Forest & Shade Tree Pathology

Hazard Trees

About this site

This is a long page. Here is an outline:

  1. What is hazard tree management?
  2. Programs
  3. Rating systems
  4. Defects
  5. Inspection procedures ←Increment Borers: Should we stop using them?
  6. Treatment
  7. Prevention
  8. Tree Failure Reporting
  9. Legal aspects

What is hazard tree management?

Tree hazard is the likelihood of property damage or personal injury from tree failure. It is important primarily in developed areas with trees:

Hazard incorporates not just the condition of the tree, but also the potential target. So hazard is:

Hazard tree management is reduction of hazard through inspection and mitigation, balancing level of hazard against the desire to maintain large, beautiful trees on the site.

Hazard tree inspection programs

An organization with multiple sites to consider should develop a policy and a program for its implementation. Following are some considerations.

Appropriate frequency and intensity of inspection

Based on consideration of such things as types of targets, frequency of use, season of use, status of the tree community (species, age, defects, etc.) and failure history, an inspection cycle and procedure should be determined for various categories of sites. For example, one extreme is rural roads closed in the winter in areas where failures tend to occur in winter. It is probably reasonable to have no formal inspection of such roads. Some year-round roads in areas of high failure potential might receive a "windshield" or roadside walk-by inspection, supplemented by closer examination as circumstances warrant. The other extreme might be a site subject to severe winds with overmature trees and buildings occupied day and night, year round. Two inspections per year might be appropriate in such a situation with one a full-blown examination.

Inspections need not all be of the same intensity. For instance, it may be determined in some circumstances that every three years there will be a thorough, tree-by-tree, pull out all the plugs, no-holds-barred inspection, and in the intervening years there will be a more cursory walk-through inspection without recording individual tree data. The latter type of inspection is geared toward finding things that changed quickly: storm damage, partially uprooted trees, widow-makers. Also, records of the most recent thorough inspection can be used to more carefully observe trees that were noted as needing watching.

Inspectors should be trained

Inspectors should be familiar with trees and tree defects and should receive documented training on tree defects, hazard rating and inspection procedures. This is often available from U.S. Forest Service forest health protection offices, state cooperative extension or other state agencies, some universities, or private sources, which may be found via the International Society of Arboriculture.

Effort should be continuous

Reacting to outbreaks of tree failures is sometimes necessary, especially when inspections have been neglected. But a proper program is continous. Annual work is planned such that sites are revisited at an interval that is deemed to be adequate as discussed above. If budget is limiting, this should be formally acknowledged, the inspection and treatment cycle adjusted accordingly, annual work rescheduled on that basis, and in extreme situations sites should be closed until they can be properly inspected and treated (see Treatment below).

Mapping/Tagging/Marking considerations

In a large population of trees, a means of finding the trees again is needed in order to implement treatment and perhaps to track changes between inspections. In some cases it may be desirable to map, tag and/or mark all trees that are inspected; in others a threshold hazard rating can be determined, above which trees will be so located. There are various possibilities:

  1. Mapping
    1. Sketch mapping. A map of the site is obtained or prepared. During inspection, trees are located on the sketch map by number. This is somewhat less accurate but faster than the following.
    2. Reference point mapping. Along with other tree data, the distance and azimuth from a fixed reference point are recorded. On large sites, multiple reference points will be needed. This is a more precise method, but takes a bit longer. Also, if a map is desired, it will be more difficult to generate from these data.
    3. A note on GPS. Although GPS would seem to be a good way to locate trees, it is still (even after the military removed selective availability) not precise enough unless: a) trees are widely spaced, or b) a radio or satellite correction is applied to the GPS signal. Without real-time correction you can generally get within a 10-feet radius, which is usually not precise enough to distinguish nearby trees.
  2. Tagging. Temporary (heavy foil) or permanent aluminum tags can be used with aluminum nails. Wildlife (often the 2-legged kind) sometimes tear off tags. If tags are used without mapping, there can be quite a job finding the tree with the right tag. Tags work best as a supplement to mapping.
  3. Marking. Ideally, decisions about treatment, such as what hazard rating levels to treat, are made after the data are in. In some situations, particularly where time is pressing, it may be desirable to decide before hand that all trees above a certain hazard rating will be treated. In that case, such trees can be marked for treatment (e.g., removal) at the time of inspection.

