Thoughts About Protection Wood, Cracks & Pruning
By John M. Phillips

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The following was written for presentation at the 2008 California Tree Failure Report Program.  The topics selected are ones that I feel have not been given enough attention at previous meetings.  It is my intention to elaborate on them so that they might be incorporated into future discussions.

While I make every attempt to draw from scientific text, my discussion does not follow the form of scientific methodology.  Instead, these are my thoughts and experiences from over three decades of study and practice.  Being a tree climber and cutter, albeit an aging one, has enabled me to see things that others might not.  I hope my insights will be of interest and benefit.


Trees have evolved with protective features and defensive mechanisms that help keep them alive and intact.   Protection wood and the process of compartmentalization are two that are discussed herein.

Protection Wood

Protection wood is wood that is more resistant to infection than healthy sapwood. 
Shigo describes eight types of protection wood:  False heartwood, branch protection zone, heartwood, discolored sapwood, discolored heartwood, reaction zone, wetwood and surface, hard dry wood.  The barrier zone can also be considered a modified type of protection wood.  There are other types of protection boundaries in bark, petioles, pith, nonwoody roots, and leaves/needles (1).

It has been commonly taught and understood that there are two basic types of wood:  heartwood and sapwood.  With the information provided by Shigo and others, it may not be that simple.

“True” heartwood is formed as sapwood cells age and die, and appears to be under genetic control.  Extractives are usually deposited that give the wood a higher state of protection from infection.  It can form in both axial and radial directions and is usually darker in color than the sapwood.   This kind of heartwood is typical in oaks and walnut.

In contrast, “false” heartwood is produced as branches wane and die.  It is formed in a longitudinal direction in the tissues below the branch.  As many branches die, their tissues may coalesce to form a column of wood that is often colored.  Its protective feature is due to the exhaustion of energy reserves in the tissues, thereby making them less desirable for certain pathogens.  This kind of heartwood typically forms in beech, ash and birch.

Compounding the appearance of wood are the effects of injury.  When trees are wounded, they respond and react by changing the affected wood tissues.  Under normal conditions, the first response is a chemical change that alters the wood to a more protective state.  A color change is usually associated with this process and is typically called “wound initiated discoloration” or WID.

Most wounds are infected by microorganisms and usually in succession.  The first group, or “pioneers”, interact with the tree furthering the alteration of the wood tissues.  As they advance into the tree, other types follow that are able to digest additional and more resistant wood tissues.

Wetwood is another form of interaction between trees and microorganisms.  It is a disease of wood caused mostly by bacteria.  In some way, these pathogens alter the permeability of cell walls such that moisture, pH and microelements are increased.  These changes make the wood less palatable to decay-causing pathogens, thereby providing a form of protection.  Wetwood is commonly found in elm, poplar and birch.  It is typically darker in color than the sapwood.

Thus we see variations on the standard theme of heartwood.  In order to differentiate them, longitudinal stem cuts are necessary.  Chemical testing may also be helpful.   These wood features help to stall inevitable infection and degradation of the stem system.  

Another important protection feature are the substances that are formed at the base of branches as they wane and die.  Trees need to shed branches that are no longer of high productivity (in energy contribution).  As they die, they become host to pathogens, both parasitic and saprophytic.  Strong individuals of most species form branch protection zones that resist the inward spread of the pathogens.  These zones can last for a long time, isolating infections as the tree grows around them. 

AN ASIDE:  Dr. Wayne Shortle and Dr. Kevin Smith, former associates of Dr. Alex Shigo at the Northeast Forest Experiment Sta., have been investigating the proposition that “all” heartwood is the result of injury (WID).   This is based on the fact that as trees get older they are always being wounded…by branch shedding if for no other reason.  The distinction between age-altered changes (heartwood) and wound-altered changes (WID) is useful most of the time.  At the extremes, it may be a distinction without a difference.  The difficulty in proving this notion is that it is impossible to do the critical experiment (2).


This is a process that resists the spread of pathogens by forming boundaries.  The concept is most often applied to patterns of wood decay, but it can also be applied to the pathology of vascular wilt diseases and the response of bark injury and infection. 

