Challenging Current Pruning Practices
By John M. Phillips
(April, 1988)

Introduction

This paper is written for presentation at the 1998 WCISA annual conference in Yosemite, California. The theme of this conference is "Sustaining Our Urban and Community Forests". It is with this in mind that current pruning practices are challenged.
Simply put, sustainability means continuity for a long time. If we are to have high quality and long lasting urban forests, maximum efficiency of their care is needed. This is especially important given that these forests and their maintenance budgets are shrinking (1).
Tree pruning is commonly accepted and frequently performed. The concepts supporting the practice have changed very little since arboriculture became a profession many years ago. Reexamination of these concepts may provide an opportunity for improving the efficiency of tree care and the role it plays in sustainability.

Summary

Pruning is a two-edged sword: it can help or it can hurt. It is a wounding treatment that can deplete the vital force of energy. It can be an alteration of structure that changes the resistance to the forces of wind and gravity. The extent of these impacts is predicated upon many factors.
To minimize the negative consequences of pruning, proper techniques must be employed. Their implementation should be preceded by a decision-making process that takes into account the many variables regarding the tree. Ideally, pruning should take place only when there is the potential for a positive outcome.
In reality, most pruning is a compromise between trees and people. The benefits tend to weigh heavy for people, while the tree bears the costs. However, if the costs are too high for too long, there may be no gain for either party.
This scenario can develop when there is excessive removal of living branches and leaves. The threshold at which this begins is difficult to determine and central to ongoing debate. It changes over time and with the health of the tree, becoming most critical at maturity.
As trees get bigger and older, pruning is frequently performed to make them smaller, lighter, and less dense. In order to do this, green parts must be removed. Such action becomes a precarious balancing act of sacrifice for preservation.
The need for excessive pruning of mature trees can be reduced by generating a plentiful supply of young trees. The new trees should be healthy, matched to the location, and raised with great care.  Ongoing pruning should be prescribed and followed long ,enough to insure strong development.
Undertaking vigorous programs of forest renewal can help make tree care more efficient. The emphasis of pruning would be on younger trees where it would do the most good. Pruning on mature trees, properly raised, could be limited mostly to dead branch removal. Those that are too weak to live next to people can be removed with less pain as there would be a continuous supply of replacements.
This strategy is a significant shift in tree care. For it to work, people must adopt the community forest mentality. Governments will have to promote and practice forest renewal. Citizens will have to appreciate the values of trees beyond their own property and be willing to participate.  And arborists will have to broaden their vision and scope of work.
As always, arborists are in the position between trees and people. They have influenced how people think and act towards trees for decades. Their skill has helped to create aesthetic and functional benefits. The challenge to instill community forestry is simply an extension of what they have done before.

Scope

1) Extent: The focus of this paper is the pruning of mature trees. It is not about the training and development of young trees or specialized pruning, e.g. fruit trees, pollarding, etc.
The practices of concern are "crown cleaning" and "crown thinning" as described in the Tree-Pruning Guidelines (ISA 1995).

2) Background: Thoughts for this paper have developed over the 25 years of this arborist's professional practice. They are a combination of personal experience and that of others. This arborist has had the honor of working with many outstanding practitioners, researchers, and educators. Some of these people were asked to contribute specific comments for this paper, and did.
By no means does this paper attempt to be an exhaustive search for relevant information. Nor does it purport to be a scientific study. It is simply one arborist's reflection on many hours of tree climbing and pruning, combined with an ongoing curiosity of trees.

Observations

1.Pruning is a frequently performed treatment. It is perhaps the most frequently performed treatment. In many cases, pruning is performed repeatedly on the same tree, sometimes on a regular, calendar basis. It may also be done when it is convenient for the pruner, rather than when it is best for the tree.

2.pruning is done almost the same way now as it was in the beginning. Except for modifications in the pruning cut, tool modernization, and improvements in climbing techniques, pruning has changed very little.

