Tree Growth and Decay

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As forest scientists observed how trees respond to wounds, pruning techniques changed and pruning objectives were clarified. 

This CMG GardenNotes provides background information on how trees grow and decay and therefore the implications of pruning cuts and structural training.  Refer to other CMG GardenNotes for additional details on pruning cuts and structural training.

Note: in this publication, the term “trunk” refers to a trunk or parent branch and “side branch” refers to a side branch arising from the trunk (parent branch).  The same relationship would exist between a side branch and a secondary side branch.


Developing Strong Branch Unions

In Colorado (and other snowy climates), the most common type of storm damage in landscape trees results from failures at the branch union (crotch) primarily with codominant trunks (adjacent trunks of similar size).  Primary objectives in training young trees are to develop strong branch unions and eliminate structurally weak codominant trunks. Figure 1]

Storm damage with codominant trunks
Figure 1.  codominant trunks (adjacent trunks of similar size) account for the majority of storm damage in Colorado landscapes.



Structural strength of a branch union is based on the development of a branch collar.  The branch collar is where the annual growth rings of the trunk overlap the annual growth rings of the side branch, like shuffling a deck of cards.  In lumber the branch collar is called the knot. Figures 2 and 3]

Branch Collar and Branch Bark Ridge
Figure 2.  Structural strength of the branch union (crotch) is based on development of a branch collar.


Branch collar
Figure 3.  The branch collar is where annual growth rings of the trunk overlap the annual growth rings of the side branch like shuffling a deck of cards.  This creates a very solid section of wood, known as the “knot” in lumber. [Line drawing U.S.D.A.]


As the branch collar develops, side branch tissues connect into the trunk in a wedge shape, making a structurally strong branch union. For the branch collar to develop, the diameter of the side branch must be less than half the diameters of the adjacent trunk/parent branch. Less than one-third is preferred. 

If the side branch is too large in diameter, prune back the side branch by 33-67% to slow growth (or remove the branch entirely). Over a period of years, a branch collar will develop. [Figure 4]

Size relationship for branch collar development
Figure 4.  As the branch collar develops, side branch tissues connect into the trunk in a wedge shape making a structurally strong collar.  For the branch collar to develop the diameter of the side branch must be less than half the diameter of the adjacent trunk/parent branch.  Less than one-third is preferred.


The size relationship between the trunk and side branch is called Aspect Ratio.  A branch union with high aspect ratio, like 1 to 1 (two trunks of the same diameter), is highly prone to failure in wind and snow loading.  A branch union with a low aspect ratio, like 1 to 3 (side branch is 1/3 the diameter of the adjacent trunk), would not likely fail due to the development of the branch collar.

A branch collar will not develop on codominant trunks (adjoining trunks of similar size) making this branch union structurally weak.  [Figure 5]

Multiple branches arising at the same location also compromise the branch collar’s structural strength.  Some tree species (like elm, maple, and crabapple) naturally develop multiple branches at one location. This predisposes the tree to storm damage if the situation is not corrected by structural training when the tree is young.  [Figure 5]

Co-dominant trunk and multiple branching leads to tree failures
Figure 5.  (Left) A branch collar does not develop on codominant trunks, making the branch union structurally weak.  (Right) Multiple branches arising at the same location are also structurally weak as the branch collars can’t knit together into a strong union.


Minimizing the spread of decay — Due to the constriction of the xylem cells where the side branch annual growth rings are overlapped by the trunk annual growth rings, the development of a branch collar significantly reduces the potential spread of decay.  Also branch unions with a right angle of attachment are more effective in preventing the spread of decay. 

To reduce the potential for decay, 1) prune to develop branch collars (the side branch must be less than half the diameter of the adjacent trunk) and 2) select branch unions with a wide angle of attachment.  In pruning, remove codominant trunks and narrow branch unions while young (smaller than two inches).   [Figure 6]


Branh union and decayFigure 6.  Branch unions that form a right angle are more resistant to decay.  A branch union with codominant trunks and a narrow angle of attachment is highly prone to the spread of decay.


