In order to assess defects and their potential contribution to hazard, an inspector needs:
- Detailed knowledge of and experience with tree defects and how they contribute to failure potential.
- Tools and opportunity to assess defects.
- A system for estimating their contribution to failure potential (usually a rating system).
For illustration, we will use a defect rating system used by the Rocky Mountain Region of the US Forest Service . Defects are rated from 0 to 3, 3 being most severe.
Dead Trees, Tops and Branches
Any dead tree that meets the minimum DBH of the inspection (often 6 or 7 inches) receives the maximum defect rating. When only branches and tops are dead, most systems attribute greater hazard values to larger pieces.
People sometimes want to leave dead trees standing near targets, saying that in the past they have cut dead trees and found them to be thoroughly sound. However:
- The condition at the stump does not necessarily tell you about the roots.
- Branches and tops start falling out of trees soon after death, and branches can be lethal.
- Dead trees are not all the same — they are unpredictable.
- When the tree dies the sapwood loses all decay resistance and decays rapidly, increasing the failure potential.
- Many people have been killed and injured by dead trees falling on them. It is the most obvious indicator we have. If for no other reason than liability, it can’t be ignored.
Some species are notorious for top dieback (often sensitive to water table fluctuations). and dropping dead branches. After a major branch does break, decay can begin there if it wasn’t already present, compromising the larger branch below.
Road construction, severing for utilities, soil erosion, trampling damage, debarking, partial windthrow, omnibus decay and root disease are the considerations here. Certain conditions are widely agreed upon as very serious:
- Any diagnosable root disease that kills and decays roots (see the Root Disease pages). This does not include decay localized around root wounds that does not progress and invade healthy tissues.
- A recent, uncorrected lean, often visible by soil irregularity associated with tipping of the root system. This indicates that a tree has lost its grip on the earth and is beginning to fall. By “correction” in this context we mean curvature of the leaning stem to correct the orientation due to negative geotropic growth.
- More than half the major roots have been severed, severely damaged, or have decay.
This does not mean that other root conditions are not serious. Any decay found in roots should be carefully examined and considered.
Poor Crown Architecture
This may be caused by poor pruning in the past. Sharp bends or crooks in the branches are naturally weak, liable to failure. If a tree is topped, multiple branches may come out just below, they are usually weakly attached.
A leaning tree may or may not be a substantial hazard. It is necessary to distinguish between two types:
Corrected or “natural” lean
Not the greatest term, but here we mean trees that have been leaning for much of their life. You can see sweep (curvature) of the stem, or maybe even a crook, where the tree corrected the lean to grow vertically at the top again. The upper stem is vertical, not leaning. There is no evidence of recent change, such as soil/root plate movement, cracking or stress bending of the stem. Sometimes natural leans can increase slowly over time as the weight of the stem increases. Corrected leans are generally not serious defects. However, failure potential is considered high when a tree leans more than 45° or when it leans and has another defect in the main stem or roots, such as root and butt rot.
Uncorrected or “unnatural” lean
This is a lean that appears to have occurred recently and has not been corrected by negatively geotropic top growth (“unnatural lean”). The upper stem in an uncorrected lean is mostly not vertical, but leaning. It is a relatively recent change in the orientation of the stem. You may see evidence of soil/root disturbance indicating that the root system has shifted in the soil. You may see cracking in the stem as it gives way. There may be recent bending of the lower stem going on, usually associated with decay, and compression failure on the side toward the lean. Trees with such a lean have already begun to fail and are extremely hazardous.
Do not put too much emphasis on direction of lean. An engineer once told me that a little wind can easily counter the force of the lean. If you have experience felling trees, you know how much a little wind and holding wood can influence the direction of fall. It would not be at all surprising to have a tree go down at right angles to the direction of lean. Under a good wind, it could even fall in the opposite direction.
Distinct from branches in that the main stem splits into two, more or less codominant stems, each asserting apical dominance. Can be a difficult situation to deal with because it can be locally very common and yet these are sites where trees tend to fail. As with leans, there are two types of fork:
No included bark
Usually this is a U-shaped fork. Often one can see a bark ridge at the union between the two stems. These forks are not serious unless associated with other defects.
With included bark
This is usually a V-shaped fork. There is usually no bark ridge and the bark dives down the crotch like a crevasse. This is a more serious hazard for several reasons:
- Where the bark is between the stems, the tree will never be able to bind the stems together by fusing the tissues. Imagine trying to hold chopsticks together at the base while someone is pulling them apart at the top. You wouldn’t have a chance. If you are able to hold them together higher up (simulating a fork whose tissues bind higher up as the tree grows in diameter) you have a much better chance of keeping them together.