3.pruning is done primarily for the benefit of people. It is done for human aesthetics and to improve tree compatibility with humans (i.e., to improve safety, control size & shape, provide clearances, etc.). These benefits may not always outweigh the costs to the tree or the customer. .

4.Determining the need for pruning can be a very Subjective process. Aside from clearance pruning and the presence of large or broken branches, there are few, if any, objective criteria for determining the need for pruning. Even when the need is obvious, the amount to be removed (dose) is an untested variable.

5.The efficacy of certain pruning treatments have very little scientific basis.  The reduction of tree density and/or branch weight are two common treatments that are implemented with a high degree of subjectivity. Whether they accomplish a net gain of benefit is largely untested.

6.Pruning standards and guidelines can, under certain circumstances, give a false sense of purpose, and may result in unnecessary or excessive pruning. The standards/guidelines may allow for a state of complacency or even a sense of license. This is most likely to occur when the pruner has an inadequate understanding of trees (biology, physiology, anatomy, etc.) and a proper decision-making process is not followed.

DISCUSSION

1.The common nature of pruning.
In this country, at least, pruning is commonly performed. For many companies and municipalities,' it is the most commonly practiced treatment. It is usually highly visible and can result in a large amount of impact to the tree.
Pruning has become an accepted treatment by most people. Precedents have been set and citizens expect their city trees to be pruned. Private owners expect pruning to be part of their maintenance package. Pruning has become the norm, and as such can become a routine that detracts from the evaluation procedure for its need.
In both the public and private domain, some trees are pruned on a repeated and regular basis. When this is done in accordance to real need and with high skill, it offers the best opportunity for minimal negative impact to the tree and maximum gain for people. Certainly, planned management is better than crisis stimulated action.
Unfortunately, most cities don't allocate enough resources to implement sound tree programs(1). This is either a choice or by the consequence of the shrinking budget. It is not much different in the private sector.
With homeowners, it is not uncommon to find extremes with regard to the frequency or amount of pruning. There are those that demand pruning to excess or at least more than the professional will recommend. And then there are those who won't do anything until something breaks.
Trees in both the private and public realm are subjected to changes in ownership or management. Sometimes properties change ownership frequently(the average was once reported as every 3 years). Municipalities often experience a high degree of personnel changes.
Such changes may help or harm the tree which outlives the people. A new owner often has different values and needs. A new manager often has a different background and ideas about the job. Ongoing programs may be interrupted or altered. The manager may hire different employees and the homeowner may hire a different tree service. .
Sometimes pruning is dictated by the convenience or need of the pruner. Most pruning tends to get done when the weather is favorable. This may not always coincide with the best moment in a tree's phenology. There can also be an emphasis to sell more pruning when work backlogs are thin. Such "desperate" sales may squeeze the allotted time to perform a treatment, which could affect the quality of the treatment.
Tree work, in general, is renown for a high degree of competitiveness. Pruning prices range widely and fluctuate frequently. Even the most skilled and ethical practitioners sometimes must sacrifice quality in order to stay in business.
The competitive nature of the trade is exacerbated by the fact that it is a relatively cheap investment to get into. Only a few basic tools are needed to climb and cut. Except in a few localities, there is little regulation to qualify or otherwise control the level of competence.

2.The traditional nature of pruning.
The understanding of how branches grow and shed, as provided by Shigo(2), has greatly improved the potential for better pruning. This has been enhanced by the invent of better cutting tools and the development of more efficient climbing techniques. Beyond this, little has changed.
Early texts, such as Practical Tree Surgery (Millard F. Blair, 1937) describe the basic concepts of "cleaning and thinning". Prune to remove the dead, dying, diseased, deformed, dense, etc. From one source to another, the words vary but the parts are the same.
While standardizing our terminology helps us communicate about the practice, it is still the same practice. Why should it be any different? Trees are basically the same as they always have been. What has changed is the available information regarding how trees live, grow, and die.
Some of this information is new, and some of it is older, but made more applicable by recent interpretations. Simply put, trees are integrated systems of highly ordered parts and processes, relying on a net energy gain to stay alive and enough sound, connected pieces to stand up. This is not new, but it is often overlooked in pruning decisions.
Examples of this are contained herein. Of course, for a detailed understanding of the physiological and anatomical points, please refer to texts by Shigo (2,3), Mattheck(4), and others.

