this specific goal by allowing and simulating natural conditions in the urban environment whenever it’s reasonable and appropriate. 

This practice implies a level of tolerance beyond our habitual cultural expectations and requires taking adequate mesures to preserve and encourage the establishment of dendromicrohabitats in order to boost biodiversity.

In recent years, the complex interrelationships between trees and biodiversity has been the object of rigorous research led by the French forest ecologist Laurent Larrieu. Larrieu et al. have described 47 types of dendromicrohabitats as Potential Indicators of Biodiversity in arboreal ecosystems, classifying them in 15 groups and categorizing them in 7 forms:

1. Cavities

2. Tree injuries and exposed wood

3. Crown deadwood

4. Outgrowths

5. Fruiting bodies of saproxylic fungi and slime moulds

6. Epiphytic, epixylic or parasitic and hemiparasitic structures

Epiphytes are plants using other plant structures as physical support; Epixylic refers to living on wood.

7. Active exudates 

These could be sap or resin

As we go through an overview of these 47 dendromicrohabitats, we will realize that we are in fact, quite familiar with most of them, given the fact that we have been referring to these features as structural defects in our tree risk assessments, a label that we take for granted even without carrying out a proper evaluation and thorough risk-benefit assessment. These morphological singularities, which we have been calling structural defects for granted, are rarely beyond an acceptable or tolerable level of risk. In some cases, these morphological singularities may be high potential tree risk features, but we have to evaluate them and perform a thorough risk-benefit assessment before coming to that conclusion.

So can we put aside the structural defect label without a proper evaluation and a thorough risk-benefit assessment? and rather embrace these morphological singularities as dendromicrohabitats, which depending on the scenario and the situation, may or may not be considered as a high potential risk features. The implementation of dendromicrohabitats in urban contexts goes hand in hand with a tree risk-benefits assessment management strategy.

Let's explore these 7 forms of dendromicrohabitats in detail:

1. Cavities.

There are several woodpecker breeding cavities from small to large 

And contiguous cavities referred to as a woodpecker "flute". 

There are trunk humus-holes, sometimes having an open door at the ground level. Sometimes exhibiting a window.

Sometimes having an opening at the top. These are referred to as chimneys, trunk humus-holes. 

There are also hollow branches.

Insect galleries.

Concavities...

Some are referred to as dendrotelms, these are concavities in the form of cups that stock rain water temporarily. Other concavities are holes from woodpeckers feeding, usually the opening is larger than the hole. 

Some concavities are quite hard, covered with bark. 

Some are concavities located between root buttresses.

2. Tree injuries and exposed wood.

Some are the result of a loss of bark or burns from fire.

The peeling off the bark may form a shelter when facing downwards, or a pocket when facing upwards.

Natural fractures on the trunk or main branches. 

Cracks, lightning cracks or split included bark junctions.  

3. Crown Deadwood:

Dead branches

Dead uppermost crown

Natural fracture stubs

4. Outgrowths:

Witch brooms and Clusters of epicormic shoots

Burls and cankers

5. Fruiting bodies of saproxylic fungi and slime moulds-

Perennial fungal fruiting bodies

Ephemeral fungal fruiting bodies

6. Epiphytic, epixylic, parasitic or hemiparasitic structures.

Mosses and lichens

Lianas, ferns, mistletoe

Vertebrate and invertebrate nests

Microsoils form from residues 

7. Active exudates.

Sap runs and heavy flows of resin

These illustrations are taken from a recent comprehensible guide on dendromicrohabitats featuring Laurent Larrieu.

The guide indicates the type of fauna associated to the dendromicrohabitat in the form of pictograms.

The 47 types of dendromicrohabitats described by Larrieu et al. are comprehensibly detailed in technical sheets.

Let’s discuss now why in specific situations and exceptional scenarios coronet cuts are a legitimate practice and not a weird and bizarre technique ?

Many of the most current practices in arboriculture are drawn from simulating nature. Root collar excavation for example, is nothing more than meticulous observation of how trees live in a natural habitat. Trees are not supposed to come out of the ground like an electrical pole like we see often in urban contexts.