There are two distinct stages in the process.  The first sets boundaries in the wood present at the time of injury.  They are commonly called “reaction zones” and are both physiological and anatomical in nature

The second stage involves the formation of a barrier zone by the vascular cambium.  This zone tends to resist the outward spread of discoloration and decay into wood formed after injury.  While this zone is relatively strong in its containment power, it can contribute to structural defect.

The concept of compartmentalization was introduced by Alex Shigo with the help of many others.  It was done to explain the patterns of wood discoloration and decay and to distinguish changes in wood due to wounding from changes in wood due to heartwood formation (3).  The model C0DIT was produced to help others understand the concept.

The concept has had its share of criticism.  There are still probably people who do not accept it as a part of the tree system.  In my experience, the lack of acceptance appears to be based on confusion and/or misinterpretation.

The CODIT model shows distinct walls that contain decay.  But this is only a theme.  In reality, there are many variations as walls and zones are not always neat.  Expecting all wounds and the consequent defect to look like the model is confusing the map with the territory (3).

It may help to think of compartmentalization more as a process rather than an end result.  And it is critical to understand that the process resists pathogens and does not stop them, at least not forever.   There are many ways that pathogens “escape” containment.  Internal wood cracks are one way that infections can spread from the inside of the tree outward.

Another area of confusion comes when tree stems are only dissected into crosscuts.  These may be useful when there is only one infection.  But when there is more than one, it can easily appear that the pathogens are spreading at will.  In such cases, longitudinal dissections are essential.  This will help reveal both the origin of the infections and how they may overlap.
And finally, the effectivity of compartmentalization depends on tree species, individual genetics, cumulative stress, tree condition (esp. energy levels), and environmental factors, along with the type of pathogens present and their relative aggressiveness.

Over the life time of a tree, there are hundreds, if not thousands, of infections.  The interaction of the tree with the pathogens is ongoing and variable.  Ultimately, all trees decay and die (or die and decay completely).  In the meantime, compartmentalization helps to reduce the degradation of structure and preserve the vascular cambium so that the tree can generate new stem tissues. 


This term is abundant in old texts and publications.  It still appears in modern day writings.  What does it mean?

Is it rot in the heart of the tree?  If so, where is the heart?

Is it rot of the heartwood?  What if there is no heartwood?

Does it imply that rot can spread at will in the heart or the heartwood?

Based on a modern understanding of trees, as demonstrated in the topics above, the term heartrot is misleading and confusing.  Rot is not limited to heartwood nor does it spread at will.

Swiecki & Barnhardt (4) use the term “stem decay” to indicate the location of rot.  This seems more accurate and appropriate.  Perhaps there are other ways to define either the location or the extent of decay that fall in line with the current understanding of tree biology.

Let’s put heartrot on the shelf with other archaic terms like deadwood, topping, drop crotching, tree surgery, etc.

AN ASIDE:  It was Denice Britton, circa 1997, that brought to Alex Shigo’s attention that the term deadwood, and the related practice of deadwooding, was inappropriate.  Shigo heartily agreed.  However, the careful reader will find the term in Shigo’s text, Modern Arboriculture as it was printed prior to that discussion.  Maybe it will be revised someday. 

Tree Profiles

Clark, etal, offers a model for tree profiles (5).  It includes various aspects of growth & development, reproduction, culture & management, and values.  Such a model provides a basis for “normal”, from which deviations could be better interpreted.

In addition to the aspects listed, I suggest that information about protection wood could extend our ability to interpret tree condition, both in living trees and for autopsy work.  For example, if I cut into a tree that doesn’t normally produce colored heartwood but the wood is dark, what does that mean?  And so on.

Other information that would be immensely useful is that similar to what is described by Schwarze, etal (6).    They show how various fungi move into and through the wood of different trees.  If this kind of information were known about our local trees, we could better interpret the potential effects of any particular fungus.  And this could be improved even further with more extensive mapping as displayed by whole tree dissections.
AN ASIDE:  I sent samples of Q. agrifolia with “SOD” to Alex Shigo (prior to quarantine).  He reported back that the vessels were plugged with tyloses.  He went on to explain that this takes time to happen and is not a normal condition in this species (agrifolia has the ability to absorb soil water whenever it’s available, which may relate to having so many vessels unplugged and its ability to grow in heavy clay soil).  The point is that if we have a thorough understanding of the tree, then we are in a better position to diagnose problems.  Have there been any discussions amongst the SOD experts about tree anatomy/biology and the possible relationship to the disease? 