3.pruning costs and benefits.
Pruning is done for human needs and desires. One could argue that if pruning helps to keep a tree from falling apart, that this is good for the tree. True, but still most trees don't need pruning to stay intact. Evidence for this follows any storm event where most trees, pruned or unpruned, have suffered no failures (or at worst, only small branches). Extreme weather events, e.g. tornadoes, hurricanes, etc., may cause extensive failures, but it is highly unlikely that pruning would have prevented them.
Given enough time, however, most any tree will fail. This is part of the natural cycle, at least in the natural setting. In the urban forest, human safety comes into play. Again, this is a people problem .
In Arboriculture…(5), Harris says that most trees grow quite well with little or no pruning. But if trees are taken from their natural settings, or if these are changed, then it can be a different story. He further points out that few trees have been properly trained, if at all, as far as their mature structure is concerned(6). This is probably more common in nursery grown trees than those that arise on their own.
Pruning for tree health may apply in certain insect and disease complications. While pruning, correctly timed and performed, may help these situations, it is not always necessary. In other words, many unpruned trees survive. Pruning, at best, would be a part of a more complex treatment program.
Furthermore, if the tree is in a "low-energy" state, pruning is unlikely to ward off insects or diseases in any case.

a.The dead. Removal of dead branches, the most basic component of pruning, could be considered a tree health treatment. Dead branches are subject to degradation by microorganisms, and under the right conditions, could move beyond the branch tissues (into the trunk). And many dead branches could mean many infection sites, the total of which could be quite detrimental. Fortunately most dead branches most of the time do not lead to this problem(7).
Arborists exist because there are trees in domesticated or compromised locations. Removing dead branches, even if just to stay on the safe side, is one of the things arborists can do to help maintain tree health and satisfy people. Since these branches will be shed by the tree anyway, what harm can be done? Probably none, provided the pruning is done with great care. It is when the significance of dead branches is overemphasized or incorrectly interpreted that such pruning isn't worth the cost.

b.The dying. Next on the priority for pruning are dying branches. These will be shed eventually. However, while this dying process is taking place, materials are recycled back into the young symplast of the parent stem, where they may be used in the formation of the protection boundaries(8). When does this process begin and how long does it take? Shigo says that most dynamic transport takes place early in the dying process(8).
A more critical question is what constitutes a dying limb? When a few twigs die? When some percentage of the leaves die? Removing all of the "dying" branches on a tree may be taking too much too fast, in certain situations (see c. below).
Some limbs may die slowly over many years. Large lower limbs that reduce their leaf area over a period of years have been called "parasitic", i.e., that they are "taking" more from the tree than they are giving. Shigo says this is not possible(8). Removing such limbs before they die might create large wounds somewhat deficient in protective materials.
If there are many dead and dying branches on a tree, then there may be a larger, more serious health problem. Pruning without a comprehensive understanding and treatment of the problem could be a waste of money. It also raises ethical
questions. '

