As forest scientists observed how trees respond to wounds, pruning techniques changed and pruning objectives were clarified.  Plants don’t heal.   Instead tissues in the injured area, called the reaction zone,undergo chemical changes to compartmentalize or seal-off the damaged area from the surrounding tissues in an attempt to suppress the spread of decay. 

This fact sheet 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 barnch and “side branch” to refer to a side branch arising from the trunk (parent branch).  The same relationship would exist between a side branch and a secondary side branch.

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 1 and 2]

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 3]

The trunk is a series of boxes or “compartments” framed by the annual growth rings (xylem tissue high in fiber that grew towards the end of the annual growth cycle) and ray cells.  Each compartment is filled with xylem tissues, tubes in which water with minerals move from the roots to the leaves.  [Figure 4]

 

CODIT – 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 effort 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.  [Figures 5 and 6]

The reaction zone response is very weak up and down the xylem tubes; otherwise, the tubes would plug, stopping the flow of water, and kill the plant.  So, decay can readily spreads up and down in the xylem tissues.  (Side note: A few diseases, like Dutch Elm Disease and Oak Wilt, kill the limb as fungal activity plugs the xylem tissues thus restricting water and nutrient flow.)  

The wall into the older xylem tissues (towards the center of the tree) is also rather weak.  The walls created by the ray cells are somewhat resistant to spreading decay around the tree (depending on species).  The wall towards the outside (made by the new annual growth ring laid down after the injury occurs) is rather resistant to decay.

Resistance to the spread of decay by the outside 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.

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 type of wood.

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). 

33 percent shell = high risk potential – Trees with a 33 percent shell or less are termed “high risk” with a statistically high probability of failure in a storm event.  For example, a six inch 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 (removal of the defective branch or removal of the tree) should be considered.

20 percent shell = critical risk potential – Trees with a 20 percent shell or less are considered a “critical risk” with a very high probability of failure in storms.  For example, a tree with a 10 inch 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 (removal of the defective branch or removal of the tree) should be taken.  [Figure 7]

 

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.

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.
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 brings 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.  All coring devices only indicate the decay potential at the point of drilling and don’t represent the entire trunk or branch.

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 ideas about the internal structure at this location.  Increment borer are rarely used today in arboriculture.

Drill with small drill bit – Drilling the trunk or branch with a 1/4 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.

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 8]

Digital Microprobe, a specialized drill bit rotating at 7,000 rpm, measures the pressure needed to drill/burn its way through tissues.  Data if 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 on thick bark trees (like old cottonwoods) may not be very effective.  However, don’t 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.

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.

Visual indicators of decay:

Large pruning wounds suggest the potential for internal decay.  Often decay may be observed within the pruning wound.  [Figure 9]

Cankers suggest the potential for internal decay.  If the canker extends down into the soil, decay organisms will always be actively developing.

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.

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. 

Additional Information – CMG GardenNotes on pruning

• #611   Tree Growth and Decay
• #612   Developing Strong Branch Unions
• #613   Pruning Cuts
• #614   Structural Training of Young Shade Trees
• #615   Structural Training– Pruning Flow Chart
• #616   Pruning Mature Shade Trees
• #618   Pruning Evergreens
• #619   Pruning Flowering Shrubs
• #620   Structural Pruning Summary

Books – Edward F Gilman. An Illustrated Guide to Pruning, Second Edition. Delmar. 2002

Web – http://hort.ifas.ufl.edu/woody/pruning/

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