Natural Notes: Seven Main Defects of Trees

This blog post consists of my notes from a lecture on urban street tree defects, presented by Nick Kuhn, Resource Management Technician with the Forest Preserves of Cook County.  The presentation was delivered on September 21, 2016 at Sand Ridge Nature Center in South Holland, Illinois.  
Introduction
All trees experience structural failures in part or whole, including healthy trees.  It is impossible to eliminate all risks associated with tree failure.  However, recognizing signs or symptoms of compromised tree health can help mitigate or prevent problems.

It is important to distinguish signs versus symptoms regarding tree health problems. Signs are physical pieces of evidence of tree health problems.  For example, D-shaped holes created by Emerald Ash Borer on a dying Ash tree are signs of an infestation.  Symptoms, on the other hand, are physical evidence that indicates a problem.  For example, excess epicormic branches emanating from the base of an ash tree could be an indication of emerald ash borer since ash trees will send out multiple shoots when under stress.

Seven Main Categories of Defects

 (1) Decay.  Decay is the structural strength loss of a tree.  Visible signs and symptoms include sunken bark, fungal growth, bulges in trunks and stems, and other signs.  Decay often emanates from a wound.  Decay becomes a high risk factor when there a wound or opening that is greater than or equal to 40 % of the tree's diameter at breast height (DBH) and/or when less than 1% of the tree's outer shell is compromised (see figure 1).  Because decay usually happens inside of the tree, it is generally not feasible to determine the extent of decay by just looking at the outside of the tree.  The use os a resistor graph, a tool used to measure tree decay, can be used to evaluate tree rot.  With the tool, one can drill a small hole into the tree and obtain a measurement of resistance.  The tool delivers an output graph showing resistance through the diameter of the tree.  Low resistance equals a lower value.  Figure 2 shows an example of a simplified output graph showing the greatest amount of decay (lowest values) towards the middle of the tree (perhaps heart rot?).

Figure 1:  High risk factors for tree decay
Figure 2:  Tree resistance graph
(2) Cracks.  Cracks can occur near the separation of bark or wood, from wind, weak branch unions, ice, or improper bud pruning.  Sometimes cracking is the only visible indicator of an internal defect, often found on an area of abnormal growth such as a seem (figure 4).  High risk factor for cracking include splits that go all the way through the tree, multiple cracks in one section, horizontal cracks, and cracks on the co-dominate leader.  Figures 3-6 show visual clues for different types of cracking.

Figure 3

Figure 4

Figure 5

Figure 6


(3) Roots.  Root problems can originate from abiotic and biotic factors.  Abiotic factors include root burial, cutting, chemical applications, and soil compaction.   Biotic factors include fungus, flooding, and root decay.  Symptom of root problems include canopy decline, a sudden lean, or stem girdling roots (SGR).  Signs include trenching, grading from recent construction activities, exposed soil and roots, sudden lean with exposed roots visible, as well as extreme flooding or drought.

Before or during construction activities, construction contractors and others will attempt to protect existing trees.  Some people will determine the area in which to protect the tree based upon the tree's drip line or a perceived critical root zone.  These criteria for protecting trees are often too vague or are inadequate for protection of tree roots.  A more specific and more effective criteria for protecting trees during construction activities would be the establishment of a tree protection zone (TPZ), which takes into account the true extent of a tree's roots as well as inherent;y limited spaces for root volume such as trees planted along sidewalks.

High risk factors for tree root problems include a recent lean, greater than 40% root loss, and greater than 40% SGR.  It should be noted that tap roots only compromise a small percentage of all roots in a mature tree.  Most roots spread well beyond the drip line and extend in all directions.  Also, fungal growth at the root color is almost always a certain indication of decay.

(4) Weak branch unions.  Weak branch unions include included bark and epicormic branching (see figure 7).  High risk factors include cracks and decay, and are most concerning on codominate branches with a v-shaped union (see figure 7).  A U-shaped union with a 45 degree angle is ideal for structural soundness of branch unions.
Figure 7:  Branch unions.  
(5) Cankers.  Cankers are caused by fungus, insects, or storm damage and can prevent normal growth.  Cankers can cause structural failure of branches (figure 8) or trunks (figure 9).

Figure 8:  Canker
Figure 9:  "Target" canker


There is growing concern about the thousand canker bug (Pityopthlorus juglandis).  This invasive beetle creates many small cankers that are hidden beneath tree bark.  The beetle is not directly responsible for the creation of these cankers but rather they carry a fungus that does.  Black walnuts (Juglans nigra) is the host tree for this insect.

(6) Poor architecture.  Poor tree growth architecture is often caused by a poor growth pattern, perhaps the result of environmental changes or damage.  It can be natural or human-caused.  Recent changes in the trunk or roots along with a sudden lean are red flags.  High risk factors include a greater than or equal to 40% lean, a lean with a crack, cankers, or decay, as well as tension and compression alongside the above factors.

(7)  Dead parts.  Dead tree parts can be high risk factors if they are hanging near a target.  Dead parts can fail unpredictably.






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