Internal wood cracks are abundant in most trees.  They come in various forms and appear to persist for a long time.  It is quite possible that more tree failures occur as a result of cracks than from decay (7).

Types of Cracks

1. Circumferential cracks (ring shake, wind shake)

These can be initiated by wounding anywhere along the stem or roots and then may extend from that point a great distance up or down (with the grain of the wood).  They tend to follow the growth increment lines (depending on time of wounding), thus the term “ring shake”.

It is likely that most (if not all) of these kinds of cracks are initiated as a result of barrier zone formation (Wall 4 of the CODIT model).  As noted above, the barrier zone can be a weak structural feature.  This could be exacerbated by the swaying motion of a trunk or branch, thus the term “wind shake”.

When viewed in cross cut, the cracks have a circular pattern (following the increment).  There can be one to many in the same vicinity.  Such cracks may contribute to whole tree failure when positioned in the trunk.

AN ASIDE:  I have cut many trees, both fir and redwood, that were remnants of a forest that was logged.  In almost every one, there was a circumferential crack that separated the wood prior to logging from the wood that formed after logging.  The growth increments in the prior wood were quite close together and those after much farther apart.  I do not know if the crack was a result of the tree being wounded during logging or if it was the result of the sudden stand opening.  In either case, the logs make lousy lumber.

2. Radial cracks

Seen in cross cut stem sections, these cracks radiate across the grain of the wood.  They may be narrow (appearing closed), wide (with spaces) and/or with discolored wood about them.   They are commonly seen radiating outward from barrier zones about wounds that have been buried in wood.  Distal from these, they may appear as only “lines”.  Longitudinal dissection is needed to trace the origin of many cracks.

Callus, and later woundwood, can roll back under itself.  This may happen with or without a cavity.  The pressure of the inrolling tissue can cause cracks to occur in adjacent wood. 

3.Woundwood cracks

Callus, and later woundwood, can roll back under itself. This may happen with or without a cavity. The pressure of the inrolling tissue can cause cracks to occur in adjacent wood.

4.Included bark

When there is no protruding bark ridge between two stems, there is usually a seam that develops.  With time, this seam can develop into a crack.  In a sense, the tree wounds itself.  It is common when the branch angle is narrow and between codominant stems.  Some trees seem to produce included bark at many stem junctions

Shigo calls circumferential cracks “primary” and radial cracks “secondary”.  He says wounds, flush cuts, cankers, branch and root stubs, and dead spots from included bark in branches and roots start the cracks.  Then some other factors such as temperature extremes, drought or wind cause the cracks to split outward (1).

Reaction zones may form about the cracks and resist pathogen activity.  Cracks are commonly home to wetwood bacteria.  And cracks are “motorways” for pathogens to spread outward from internally compartmentalized wounds (6).

Sudden Branch Drop

Given enough time, most branches will break.  What causes them to break before they are dead or obviously defective has been an ongoing question for many years.  Branches have been known to “suddenly” break and drop (SBD).  The current understanding of this phenomena is summarized by Costello( 8) and Harris, etal (9)

Shigo puts forth the hypothesis that SBD is the result of internal wood cracks failing due to dryness(10).  As stated above, cracks are commonly inhabited by wetwood organisms.  Shigo observed that trees with cracks and the associated wetwood seldom failed.  It was only when the wetwood dried out that the cracks materialized into a stem or branch failure.

To test this hypothesis, he bent stems with artificial cuts to simulate cracks.  It was only when heat was applied to the cuts (to force drying) that the stems broke.  He admits this was a crude experiment, but that it would be refined and further tested by engineers at the Univ. of N.H.  I have not been able to find out if this was ever done.

The other part of this discussion is that as cracks progress as a result of loading, the branch or trunk tends to split into two “beams”.  One beam may slide or move over the other.  The wetwood may act as a lubricant.  If and as it dries out, the friction may be more than the two beams can handle and fracture occurs.