c.The rest. In addition to the dead and dying branches, crown cleaning (according to the Guidelines) includes the diseased, crowded, weakly attached, low-vigor branches and watersprouts. The terms here imply unnecessary and perhaps hazardous growth. What value could they be to the tree? Why are they there? "
What is a diseased branch? One that is dying? Is there any difference between one with parasites versus saprophytes (with reference to the term "diseased")? Does a branch with a twig blight infection(Cryptocline sp.), for example, require removal? Such infections often have about 1/2 living and 1/2 dead twigs per branch throughout the crown. Removing all of the diseased branches would be removing the tree! And finer detail of pruning (infected twig removal) doesn't prevent reinfection.
When is a branch crowded? When it is touching another? Is that detrimental? Does a tree know crowded? Or is that a human thing?
A weak attachment is usually a bark inclusion (or a sprout). While this is an obvious defect, it is not always clear how to deal with it. On some trees, there are dozens if not hundreds of them. Obviously, removing one branch portion with such attachments could denude the tree. This is a problem best corrected when the tree or branch is very young.
What is meant by "low-vigor"? Is it dying? Or is it just suppressed within the crown? Or are these the same? The term "vigor" needs clarification. Shigo uses it in reference to genetic capacity(3). Perhaps "vitality" is more appropriate in this context of crown cleaning.
Finally, we have watersprouts, or epicormic shoots, depending on the term chosen. As with dead/dying branches, it is important to understand why these types of shoots are present. According to Shigo(2,3), they arise in response to energy depletion. Causes for this include root-related problems, defoliation, and overpruning. Removing these shoots prior to their decline could further exacerbate the energy supply.
The point through all of this is that crown cleaning may remove branches that are of benefit to the tree. So-called "unnecessary" or "harmful" branches may be so only in the eyes of people. The term "cleaning" alone has a very popular human appeal.

d.Crown thinning. Removing branches of potential value to the tree becomes even more pronounced when pruning progresses to "crown thinning". It is here where some concentration of living, healthy branches are removed. The purposes for this treatment, as described in the Guidelines, are to decrease the density of the crown for better light penetration, air movement, and weight reduction. The intent is to maintain natural shape, inner branches, and sound structure. 
Maintaining a sound tree structure is mutually beneficial for trees and people (given the notion that longevity is important). Retaining inner, lateral branches helps to build stronger parent branches. But branches can die with or without light penetration. The tree "knows" when to shed.
Reducing weight, carefully and periodically, may reduce branch failures (see below). But any changes in a tree's crown are just that: changes. These may not always be in the best interests of the tree.
It is important to remember that pruning is a wounding treatment. Even wounds properly made may lead to internal defect. When improperly made, some extent of defect is almost inevitable. And it is very easy to make a poor pruning cut or an
unintended wound.
The most skilled tree pruners, being human, will always have some degree of error. This is probably small and insignificant on a "good" day (Shigo's "90%"). But there are many variables that affect pruning accuracy. Tool efficiency, climbing conditions, worker disposition, available time, etc., are just some of the factors involved in pruning outcome.
Another important aspect of the removal of living branches is the reduction of energy producing parts. Photosynthetic tissues, in both the leaves and the stems, are reduced. How much energy can a tree spare? This is complicated and highly variable.
It is important to note that on most trees, most of the photosynthetic tissues are in the outer portions of the crown. This is the same portion of the crown where much of thinning takes place. It is thought that the least negative impact will take place here because many small cuts can be made, spreading out the effect, rather than concentrating on fewer and larger diameter cuts. In either event, the pruning is affecting a most vital part of the tree.
In addition to the reduction of the productive capacity, there will also be a reduction of the energy storage capacity. Energy is stored in living cells, primarily located in the growth increments of the stem and root. Energy is needed for all of the tree's processes, and stored energy is especially critical for the initiation of growth and for defense.
Beyond the wounding and energy impacts, there is a potential for the alteration of the tree structure or architecture. Simply put, trees grow where and how they can. This means that any given branch grew as and where it could. It had to do something right to get there. It laid down wood cells to counter the forces of gravity and wind. It produced laterals that contributed more wood cells and taper, allowing the branch to flex, twist, and turn without breaking.  This all took time to develop and can be quickly changed by pruning.