Minimum DBH

A minimum DBH (Diameter at Breast Height, 4.5 feet) for inspection must be decided. Usually minima are in the range of 6-8".


All decisions, training, inspections and treatments should be documented. Documentation of inspections should include results. In event of an accident that results in litigation, this helps to demonstrate organized, detailed efforts to reduce hazard and can provide evidence that particular trees were inspected. It also can help track the progress of the program, detect trends in disease and hazard development in the recreation sites. These records are more likely to help than hurt the organization in court. If there is no record of inspection, for legal purposes it was not done.

Rating systems

A hazard rating system is essential to ensure that appropriate and reasonably consistent criteria are used to evaluate the relative hazard of a tree. But hazard rating systems are like belly buttons - everybody has one. At first it may seem surprising that a single officially sanctioned, scientifically tested rating system is not used everywhere. Here's the problem:

Rating system components

All systems incorporate some measure of the degree of tree defect and a measure of risk: likelihood and value of target loss if the tree fails. These two values are then added or multiplied to get a hazard rating.


Dead Trees, Tops and Branches

Any dead tree that meets the minimum DBH of the inspection (often 6 or 7") receives the maximum defect rating. For branches and tops, most systems attribute greater hazard values to larger pieces.

I have heard people talk about leaving dead trees, saying they have cut them and found them to be thoroughly sound. However, the condition at the stump does not tell you much about the roots. Also, branches start falling out of trees soon after death, and branches can be lethal. Furthermore, dead trees are not all the same - they are unpredictable.

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.

Root Problems

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:

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. Failure potential is considered high when tree leans more than 45° or when it leans and has another defect in the main stem. A significant lean that appears to have occurred recently and has not been corrected by negatively geotropic top growth ("unnatural lean") should be taken seriously.


A leaning tree may or may not be a substantial hazard. It is necessary to distinguish between two types:

1. 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. 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.
2. Unnatural lean
Here we mean a lean that is due to 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 even be bending of the stem going on, usually associated with decay. The upper stem in an unnatural lean is mostly not vertical, but leaning. Trees with an unnatural lean have already begun to fail and are extremely hazardous. I would suggest closing sites immediately until the tree can be removed.

Some specialists suggest that any lean greater than 15°, particularly if it is in the direction of the target, is probably cause for removal. Be careful not to put too much emphasis on direction of 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.


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.
With included bark
This is usually a V-shaped fork. There is usually no bark ridge and the bark dives down like a crevasse. This is 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 a fork, 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.

wagener graph

Method for measuring relationship of sound shell thickness to radius (t/R) when the decay column is excentric, according to Mattheck and Breloer (1994).


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 Decay page 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 (1963) 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 (1994), who studied 800 broken and standing trees. Expressing the relationship between decay and sound wood as the ration of 't' (sound shell thickness) 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, 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).

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 cracks, cankers, or leans. Such cases are rare 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.

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 treatment. 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.

Inspection procedures

Hazard tree inspection is an art. Rating systems, procedures and guidelines have been developed for inspection and decision-making, but knowledge, judgment, common sense and experience are an important part of the process. As an inspector grows in knowledge and experience, decision-making and interpretation of defects become more nuanced and complex.

Assessing internal decay

In order from low- to high-tech, here are some tools for detecting and measuring internal decay in stems and large roots.