In a discussion of fracture mechanics, it is reported that plant tissues are more sensitive to water content and temperature (than metals). Also, fracture toughness across the grain can be 20 times greater than toughness parallel to the grain (11).  These thoughts may add some background to Shigo’s work.

After cutting thousands of stem parts, it is obvious that internal wood cracks are abundant.  What is not so obvious is why don’t more limbs break?  This may be answered, at least in part, by the fact that wood typically fails gradually and retains overall integrity well into the realm of irreversible condition before it actually breaks apart.  Partial failures may exist for a long time (12).

AN ASIDE:  For those that climb into trees, the presence of cracks can be “thought-provoking”.  Most of the time, cracks are not visible from the outside.  However, sometimes we can see evidence of them as stem bulges and other types of deformations.  Fluids leaking out of stems are probably a very sure sign of internal cracks (fluids are not supposed to leak out of non-
wounded stems). 

The thoughts that go though my mind when aloft center around failure thresholds.  When or what is going to make this limb break?  What if I am the straw that breaks the camel’s back?  Indeed this has happened to at least one guy that I know…who happened onto a limb of approx. 24 in. diameter which broke as he approached the midpoint.  Fortunately, his tie-in was high above him and his line was snug…he did not suffer serious bodily injury.


Our understanding of tree biology has improved significantly in the last three decades.  Perhaps the biggest contribution of this to pruning practices is the knowledge of how and where to make good cuts.  Beyond this, not much has changed.

Decisions to reduce crown density and size are still based primarily on personal experience and observation rather than from scientific methodology.  We do have strength loss guidelines that help us predict when stems will break, but these give us little to determine pruning applications.  In other words, we may have a read on some defect but actions to offset the problem are based on a high degree of subjectivity.

I do not mean to imply that our lack of objectivity should prohibit our pruning of trees.  Certainly we can continue to provide benefits to people, and to a lesser extent, the trees themselves.   But I do think we need to remember that there’s a fair amount of “by guess and by golly” and that caution may be prudent.  We should never lose sight of what we don’t know.

The following discussion is focused on mature trees.  Pruning trees when they are young can be done to a greater extent and is a time when a lot of improvement can be made.  The older ones require special considerations.


The basis for understanding the effects of pruning mature trees is founded, at least in part, upon the changes associated with aging.  Young trees have a very high percentage of living cells.  When healthy, young trees also have a high amount of foliage in relation to the amount of woody tissues.

Leaves harvest energy and living cells store it.  Energy comes from storage to make and maintain new growth, make reproductive parts and defend the tree system.  The ability of the tree to capture and utilize the energy is limited by available water, elements and space.  Most of the time, trees regulate their growth within these limits.

If they don’t, it is possible to grow beyond their means.  A tree subjected to abnormal growing conditions (e.g. high amounts of nitrogen, water, etc.) or one that with a sudden increase of space (e.g. stand disruption, etc.) may put on more growth than it can sustain over the long haul.  The tree has to put sufficient energy into storage for future needs, otherwise it may be overly affected by the things that would like to have a share of its energy (insects, diseases, etc.).

With younger trees, the abundance of energy and living cells allows them to grow and store energy without much restriction.  But as they get bigger and older, this changes because the amount of energy needed to sustain their systems gets disproportionately greater.  As the mass of the tree increases the energy needed becomes exponential.

The tree has two choices:  capture more energy or reduce the need for it.   The tree can only capture more energy if there is sufficient water, elements and space.  These usually become more limited with time as the tree gets bigger and/or competition with other plants increases.  As the tree approaches maturity and the limitations are met, growth slows. 

Along the way, the tree further conserves energy by reducing the number of living cells.  It either sheds parts (branches, nonwoody roots, etc.) or converts sapwood to protection wood.  The amount of dynamic (living) mass changes in proportion to static (dead) mass.  This changing ratio is one way of explaining how trees age.

Trees are said to be mature as the ratio approaches 1:1.  The “approach” can last for a long time if the tree is efficient at regulating its energy needs and can store an adequate amount for unforeseen draws as it endures its future.  If the tree gets close to 1:1, it has to “shrink” its system drastically or it will decline.  At 1:1, the tree will be dead.