4.The need for pruning.
Pruning for clearances and aesthetics are human needs (or desires). Pruning to maintain a strong or "safe" structure could be considered, a need of both people and trees. While trees don't "care" about safety, they benefit if they don't break apart (again, assuming that longevity of the individual tree is good). The critical question is when and where does pruning help prevent breakage?
Tree parts fail when the forces of wind and gravity exceed the limits of strength. Trees produce wood in places to counter these forces(4). Barring sudden and extreme weather, trees generally have enough time to gain sufficient strength. Why then, under normal circumstances, would a failure occur?
Strength is affected by defect. This could be a degradation in the branch tissues or an inherent weakness, such as a bark inclusion. Defects usually get worse with time. Evaluating these defects is quite difficult.
There are a few tools available for measuring wood strength. When properly used, they can tell us something about branch or trunk conditions at or near the point of measurement. Determining the failure threshold is another matter.
Some arborists claim they can tell when a branch is too heavy or a tree too dense. This ability may come from having made a large number of observations, before and after failures. While this is valuable experience, it is very subjective and far from 100% accurate. There are plenty of examples of trees that broke that didn't appear to have a problem or of trees with obvious defect still intact.

a.Defect. Obvious defects are those we can see symptom or sign of. Mattheck describes "body language" that could indicate tree deformities and potential failure-points. Shigo has shown that flush pruning cuts (among other features) are a common initiating point of weakness. Wounds in general are suspect points, for both wood decay and internal cracks. Shigo maintains that such cracks are the most significant type of defect in terms of failure potential, and these are problems that often remain out of sight until failure occurs.
Most, if not all, trees have some degree of defect. Things happen, even without people. Yet trees resist the forces of failure because they have internal defense mechanisms and the ability to produce new wood. It is when these processes are diminished or otherwise compromised that strength weakens.
This can occur when a tree is subjected to a radical change in its growing conditions. Trees that suffer a sudden change in their surroundings are likely to experience new environmental forces. An obvious example is where a tree is isolated from its former group. .
Strength can also be affected when a tree is altered or suffering. Severe pruning of one portion of the crown may affect another portion. Root severance can affect stability both in the short and long term. Any decline of health could lead to loss of strength by retarding the processes of tissue production and defense.

b.Foreign conditions. Species grown out of their native habitat may also be subjected to conditions that alter their ability to live or stay intact. Environmental deficiencies (water, temperatures, minerals, etc.) usually cause health problems. Excesses may too. Sometimes the excesses cause a tree to grow "too" well.
Trees such as Monterey pine and eucalyptus, grown out of their native regime, may have failures more often than when they are at "home". This could be due to a change in the wood structure and/or a greater production of foliage. Clark(10) claims that trees such as Monterey pine have to be pruned when grown in the typical non-native landscape, or breakage is inevitable.
When trees grow too well they can "outgrow" themselves. In addition to the physical or anatomical changes, there can be physiological effects. Trees that allocate an excess of energy to growth may not have enough in storage. Low energy reserves decrease the tree's ability to be healthy during "lean" times, thereby increase susceptibility to insects and pathogens. Or there just may not be enough energy for defense at any time.
Whether pruning can offset a tree's "overproduction" is questionable. The amount and distribution of weight can be altered. In doing so, the energy supply is affected. Does this "dwarfing" process maintain health?