How Trees Grow

Xylem tissues – Each year a tree puts on a new outer ring of wood (xylem tissue) under the bark resulting in the increased diameter of a trunk or branch.  The number of rings indicates the limb’s age and the width of individual rings indicates that year’s growing conditions.  [Figures 7 and 8]


Cross section of tree with bark, phloem, cambium and xylemFigure 7.  Tree cross section
Bark – outer protective covering
Phloem, inner bark(red in drawing) – Photosynthates (sugars and carbohydrates produced in the leaves by photosynthesis) move throughout the tree in the phloem tissues, including down to feed the roots.
Cambial Zone (yellow in drawing) – layer of active cell division.
Xylem (brown layers in drawing) – Each year the cambium adds a new ring of xylem tissue just under the cambium layer, resulting in a growth in limb diameter.  Xylem tissues are the technical name for the “wood”.


Wood showing annual xylem growth rings
Figure 8.  The “wood” of a tree is the xylem tissue.  Xylem tissues that grew in the spring and early summer enlarge and are the tubes in which water with minerals flows from the roots to the leaves.  In a cross-section of the log, these are light colored rings.  Xylem tissues that grew mid-summer, at the end of the growth cycle, are higher in fiber content creating a wall to the outside.  In a cross-section of a log, these are the darker colored rings.


Younger annual growth rings of xylem tissues (living cells active in water transport and storage of photosynthates) are called sapwood.  Depending on the species and vigor, sapwood is comprises the five youngest (outer) annual growth rings.   Heartwood, the older annual growth rings no longer active in water transport, is very susceptible to decay organisms. Due to chemical changes in these non-living cells, heartwood is often darker in color.  [Figure 9]

Sapwood and heartwood
Figure 9.  On this Douglas fir log, the sapwood is the light colored annual growth rings active in water transport and storage of photosynthates.  The darker colored heartwood in the center has no resistance to decay.



Ray cells grow through the annual growth rings functioning like staples or nails to hold the growth rings together.  Ray cells also function as the path to move photosynthates in and out of storage in the xylem tissues.  On some species, ray cells are not readily visible.  On other species, ray cells create interesting patterns in the wood.  [Figure 4]

ray cells
Figure 10.  The cracks on this willow stump show ray cells. 



The wood is a series of boxes or “compartments” framed by the annual growth rings and ray cells.  Each compartment is filled with xylem tubes in which water with minerals moves from the roots to the leaves.  [Figures 11 and 12]

Cross section of tree showing annual grown rings and ray cells
Figure 11.  The xylem tissues (wood) is a serious of compartments or boxes created by the annual growth rings and ray cells.


Comparmentalizaiton in tree with xylem tubes
Figure 12.  Each compartment or box framed by the annual growth rings and ray cells is filled with xylem tubes.  Water moves in the xylem tubes up from the roots.


CODIT: Compartmentalization of Decay in Trees (How Trees Decay)

Unlike animals and people, trees don’t replace damaged tissues.  Rather, cells in the damaged area undergo a chemical change in an method to seal off or “compartmentalize” the damaged area from the spread of decay.  This area of chemical change is called the reaction zone.  In most species, a reaction zone appears as darker colored wood. 

The spread of decay is related to this compartmentalization of the xylem tubes in a box-like structure created by the annual growth rings and ray cells.  In this box-like structure, the four walls differ in their resistance to the spread of decay.  [Figures 13 and 14]

  • Wall 1 – Resistance to the spread of decay is very weak up and down inside the xylem tubes.  Otherwise, the tubes would plug, stopping the flow of water, and kill the plant.  From the point of injury, decay moves upwards to a small degree, but readily moves downward.  The downward movement may be 20 or more feet and can include the root system.

  • Wall 2 – The walls into the older xylem tissues (towards the center of the tree) are also rather weak allowing decay to readily move into older annual growth rings.

  • Wall 3 – The walls created by the ray cells (being high in photosynthates) are somewhat resistant to decay organisms.  This may help suppress the spread of decay around the tree. 

  • Wall 4 – New annual growth rings that grow in years after the injury are highly resistant to the spread of decay.

Resistance to the spread of decay by the new annual growth ring and ray cells creates a pipe-like structure, with a decayed center.  This concept of how decay spreads in a tree (as controlled by the annual growth rings and ray cells) is called CODIT, for Compartmentalization Of  Decay In Trees.  [Figures 13 and 14]

Decay walls in tree
Figure 13.  Spread of decay in trees - The spread of decay in trees is suppressed by the four walls created by compartmentalization of the annual growth rings and ray cells.
In the drawing, injury occurred three years ago when the yellow colored annual growth ring was the youngest. That year and everything older (grayed annual growth rings) are subject to a reaction zone and decay.  The two new annual growth rings (brown color) that grew in years after the injury are highly resistant to decay. 