Pruning of mature trees is somewhat of a Catch 22.  It can help the tree by reducing the amount of living cells that use up energy.  At the same time, it reduces the amount of energy available (less foliage) and reduces the amount of space to store energy (because there is a high amount of dynamic mass in branches as compared to the trunk).

Failure Thresholds

Part of the reason we prune mature trees is to reduce structural failures.  Trees break and fall over.  We think that if we reduce weight, density and/or size, these failures can be reduced.  It makes intuitive sense and is successful at least part of the time.

The problem is we just don’t know when a tree, or a part of it, is going to fail.  Certainly there are situations that present likely failures.  Obvious and severe defects or sudden changes in a tree’s surroundings may be predictable predispositions to failure.

But for the most part, failures are elusive until they happen.  Most trees don’t fail most of the time.  And since most mature trees have some degree of defect, even trees with defect don’t fail, most of the time.

We have established, through testing with small trees, a breakage threshold of one-third trunk (or branch) soundness.  However, there are many trees that stand with way less than this amount.  There are even more trees that break before this threshold has been reached.  Do they break because of the lack of wood or because there were internal wood cracks?  Most of the time, we have no idea.

Limbs don’t break and trees don’t fall over without a reason.  They fail when loading exceeds strength, with strength generally degrading over time.  As trees get older and bigger, they are subjected to an increasing number of infections that can lead to defective structure.  So the first part of the problem is evaluating strength; the second is determining when it cannot support the load.  We are a long ways from having a reliable method for either part.


Once we determine that there is a failure potential worth treating, the next question is how much mass needs to be removed in order to reduce that potential to some reasonable or significant point?  Again, we really don’t know.

Likewise, we don’t know what the extent of the effects will be on the tree, other than it will have less weight or wind resistance.  Very few people, if any, measure the energy reserves prior to pruning, let alone years after.  As discussed above, these energy reserves are critical to long term health and survival.

Our standards say we can remove up to 20 – 33% of foliage per pruning.  These limits have very little scientific basis (13).  Shigo simply says that as trees mature, there should be less living portions removed with an increase of dead parts (1).

In any case, removing some percentage of foliage does not necessarily mean there will be an equivalent amount of weight or size reduction.

1. Terminal Role?

The amount we can cut out of a tree is somewhat predicated on the size of the branches we cut to.  To avoid the problems associated with heading, we try to cut to substantial lateral branches.  This is fairly straight forward when removing a lateral to the parent stem (thinning or branch removal cut).  When a branch is to be cut back to a lateral (reduction cut, formerly drop-crotch cut), it should be done such that the remaining portion assumes the terminal role.

Our guidelines recommend that the residual part be at least one-third the diameter of the part being removed.  Raimbault, etal. says that the residual part be equal to or greater than the part being removed (14).  (If the residual part is greater, then it is probably the parent stem and therefore would be a thinning cut).

I have yet to find a clear definition of terminal role.  Is it to suppress sprout growth?  If so, reduction cuts do not do this on many species.

Is it to provide sufficient energy to the stem portions once fed by the removed part?  If so, how can a smaller lateral take the place of a larger one?  Even when the residual portion is the same diameter as the removed part (codominance), there is probably a loss of energy.

2. Repeated pruning.

Things that are sudden and repeated have the most potential to negatively impact trees.  Pruning is a sudden event.  If this is done repeatedly, the negative effects are more pronounced.

There are ways to keep a big tree small with repeated prunings and not severely harm the tree.  These must be started early in the life of the tree and as it approaches the desired size or form.  Follow-up prunings should be done at regular intervals, cutting only small amounts of young growth off at each time. 

Attempting to make a big tree smaller after it gets beyond the desired size comes with well-understood structural and health problems.   Crown reduction is an unnatural act.  It is forcing the tree to go backwards.  And while this may have a desired dwarfing effect, it may also accelerate aging and possibly a decline of health.

Certainly the negative impact is greatest when heading (topping) is applied.  At what point does crown reduction, using reduction cuts, become topping?

Even if it is possible to return to a tree on a regular basis and take very small amounts off at each pruning, there may still be undesirable long-term effects.   It is relevant to remember that pruning in the outer crown reduces dynamic mass more than static mass.  This may slow growth, but it can also make the tree less able to defend itself.