c.Dose. Part of the answer has to do with the amount pruned out of a tree. Shigo refers to this as "dose"(3). He claims that the dose is relative to tree age and as trees get older(more mature) the pruning dose of living parts should be reduced. He says a tree is mature when the genetic code has been "played out" and the dynamic mass to static mass ratio is such that it optimizes high quality survival within the environmental conditions present(8).
This would dictate that expendable leaf area is little to none on mature trees. Yet the Guidelines allow up to 25%. Other sources prescribe up to 33%. Are these limits tested? Where do they come from?
One apparent source is from early studies done with pine plantations(11). It found that the removal of 30% of the live crown (from the bottom up) had little effect on height and diameter growth. These were 15 ft. trees and have little comparison to the discussion here, or to the purpose of the Guidelines.
In a very recent treatise on the management of ornamental trees by Raimbault, et.al.(12), a 20% allowable dose is prescribed. This is the same amount as recommended by Gilman in his text (13). Gilman says that he knows of no research to support this(14). It is unclear how Raimbault arrives at that limit.
The Raimbault text is an extremely detailed analysis of tree growth and morphology, leading to an equally detailed and complex methodology for pruning. It divides the tree's life into four phases, subdivided into ten stages, that reflect early development through decline and mortality. Pruning is correlated to each phase and stage, changing in application to accommodate the age of the tree.
Of interest here is his differentiation of chronological age from physiological age. The latter is influenced by the specifics of a tree's environment, past treatment, etc., as opposed to how long it has been living. This means that maturity is a variable and cannot be predetermined by the calendar. He cites one feature of maturity as the "accumulation of wood mass increasingly out of proportion to the leaf activity".  This is what Shigo refers to as the increase of the mass/energy ratio(3).
Prior to maturity, in stages 5-7, Raimbault states: "pruning of about twenty percent of the branches increases the duration and the length of annual growth for several years; this increased vigor lessens to almost disappearing before the tree totally regains its lost volume. This reaction can be accounted for as a momentary renewal of vigor rather than a physiological rejuvenation of the tree. But we have only worked on trees that have virtually never been pruned; we don't know whether the trees that have undergone regular pruning would react the same way.”
After these stages (early through mid-life), a similar intensity of pruning (20%) will have a negative effect, initiating a decline of the crown and accelerating the aging process. He also has noticed that even a tree that has been only slightly pruned will not reach the volume of the unpruned tree, but questions whether the pruned tree will live longer.
This discussion of invigoration versus dwarfing is very similar to that given by Harris(5). A pruned branch maybe invigorated, but the overall plant may be dwarfed as compared to an unpruned plant. This would especially apply when many
branches are pruned.
Raimbault's recommendations roughly parallel the concepts and techniques given in the Guidelines. The terminology and the degree of detail are different and much more complex. Of significance here is his concept of "long pruning" which translates to a "thinning cut" in the Guidelines. He says that in order for a pruned branch to assume the terminal role, it should be equal to or greater than the branch removed.  The Guidelines offer 1/3rd the diameter or greater.
Of further interest (from Raimbault) is the notion that as trees age, there are more branches with less growth per branch. These fill out the volume of the crown, with less increase of overall size. Apical dominance lessens and the crown becomes more rounded. And while each branch never stops elongating, the tree is not increasing in size at the rate it did in the earlier years.
 This helps to explain and support the categorization of trees into relative mature sizes as is commonly done in design and selection. It could also contribute to the basis for assessing maturity and prescriptive doses.

5.Untested common theory and practice.
Crown thinning is frequently done to take weight off of branches and reduce crown density. One is intended to reduce the branch "load", the other the tree's "windsail". While this may seem a logical approach to reducing breakage and uprooting, it is an untested concept (on large, mature trees).
Unfortunately, science can't always keep up with our need to take action. Deductive reasoning and educated guessing are often the best basis we have for making decisions. Problems can arise, however, when certain concepts from one arena are crossed with another.
It wasn't so long ago that arboriculture borrowed from the practices of human medicine and dentistry. Improvements in the understanding of tree biology have since done away with wound dressings and cavity treatment. Maintaining the biological perspective is imperative to giving sound treatments.
Mattheck(4) has applied principles of engineering to trees. He describes the tree as a "chain of links", where the wind is received by the crown and transferred through the stem into the root ball and surrounding ground. In an unpruned, healthy tree, these three components are perfectly matched to the wind load as "measured" by the tree. 
When a tree is pruned, the chain is broken. Because the tree optimizes the use of its resources by minimizing their allocation(no oversized links), a weakness is possible, i.e. there isn't much to spare. The extent of the weakness may be proportional to the break. Its repair may take place given enough time and available resources.
Mattheck also provides some basis for reducing limb breakage and uprooting with his Law of the Minimal Lever Arm. This is where the tree does what it can to avoid a bending load. It will grow away from gravity and wind to avoid their stresses. Pruning might assist the tree in either of two ways: by reducing the size of the tree or length of the branch (crown reduction); or by reducing the density of the crown or branch (crown thinning) .
Which method is more effective is debatable. In one of his lectures(15), Mattheck said the crown or branch should be reduced in size (pull the sail in). This is consistent with the lore of several "old-timers", as told to this arborist. Other arborists claim it is better to thin the crown (poke holes in the sail) .
Both methods probably accomplish the intended purpose, at least immediately. How long the effects last and how much they may be offset by negative consequences are important questions. The dose or amount of live crown removal is a critical factor. The less foliage removed, the least health impact. Conversely, the less size or weight removed, the less significance of the action.
Attempts to answer these types of questions would involve very complicated and long-term experiments. It may not even be possible, with present technology, to completely evaluate treatments. A tree's environment has many variables and every tree is different.