The application of mulch, for instance, is also a product of observing how a tree grows and develops in its natural habitat. A tree functions like a recycling factory, the thin layer of debris that falls on the ground’s surface preserves the quality of the soil; protects the tree's rhizosphere, which bears the microbe storehouse of the tree; restores compacted soil; and among many other benefits, returns organic nutrients to the soil for the tree to keep on growing and developing.

Our conventional natural target pruning cuts, are also the product of meticulous observation of natural pruning and putting cuts to the test. The proper angles that are applied in pruning cuts, simulate the way trees isolate their branches in natural pruning.

Natural failures on dead or living branches induce natural fractures in axes. In an analogous way, coronet cuts simulate those natural fractures in axes. 

Small branches are manageable to break, but in branches with a large diameter a coronet technique is necessary, making random spikes in the form of a crown. The larger the diameter of the cross-section, the more challenging it gets.

Coronet cuts belong in the arborist's toolbox, just as much as natural target pruning cuts do. They have a place of their own in specific situations. For instance, in the case of trees presenting decline, in a physiological reactionary state, pruning interventions should be avoided.

These may be stressed trees, in a state of emergency; 

or resilient trees, relaunching a normal architectural dynamics after coping with decline, not to be confused with resistance;

or trees facing an uppermost crown descent, building a new crown under the former upper most crown; 

or yet retrenched trees, unable to reiterate, settling to a reduced retrenched crown. 

If potentially dangerous dead wood must be removed for security reasons, in these specific situations, removing the dead axes by leaving stubs with coronet cuts without affecting the remaining healthy parts, respects the integrity of the tree without additional trauma. The dead axes will end up breaking naturally in the course of time anyway.

In senescent trees, dead or broken axes are never replaced. In urban areas, when it becomes necessary to remove potentially dangerous dead wood for security reasons in senescent trees, coronet cuts executed without affecting the living parts that remain, ensure a slow and progressive death of the tree without additional trauma that may accelerate its demise.

When leaving snags for biodiversity, from dead trees or trees presenting irreversible decline,

there’s no reason to leave unnatural straight heading cuts. Coronet cuts guarantee a natural appearance simulating dendromicrohabitats induced by natural fractures.

In exceptional scenarios, where extreme measure must be taken, coronet cuts have a place of their own. They are an alternative where proper cuts are limited regarding the species capacity to compartmentalize.

When referring to proper cuts that simulate the angles of natural pruning, one thing that is often underestimated, is that compartmentalization is proportionate to the size of the cut. 

Unfortunately, at one time or another, we are all confronted to exceptional scenarios, where we have to take extreme measures and execute cuts exceeding the species capacity to compartmentalize, taking mitigation measures and executing drastic reduction cuts in order to meet an acceptable or tolerable degree of safety. Sometimes we see straight heading cuts in mutilated branches, and shocking as it may seem, sometimes it happens to be a more judicious alternative than removing the full branch and affecting the parent branch or the trunk, knowing that compartmentalization is proportional to the size of the cut.

Occlusion is often regarded as proof of compartmentalization by tree pruners. However, occlusion is not indicative of compartmentalization.

Compartmentalization and occlusion are two different processes carried out by distinct tissues.

Compartmentalization is an inside process, where boundary layers encapsulate dysfunctional or damaged tissue, while occlusion is an outside process where meristematic tissue forms callus and woundwood over wounds.

The callous formed compensates and reinforces the structural and mechanical loss, it does not stop decay.

For instance, a poplar may produce vigorous callous around the cut, while its compartmentalization is weak; conversely, a pine may produce little callous, while its efficacy compartmentalizing is outstanding.

Therefore a strong callous formation and fast occlusion do not translate to efficacy compartmentalizing, as indicated in research by Dujesiefken et al., which happens to be the most reliable research to date regarding pruning cuts.

Dujesiefken et al. research indicates that large diameter branches surpassing the species capacity to compartmentalize should not be cut, but rather be reduced. That is, cuts that surpass 5 cm (2 inches) in weakly compartmentalizing species, and 10 cm (4 inches) in strongly compartmentalizing species.

In exceptional scenarios, when we must break natural laws, often as a result of mismanagement, making large cuts that exceed the compartmentalization capacities of the species, the only alternative that research gives us is not to remove completely the branch, but to reduce it partially. 