Finally, pruning of any kind should be done for a good reason.  So called “regular” pruning may make money but may be a hardship for the tree.

AN ASIDE:  The City of Sebastopol has recently adopted an ordinance that requires a Removal Permit when a tree is topped (removal of one-third or more of the crown).  It appears that some people are recognizing that such pruning is what I call “first stage” removal.  How many trees have been cut back “one-third” thinking that they were following the guideline of the allowable foliage reduction?

Structural Modification

If most trees do a pretty good job of making a crown, then what happens if it is modified?  Does taking weight off one portion of a limb cause undue stress on another portion?  Does thinning out of a crown allow for an increase of forces on the remaining portions?  What are the dynamics of a crown and what happens when it is altered?

Trees grow in small increments over a long time.  This is even more pronounced as a tree enters maturity.  As said before, branches, trunks, and roots grow in proportion to what is available (water, elements, space, etc.) and in relation to each other.  A mature crown (and root sytem) is usually a very complex matrix.  And every branch has its own character.

Removing portions of a branch may affect how it reacts to wind and gravity.  It may twist, turn and sway differently.   This is compounded over the whole tree when the crown is thinned or reduced.   To my knowledge, the concept of tree dynamics is rather new and offers a whole lot of future for discussion and consideration. 

Pruning cuts have the potential for long-term changes in structural integrity.  If the cuts are small, properly made and the tree is in good health, the impact should be minimal.  As the size of cuts and/or the degree of error increases, there is a greater likelihood for wood and bark degradation. 

All pruning cuts are wounds.  They all require energy for closure and protection.  Poor cuts draw even more energy for protection.  At the same time, improper cuts can reduce energy storage space and initiate internal cracks.  Poor tree health will exacerbate these problems.
Dead Branch Removal

Removing dead branches is done for both the health of people and trees.  All dead branches will at some point break and fall.  While they are attached to the tree, they can increase the incidence of infections because they give fuel to potential pathogens (1).

While this justification for pruning is given many times throughout his texts and talks, Shigo also has said that most trees, most of the time, do a fine job of protecting themselves (7). Trees did not evolve with humanized pruning.  They did evolve with protection mechanisms that enabled them to shed branches without succumbing to an onslaught of microorganisms.

Those species that could not effectively protect themselves, perished.  Individuals of surviving species that come with inherent protection deficits tend to rot and collapse quicker than their counterparts. 

As branches die, they are inhabited by microorganisms.  Perhaps they will build up in force and travel into the tree.  Or they may just degrade the branch so it breaks away.  At the same time, substances are deposited at the base of the branch and within the trunk.  These help to resist the inward spread of the microorganisms.

Since it usually takes several years for the shedding process to complete, the branch collar can enlarge as the trunk keeps growing.  I wonder if this enlarged collar is more protective than if the branch were cut soon after it begins to wane.  In places where I have cut dying branches, I have noticed dead areas below the cut on the trunk or parent branch (one year or longer after the cut was made).  These dead spots did not occur where branches were allowed to die completely before pruning.

Certainly this is only a casual observation and doesn’t occur all of the time in all trees.  But still, I wonder if there are advantages (to the tree) of allowing branches to completely die before removing them.
Margins of Error

1.The Pucker Factor.

Just as crown reduction is an unnatural act for a tree, accessing the outer regions of a crown can be contrary to climber safety.  Climbers need a high tie-in point to support their weight.  This becomes most critical when advancing out to a branch end.  Progressing upwards, the benefit of the tie-in decreases as the angle of support widens.

Skilled climbers look for ways to offset the loss of support.  Redirects and secondary lines are two of these.  Regardless of how clever and tough the individual, there almost always seems to be parts of a tree that raise the hair a bit.  It’s times like these that compromises are often sought.

Don Blair (in some memorable talk at some forgotten place) said that half the job is getting into position to make a good cut; the other half is making the cut.  When we struggle to get into position, our cuts can be less than perfect.  If we’re a little fearful, the level of care can become secondary to our own self.