Hollow limb from decay
Figure 14.  Decay in a tree creates a pipe-like structure with a hollow center.  The light colored wood is new annual growth rings that grew after the year of injury.  The darker colored ring is a reaction zone created in the sapwood.  The heartwood has completely decayed away.


Evaluating Decay

A trunk or branch with some internal decay is not necessarily at risk for failure.  Structural strength is based on 1) the minimum thickness of the healthy wood (xylem tissues) and 2) the strength of wood (species).

In evaluating potential hazards, arborists (tree care professionals) work with a technical term called percent shell.  Percent shell is calculated by dividing the thickness of the healthy wood at the thinnest point (not including bark, reaction wood, or decaying tissue) by the radius of the trunk/branch (not including bark).

  • Thirty-three percent shell = high risk potential – Trees with a 33% shell or less are termed “high risk” with a statistically high probability of failure in a storm event.  For example, a six-inch diameter (three-inch radius) trunk with only a one inch thick ring of healthy wood would have a 33% shell with a hollow center.  If injury or property damage would occur upon tree failure, corrective action (such as removal of the defective branch or removal of the tree) should be considered.

  • Twenty percent shell = critical risk potential – Trees with a 20% shell or less are considered a “critical risk” with a very high probability of failure in storms.  For example, a tree with a ten-inch diameter (five-inch radius) trunk with only one inch ring of healthy wood would be considered a “critical risk”.  If injury or property damage would occur upon tree failure, corrective action (such as removal of the defective branch or removal of the tree) should be taken.  [Figure 15]

Hollow limb from decay
Figure 15.  This cottonwood branch has a 25% shell, putting it at “high risk” for potential failure.  Percent shell is measured by dividing the thickness of the healthy wood at its narrowest point [not including the reaction wood (darker ring towards the center) and the bark] by the radius of the limb (not including bark).


The Percent Shell Formula is valid only when the decay column is centered in the trunk/branch.  Researchers are developing other formulas to evaluate off-sided decay and open cavities, which are significantly weaker.

On older mature trees, these percent shell standards may overstate the thickness of healthy wood needed to be structurally acceptable. Additional research is needed to better clarify this standard for older/mature trees.

Measuring Decay (Percent Shell)

So, how thick is the healthy wood in a trunk or branch?  Researchers are working to address this big question.  At the present time, arborists are limited in their ability to measure and evaluate the internal structure of a trunk or limb.  The following are procedures with limited potential to evaluate the internal structure of trees.

Visual indicators of decay
  • Large pruning wounds suggest the potential for internal decay.  Often decay may be observed within the pruning wound.  [Figure 16]

Decay in pruning wound
Figure 16.  The black material in the pruning cut is decay fungus.  Notice the cracking; it also raises flags of structural integrity.



  • Cankers suggest the potential for internal decay.  If the canker extends down into the soil, decay organisms will always be actively.
  • Valleys, ridges, cracks, and splits along the trunk/branch suggest the potential for decay.
  • Wildlife living inside the tree is a sign of decay.
  • Abnormal swellings or shapes could be a sign that the tree is growing around a decayed area.
Coring Devices

Note:  All coring devices have a small potential to spread decay, as the coring tools break the strong exterior wall of a reaction zone and bring decaying tissues out though healthy wood in the removal.  Thus they are generally not used on living trees except when there is a special need to evaluate risk potential.  Coring devices only indicate the decay potential at the point of drilling and do not represent the entire trunk or branch.

  • An Increment Borer is a hand tool that removes a small core from a trunk or branch.  The relative effort it takes to drill the borer through various layers of the tree and examination of the core removed gives the arborist some idea about the internal structure at this location.  Increment borers are rarely used today in arboriculture.

  • Drill with small drill bit – Drilling the trunk or branch with a 1/8 inch fully fluted drill bit is a tool used by some arborists.  Pressure to push the drill through the annual growth rings and examination of the sawdust removed gives the arborist some idea about the internal structure at this location.  An experienced arborist can be rather accurate in evaluation by drilling.  Drilling has little value, however, for the inexperienced person.  [Figure 17]

Drilling tree to evaluate decay
Figure 17.  Examination of the sawdust and the pressure to push the drill through the annual growth rings give the experienced arborist a handle on internal decay.  The yellow ear plug on the drill bit helps the arborist know the depth as he works the drill bit in and out.  With experience, the arborist could estimate percent shell for the spot drilled.