- To make matters worse, the included bark often remains alive and growing. As the stems grow in diameter, they force the fork apart. The point below, where the two stems are really joined, is unable to adjust to this pressure and tends to crack.
Once you see a fork with a crack below it, you know it has already begun to fail and the tree undoubtedly should get the highest hazard rating. However, be sure you can distinguish an occluded crack from a bark ridge, which is indicative of a stronger union.
Decay in stems and branches is a major cause of failure. Conks and decayed knots (punk knots) or stubs are definite indicators of advanced decay. Indicators that suggest decay may be present include cracks, seams, butt swell, and large old wounds. See the wood decay and stem decay pages for more details.
Some decay, especially in the interior, is tolerable. Most of the strength of a tree, like a pipe, is on the outside, so interior wood can be removed without great effect on strength. Efforts continue to define amounts and types of decay that can be safely tolerated under various circumstances. Wagener  proposed that strength loss is proportional to d3 ÷ D3, where d and D are the diameters of the decay column and the tree, respectively. This results in a relationship as shown in the figure. Wagener reasoned that a tree could withstand up to 33% loss of strength without becoming an immediate hazard.
This relationship was essentially confirmed by Mattheck & Breloer , who studied 800 broken and standing trees. Expressing the relationship between decay and sound wood as the ratio of ‘t’ (thickness of the sound shell around the decay column) over ‘R’ (radius of the tree), they found that trees rarely failed when the t/R was greater than 30%, but usually failed when the ratio fell below that threshold. When the decay column is excentric and large, the sound shell thickness is measured at its thinnest point and the radius is measured as the radius of the decay column plus the remaining wall thickness (see figure, off-center, large decay column). This has become a widely accepted rule of thumb in hazard tree work, one of the few relationships for which hard data exists.
However, it should be recognized that this applies most especially to conifers, and to cases of uniform decay columns in the absence of complicating defects such as cavities, cracks, cankers, or leans. Such complications are not uncommon in the real world. Failure potential is considered high when there is canker-rot in the main stem or decay associated with weak branch union or open crack. Any decayed branch also has a high failure potential.
The following method figure includes three situations: a) and ideal centered decay column; b) a decay column that is exposed to the outside through a cavity; and c) an eccentric decay column, using the approach of Mattheck and Breloer .
Cankers and wounds
Cankers and wounds must be considered not only from the perspective of their direct contribution to failure potential, but also as infection courts for stem decay fungi. See the Canker page for more the whole story. A canker (or scar or cavity) that involves more than 120° of the circumference is serious. Generally a tree with half or more of the circumference occupied by an individual wound or canker should be considered for mitigation. The potential for failure is higher if a canker is accompanied by decay or if it is connected to another defect.
Vertical cracks in the trunk, often accompanied by callus that may or may not be ruptured, are taken pretty seriously. They are an indication that tree failure has already begun. Some consider them the number one indicator of hazard. They arise from stress related to internal defect such as decay or buried scars or from strain associated with forks. They may be triggered by wind or cold temperatures (thus the term “frost crack”).
Callus forms at the margin of injured cambium and grows outward to cover the wound. If all is well, the callus margins will meet and seal over the wound. Sometimes, the callus tends to curl inward, or inroll, during growth. This is sometimes called “ram’s-horning.” If this happens, the callus never meets and seals properly. Instead, the bark-covered surfaces of the callus rolls meet. As growth rings are added, the callus sides push against one another, leading to formation of cracks. Also, tension generated in the stem can lead to formation of secondary cracks elsewhere. Inrolled callus might be expected where the wound surface is concave. The callus margin follows the concavity as it grows, increasing the likelihood that the callus surfaces will meet before the margins.
- 1.Blodgett JT, Burns KS, Worrall JJ. 2017. Guide to Hazard Tree Management. Technical Report R2-69. USDA Forest Service, Rocky Mountain Region, State and Private Forestry and Tribal Relations, Forest Health Protection <https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fseprd572690.pdf>.
- 2.Mattheck C, Breloer H, Lonsdale D, Robert S. 1994. The Body Language of Trees: A Handbook for Failure Analysis. Translation of: Handbuch der Schadenskunde von Bäumen. Strouts R, transl.; Lonsdale D, ed. Norwich, UK: The Stationery Office Ltd, Publications Centre. 240 pp.
- 3.Wagener WW. 1963. Judging Hazards from Native Trees in California Recreational Areas: A Guide for Professional Foresters. Research Paper PSW-P1. USDA Forest Service, Pacific Southwest Forest and Range Experiment Station <https://www.fs.usda.gov/treesearch/pubs/29102>.