Making a good cut requires placing the cutting edge perpendicular to the branch.  This is best achieved when the hand holding the tool is close to the cutting edge such as with a hand saw, hand pruners or chain saw (although chain saws have a mind of their own).  In order to lessen the strain of getting to the outer fringes, we turn to extension tools (e.g. pole pruners, pole saws, etc.).  It is just about impossible to get the cutting edge of these tools into optimal position for all of the various ways that branches grow.

2. Lost in Translation.

There can be a gap between the person who determines the need of treatment and the person implementing it.  Usually the first person is of higher qualification, possibly a consultant.  Job specifications are written and the second person takes them to the tree.

If these specifications are precise, the margin of error is more likely to be small.  When they say something like “remove all dead branches over one inch in diameter”, it is fairly clear what the directive is (do we ever get them “all”?).

On the other hand, if the instruction is to “lighten all heavy limbs” or “reduce branch end weight”, there is a lot of room for interpretation.  What the person on the ground, who made up the prescription, had in mind can be very different than what the cutter will do.

This sort of problem is at the extreme when a consultant of good professional standing writes specifications for a job that goes out to bid.  Most bids are awarded to the low price contractor.  Often (not always) the low price means that something has to be compromised.  If it’s a time factor, then corners will be cut.  If it’s a matter of a lesser skill level, then the quality of pruning may be sacrificed.

Even when contractors are “pre-qualified”, there is no guarantee that pruning quality will be high.  It is not uncommon for the lead person of a firm to be eminently qualified, while  the people that do the actual cutting be less so.  There has always been a demand for skilled pruners.  This will probably not change as long as pay scales remain disproportionate to the difficulty of the trade, and the trade itself is under valued.

3. Jobs are about money.

Rarely is tree pruning done just because we care.  Occasionally services are donated.  But most of the time, we are on a job because we get paid.  This pay usually defines the limits of our efforts and the amount of time taken to deliver them.

Once in a great while, the customer will say they will do whatever it takes to help the tree (and really mean it).  The rest of the time we do what we can with what they will give us.  It’s almost always a compromise.  Sometimes this means we will do less; other times it can mean we will make more errors as we try to get the job done before it gets dark. 


The above discussion is based on observations and research done primarily on native forest trees and remnants thereof.  These trees, and their associates, are an evolutionary product spanning a long time, being modified to some extent over the last 10,000 years by Native Americans.  In the last 200 years, our forests have undergone some changes that may impact our tree care practices.

Perhaps the most significant change has been the removal of the biggest and oldest trees.  Very few of the pre-European settlement trees are left.  In a very short time, our forests were high-graded.  What is left behind may be the low end of the gene pool. 

If this is true, then we may have a lot of trees that are less able to withstand the insects and diseases that were part of the forests.   An example might be with the ability of trees to resist infection via dead and dying branches.  Shigo’s work revealed that this ability was under strong to moderate genetic control.  Gene pool shifts could affect this ability.

Another change that is beginning to get some serious attention is the suppression of fire.  Many of our forests were altered and managed with fire by the Native Americans.  This practice modified the preexisting landscape in ways that favored their cultures.  

Not long after European settlement, the native people were forced to abandon the use of fire as their land base was reduced.  And not long after this, the suppression of wildfire began.  In places where logging occurred, the changes in forest composition and health have become even more dramatic.

We have what appear to be unprecedented forest declines and death.  There is hardly a region in the western part of this hemisphere (at least) that doesn’t have an example of these forest losses.  Whether this will have a direct impact on our urban landscapes is unknown, but it could certainly have an indirect effect.  At a minimum, most of our urban water comes from rural watersheds, so what happens out there affects downstream.

The line between rural and urban landscapes is becoming increasingly blurred.   More people are living in this interface than ever before.  With them come exotic plant species, followed by their associated insects and diseases. 

As the old forest remnants disappear, more nursery products are introduced.  Whether grown from seed or by cloning, there is bound to be some loss of genetic strengths. 

Add to these changes all of the things that people do to live.  We use a lot of water, affect the air and soil, take up space, harvest wood…all things that can have an impact on trees.  As the world gets smaller, trees and their habitats are getting increasingly pressured. 

Tree care people work hard to keep trees healthy and intact.  But it may take more than hard work to keep up with our fast changing landscape.  A more comprehensive vision, supported by sound research, are two things that could help us with our mission.