The ideal way, may be by removing several smaller branches to spare the branch from a larger cut. Failing that, we may reduce the whole branch to a potential relay, but that option is not always available. Unfortunately, sometimes mutilations are unavoidable to meet an acceptable or tolerable degree of safety.

These are exceptional scenarios, and as such must remain exceptional. We cannot extrapolate coronet cuts to replace adequate pruning techniques that employ proper pruning cuts.

When we are taking these extreme measures, we are basically confronted to two choices: making a straight heading cut or making a coronet cut. However, the choice remains largely empirical and its evident that we are lacking rigorous research regarding coronet cuts.

We need a global approach that can guide us to make future decisions. A global approach in research taking into account not only the biodiversity potential of coronet cuts, but also the effects regarding compartmentalization, biomechanics and tree architecture, we need also research putting the various techniques that simulate natural fractures to the test.

I'm glad to announce, that there is hope concerning a global approach to research on coronet cuts. Christophe Drénou is actually examining possibilities to undertake rigorous research on coronet cuts, backed up by Laurent Larrieu; so the future is promising, research will begin sooner than we may expect.

Christophe Drénou is an unavoidable researcher at the service of our arboreal patrimony. Ph. D. in tree architecture, specialized in decline and senescence and the management of mature, old, senescent and remarkable trees. Pioneer of the application of tree architecture in arboriculture.

He is behind the revolutionary ARCHI Method, which stands for: architectural diagnosis of the vitality of trees.

Drénou specializes also in tree biology and is the author of the still incontestable and unavoidable pruning guide: -The pruning of ornamental trees: from the 'why' to the 'how'-. Unfortunately unavailable in English, but available in French and Spanish. There's an update edition that will be released soon. 

Laurent Larrieu is an unavoidable researcher, having done a doctoral thesis on dendromicrohabitats and having led and participated to the most advanced research on the subject.

To further inquire into coronet cuts, there’s extremely valuable documentation published by Neville Fay that is definitely worth checking out and digging into. 

You may also want to check out the VetCert project, their site gathers valuable information concerning coronet cuts.

I want to make clear that I’m not advocating for veteranization at all.

To rip branches and damage isolated trees in the name of the current massive loss of biodiversity, is highly unethical and highly questionable.

Veteranization is not to be performed in all trees. Especially not in old trees or trees already presenting dendromicrohabitats. Veteranization techniques have a place of their own, sacrificing certain trees in specific scenarios; specimens belonging to young heterogeneous populations, where degradation is not yet present. In certain scenarios, veteranization can replace forest and woods clearings. Instead of felling selected trees, trees can be sacrificed and damaged standing.

We must be very cautious and not ever fall for the trap of replacing old trees by young artificially damaged trees, pretending that they can replace the exceptional morphological singularities found in old trees. Old trees are irreplaceable and veteranization is no excuse to deprive arboreal ecosystems of the unique qualities that old trees provide; old trees are the pick of biodiversity.

The various techniques executed aim to replicate the morphological singularities of -dendromicrohabitats- found in old trees over time, "using tools instead of time".

However, even if the various techniques succeed accelerating decay, the natural degradation achieved over time is far too complex to be replicated simply using tools, time still plays a fundamental role in the equation.

Be aware that the degree of tolerance to dead wood, natural fractures and coronet cuts varies from person to person. With certain individuals, attempting to encourage them to tolerate and allow natural conditions can be more than challenging. Keep in mind that aesthetics is subjective and that an apparent risk is not an imminent risk. 

You may remind these sceptic individuals that in natural conditions trees live in mutuality with deadwood and natural fractures and you may reassure them by pointing out that exceptional interventions in urban contexts require follow-up with periodic inspections anyway. And last but not least, you may point out to them that in order to profit from the numerous benefits that trees offer to the environment and our quality of life, a reasoned and proportionate level of risk must be accepted. 

ADDITIONAL REFERENCES

Drénou, Christophe, 2016. L'arbre - Au-delà des idées reçues

Drénou, Christophe, 2019. Face aux Arbres (Nouv. Ed.)

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