6.State of the art.
The Tree-Pruning Guidelines and the ANSI A300-1995 define the application of pruning. They were preceded by the Tree Pruning Standards (WCISA 1988). Some of the information and techniques were drawn from earlier versions of the standards
provided by the NAA and those written by the British standards Institute. The earliest version of the practice that this arborist could locate is in the "Arboriculture Code of the National Shade Tree Conference", 8/30/46.
In the preface of the Guidelines, it is indicated that they are not intended as a complete training manual for pruning or climbing techniques". This implies that the user must have other and more complete experience or knowledge. Within the Purpose section of the Guidelines, it is also recognized that trees differ and their pruning needs may not always fit strict rules.
In the foreword of the ANSI standard, it disclaims itself to be a "guideline to illustrate how to prune trees". Further, "Specifications for tree work should be written and administered by an arborist. An arborist is a professional who possesses the technical competence through experience and related training to provide or supervise the management of trees".
Thus, both of the most modern, professional guides for pruning are to be used under the auspices of a qualified professional. The preliminary decisions for the appropriate application is made prior to the treatment. This helps to insure that, the guidelines won't be misconstrued and the pruning illperformed.
Sometimes this important qualifying step is not taken. Anyone can use these guidelines and standards. At least two scenarios have been witnessed by this arborist.
One is where an agency, such as a municipality, includes the Guidelines in a job bid. The specifications say that the job shall be done according to the Guidelines. This can be the end of the job description, or there may be a "degree of detail" included. 
The process falters when field personnel are lacking to make specific interpretations of the Guidelines and associated decisions. Even if a "consulting arborist" is employed to write job specifications, this person is not always retained throughout the course of the job. This is all made worse when the job is awarded to a typical "low-bid" contractor.
The other situation is where a practitioner applies a pruning treatment simply because it is defined in the Guidelines. In other words, they exist so it must be okay. In a sense, the Guidelines give a license to prune. They could even allow a person to become complacent in pruning, i.e., pruning from tree to tree with disregard for each tree's uniqueness.
This is not much different from the tree operators who apply a "standard" prune to all trees. These people are either unaware of professional practices or just don't care. Tree care is apparently not exempt from the universal demons of ignorance and greed.
When a guideline or standard becomes a simple recipe, rather than a treatment following an experienced decision-making process, the benefits of pruning are quickly diluted. Shigo calls this process "DTA", or diagnosis, treatment, and
assessment. He has an (as yet) unpublished list of 128 facts a person needs to know in order to make a proper decision (he says he will reduce it to 50 for the tree expert). He also emphasizes the use of records, so that treatments can be compared over time and with other trees. This would be very useful for establishing species profiles and furthering the state-of-the-art.

Conclusions

1.Tree pruning is commonly accepted, requested, and expected. When money is not the limiting factor, pruning may be used to excess. When money is limited, quality may be sacrificed, which can mean the tree is compromised. More often than not, pruning is erratic rather than carefully planned over the life of the tree.

2.Crown cleaning and thinning have changed very little over the years. Except for a minority of professionals, practices have not kept pace with the improvements in the understanding of trees.

3.The costs of pruning to a tree may be greater than the money spent. Benefits from pruning are often greater for people than for the trees. The net results can be the benefits of pruning are often overrated.