Some suggestions for improving tree care

1. Develop species profiles.
            Include information about protection wood.
 Which trees form heartwood/false heartwood and in what color?
 Where is wetwood likely to be found?

2. Develop pathogen profiles.
            Adopt the work of Schwartz, etal.,  to our local trees.
            Include longitudinal dissections, on both low and high vitality trees, to map fungal extent and interactions.

3. Research cracks.
            Investigate the hypothesis that crack drying leads to failure.
            Study the patterns and prevalence of cracks.

4. Define maturity.
            What outward signs indicate when a tree is mature (e.g. growth rates, form changes, etc.)?
            What internal conditions indicate maturity (e.g. number of sapwood increments, amount of protection wood, etc.)?
            Use maturity as basis for determining pruning dose.
6. Establish failure thresholds.
            Expand our current experience with strength loss ratios to include whole tree analysis.
            Integrate tree size, density and crown configuration with environmental factors.
            Such analysis would be quite complicated and would require quantum leaps in our technologies.
            In addition to looking at why trees fail, look at why trees don’t.  How come those trees that should break or fall down, haven’t?

7. Test pruning treatments.
            Use the failure thresholds to test various types and degrees of pruning.
            Evaluate the effects on tree health, both short and long-term.

8. Propagate strong trees.
            Find ways to select stock for low incidence of bark inclusion, strong branch protection zones, and any other traits that could help reduce failures.

9. Study original forest conditions for clues today.
            Look at relationships in these forests for possible answers to what we see today. 
            Consider the role fire played and ways to reproduce its benefits.          
10 .Understand and disclose limitations.
            Recognize the limitations of pruning and that it may compromise the health and longevity of trees. 

1.Shigo, A.L. , 1993.  A New Tree Biology.  Shigo and Trees, Associates, Durham, NH.
____________, 1991.  Modern Arboriculture.                            “
____________, 1986.  A New Tree Biology Dictionary.              “

2.Smith, Kevin T., Personal Communication.

3.Smith, Kevin T.,  “Compartmentalization Today”, Arboricultural Journ. 29:173-184.

4.Swiecki, Tedmund J. &  Elizabeth Bernhardt, 2006.  A Field Guide to Insects and Diseases of California Oaks.  Gen.Tech Rep. PSW-GTR-197.  Albany, Ca.

5.Clark, James R., Nelda Matheny & Joe McNeil, 1990.  “Developing A Species Profile”, J.Arboriculture 16(5):101-107.

6.Schwarze, Francis W.M.R., Julia Engels & Claus Mattheck, 2004.  Fungal Strategies of Wood Decay in Trees.  Springer-Verlag, Heidelberg, Germany.

7.Shigo, Alex, Personal Communication.

8.Costello, L.R., 2005.  “Sudden Branch Drop:  A case for closer inspection”, Western Arborist, 31(4): 14-15.

9.Harris, Richard W., James R. Clark, & Nelda P. Matheny, 2004.  Arboriculture:  Integrated Management of LandscapeTrees, Shrubs and Vines.  Prentice Hall, Upper Saddle River, NJ.

10.Shigo, A.L., 1989.  “Branch Failures:  A Closer Look at Crack Drying”, J.Arboriculture 15(1):

11.Farquhar, Tony & Yong Zhao, 2006.  “Fracture Mechanics and its Relevance to Botanical Structures”, American J. of Botany, 93(10):1449-1454.

12.Koehler, Lothar & Frank Telewski, 2006.  “Biomechanics and Transgenic Wood”, American J. of Botany, 93(10):1433-1438.

13.Phillips, John M., 1998.  “Challenging Current Pruning Practices”, WCISA Annual Meeting, Yosemite, Ca.

14.Raimbault, P. (et al), translated 1998 by Scott & Manuele Mayer.  “Management of OrnamentalTrees”, Revue Forestiere Francaise, 1993 & 1995.

Northcoast Tree Care - Essays & Articles

Why Do Trees Die?

Flush Pruning Cuts

Challenging Current Pruning

Thoughts About Protection Wood, Crack & Pruning - Text

Thoughts About Protection Wood, Crack & Pruning - Photo Album