  • A Resistograph is a specialized drill that graphs the pressure needed to push a small drill bit through various layers of annual growth rings.  The graph gives a visual indication of internal structure at this location.  Due to cost, few arborists have a Resistograph.  [Figure 18]

Resistograph printout

Figure 18.  Sample printout of resistograph – This tree has a decayed center at 4½” from the outside bark.


  • A Digital Microprobe, a specialized drill bit rotating at 7,000 rpm, measures the pressure needed to drill/burn its way through tissues.  Data is fed into a computer database for evaluation and printout.  This equipment is new to the industry and cost prohibitive for most arborists.
Listening and radar devices
  • Rubber mallet – Tapping the trunk/branch with a rubber mallet and listening for a hollow sound may give some indication of critical internal decay.  It won’t give any percent shell to help evaluate risk potential and may not be effective on thick bark trees (like old cottonwoods).  However, do not totally discount this technique, as it is often all that is available.

  • PiCUS Sonic Tomography is a new device that listens to how sound waves move through the trunk/branch.  A series of listening devices are attached around the trunk/branch and connected to a computer.  When the tree is tapped with a mallet, the computer measures how the sound moves through the wood and creates a graphic cross-section of the trunk/branch interior.  Measurements taken at multiple heights up the trunk can generate a three-dimensional image.  This type of equipment has the potential to totally change tree care when it becomes available to arborists.  Currently the cost is prohibitive for most arborists.

PiCUS Sonic Tomograph
Figure 19. PiCUS tomography listens to the movement of sound through the tree and draws a picture of the tree's internal structure.



  • Electrical Impedance Tomography is similar to sonic tomography and measures the distortion of the electrical field by wood conditions.  Electrical impedance tomography is better at detecting “Y” crevices and cracks and thus is often used in conjunction with sonic tomography.

  • Tree Radar – A hand held radar device is run around the trunk/branch.  The computer database is sent to the company for evaluation.  Currently the cost is prohibitive for most arborists.

Tree Radar
Figure 20. Tree radar taking a look at the tree's internal wood structure.

Breaks in the Pipe-Like Structure

When a wound or pruning cut breaks the pipe-like structure of a trunk/branch, the tree is especially weak at this location creating a higher potential for tree failure.  [Figure 21]

Tree decay
Figure 21.  Structural strength is significantly compromised when the pipe-like structure of a trunk has a break in the cylinder wall.


Lack of Trunk/Branch Taper

Branch failure (often breaking a few feet to 1/3 of the branch length out from the branch union) is a common type of storm damage.  Branch failures often cause minimal damage to the tree.  However, failure of a major branch may create holes in the tree canopy, introduce decay and cracking, and make the tree look unacceptable.  Trunk failure refers to breaking of the lower trunk, above ground level (not at a branch union).

Branch and trunk failures are associated with lack of trunk/branch taper.  That is the trunk/branch does not thicken adequately moving down the trunk/branch.  This can be caused by pruning up the trunk too fast and by removing small branches and twigs on the lower trunk or lower interior canopy of the tree.

Very upright branches without a lot of side branches also typically fail to develop adequate taper.  For structural integrity, shorten these branches with appropriate reduction cuts. 


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Additional Information

CMG GardensNotes on pruning

Books (available from the International Society of Arboriculture)

  • An Illustrated Guide to Pruning, Third Edition. Edward F Gilman. Cengage Learning. 2011
  • ANSI A300 Pruning Standards, Part 1. American National Standards Institute. 2008
  • Best Management Practices (Revised 2008). Edward Gilman and Sharon Lilly. International Society of Arboriculture. 2008.

Web


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Authors: David Whiting (CSU Extension, retired) with Carol O'Meara (CSU Extension). Photographs and line drawings by David Whiting; used by permission.

  • CMG GardenNotes are available online at www.cmg.colostate.edu
  • Colorado Master Gardener/Colorado Gardener Certificate Training is made possible by a grant from the Colorado Garden Show, Inc.
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  • Colorado Master Gardener LogoNo endorsements of products mentioned is intended nor is criticism implied of products not mentioned.
  • Copyright. 2010-14. Colorado Master Gardener Program, Colorado State University Extension. All Rights Reserved. CMG GardenNotes may be reproduced without change or additions, for nonprofit educational use.

Revised October 2014

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