4.The need for pruning is greatest when defect in the tree system presents a failure potential. Determining the threshold for failure, the curative dose and the place(s) to apply it can be very difficult.

5.Branch weight and windsail reduction are two common pruning goals. The dose which is effective in achieving these goals may inadvertently decrease the health of the tree. Neither treatment has substantial scientific basis.

6.Despite the best of intentions, standards and guidelines may encourage unnecessary or excessive pruning. This could be avoided if a consistent and objective methodology for decision making was adopted.

Recommendations

1.Plant and raise more trees.
The urban forest is shrinking in stature and extent. More trees would help solve this problem. A continuous renewal of the forest would also lessen the pain of removing trees.
Arborists need to take an increased role in fostering new trees. Historically, arborists took charge when the landscape contractor gave up. This often resulted in having to cope with substandard specimens and inadequate care. Raising healthy and strong trees helps to reduce the need for excessive pruning later.
Customers need to realize the need for good care in the beginning and be willing to pay for it. They need to understand the benefits of ongoing care and commit to long range development programs. Arborists should be ready and able to implement these programs.

2.Propagate better quality trees.
It has been a common and long-standing cry that we can't get good quality trees. While this is changing, it hasn't happened fast enough. Campaigning for a role in forest renewal without the product is pointless.
We must continue to work with nursery people toward high quality trees. They have to know what is needed and that the product will sell. Once again, arborists are a necessary link between trees and people.

3.Prune efficiently.
Prune when necessary and when the benefits outweigh the costs. With mature trees, minimize the removal of green and prune with high skill. Scale the job to the amount of time/money available. A pruner should always have the time to get into position to make a good cut(Don Blair).
Keep pruning in perspective. Understand its limits and integrate them into the decision-making process. It's better to lose a sale than to make false promises.
At the same time, expand the focus to the whole tree and any related problems. Prioritize treatments. Pruning is not the only practice that supports a, tree business and most customers are capable of appreciating good tree health. More often than not, roots need attention too.

4.Remove trees wisely.
We spend a lot of time putting band-aids on large, weak trees. Yes, these trees add character to our landscape. Yes, they give some people great pleasure. But how long can we keep elephants standing on one leg(Doug Hamilton)?
While this is a test of arboricultural skill, it may not be the best use of our resources. Water, minerals, and space are limited, and becoming more so. There are less trees and time for their careful management.
It may be better to remove more suspect trees, on a proactive basis, and create more renewal opportunities faster. With good planning, diversity of size and age would develop, allowing for better' continuity. Such deemphasis of mature trees could be underscored by more appropriate tree ordinances and other types of guidelines. 

5.Promote research.
We need to know more about pruning mature trees. Is 'pruning an effective way to manage density, structure, and volume? What is the optimum schedule and dose? Consider the work of people like Raimbault as management models. '
Where and what defects can be compensated for by pruning? Does the benefit outweigh the costs?
In order to answer these types of questions, we need to understand the many variables that enter into the decision-making process. We need to know more about our species and how they perform under different conditions and with the various
treatments. 
We need to track our performance, question it, and work on the weak links.

6.Promote people with trees.
One key to forest sustainability is appreciation and support from the broader community. We need people to want trees and their benefits. In order to get this, we need to continue to help allay tree fear, false understandings, and general apathy.
 At the same time, we need to extend our vision beyond the traditional realm of one or a few trees. We need to preach community forestry. This will help to sell adjustments in our practices and the appropriate shifts of emphasis.
 As always, we remain in a key position to influence people about trees. The demise of the older trees gives us great opportunity. Being more comprehensive helps us to maintain our niche.

WCISA Yosemite 1998

(The author apologizes for losing the reference list; please contact if help is needed in finding a publication).


Northcoast Tree Care - Essays & Articles

1.
Why Do Trees Die?

2
Flush Pruning Cuts

3.
Challenging Current Pruning

4.
Thoughts About Protection Wood, Crack & Pruning - Text

Thoughts About Protection Wood, Crack & Pruning - Photo Album