Strike the tree sharply with the hammer side of a hatchet or a mallet. A very hollow tree will be easy to detect by the sound. Trees with decay but no actual void may be more difficult. Sound varies among species independent of decay. Practice sounding lots of trees to tune your ear to the sound of decay. Sounding is an art, and some people are better at it than others, but it can be a useful part of initial examination.
Increment boring
An increment borer is a long, hollow, steel tube with threads on one end. After screwing the tube into the tree, an extractor is used to remove the wood core from inside the tube. Often decay is detected by the fact that the tube spins freely when the threads enter a void. Also, the core can be examined for symptoms of decay. The thickness of sound wood can be measured fairly accurately. Increment borers provide good information but: a) create a somewhat significant hole (up to a cm or so) that could lead to problems, and b) borers require much maintenance to avoid problems and are difficult to use on some hardwoods. Trees should not be cored unless there is reasonably strong suspicion of decay.

Cordless drills
A cordless drill is very worthwhile to carry (carrying is easier with a drill holster). Inspection crews learn to appreciate them quickly. You can quickly and easily drill trees that have indicators or suspicious sounding results. Because it is easy and the wound is slight, it reasonable to drill trees that you might be reluctant to core, and you may quickly drill several radii to get a better "picture" of the decay column. Cordless drills have come a long way. The 18-volt systems are powerful and modern battery packs (XR+) hold a good charge all day and have circuits that prevent "memory" development. See sidebar for a recommendation.
This machine is a sophisticated cordless drill with a device that measures and records changes in torque as the bit goes in. It produces a paper and/or digital graph (depending on the model) of the strength of wood along the bit path. It is accurate and allows easy measurement. It is so sensitive that you may have difficulty distinguishing from changes in tree growth rate (wood density) and pockets of early decay. The only drawbacks are the expense (around $4000 and up) and bulk/weight. For rapid inspection of large numbers of trees it may not be worthwhile, but the resistograph would likely provide useful additional information for intensive inspections of few, high-value trees.
Other machines are available for internal inspection of trees, but their usefulness and power to distinguish decay from other internal variables in a tree (such as moisture content, wood density, resin, branch traces, etc.) is not yet clear. Among them are the Arbotom, a complex system of impulse tomography (almost like a CAT scan but it uses stress waves from striking the tree instead of x-rays). Another instrument is the Arborsonic decay detector, Fujikura Europe, Ltd. It is an ultrasonic instrument, which measures the strength and speed of propagation of high-frequency sound waves through the wood. Knowledge, experience and common sense are much more important than expensive equipment.

Inspection hints and tips

Here are some tools often used in hazard tree inspection:
  • Forms (inspection and failure report forms)
  • Site maps
  • Compass (for mapping)
  • Diameter tape (to measure DBH)
  • Measuring tape (100' or so)
  • Clinometer (to measure tree height)
  • Tree tags and nails (aluminum)
  • Pulaski (see sidebar)
  • Hatchet
  • Camera (for important trees)
  • Binoculars
  • Cordless drill (see sidebar)
  • Increment borer
Determining if target is in striking distance

If you are not sure whether a tree is within striking distance of a target, here is a shortcut used by John Pronos that saves the trouble of actually measuring the height of the tree and distance to the target. Stand at the near edge of the target and sight the top of the tree with a clinometer. If the angle is less than 45° (100% slope) the tree is too far away and need not be considered. A tree that is just close enough for the tip to brush the target would form an isosceles triangle. The height of your eye from the ground won't make much difference, but you can take a few steps toward the tree to compensate for that if you wish.

  1. Work the site in a logical, consistent sequence so that tree numbers are in a logical sequence that can be followed later.
  2. Assess and rate the target for each tree. If the tree would not hit a target, ignore it.
  3. Look at the tree from 2-3 perspectives, close and far and all around. When you are learning, it will help to go through each defect and decide if it applies.
  4. Based on indicators and experience, it may be necessary to check for root disease, usually using a pulaski. Examples of indicators would be thinning crown, dieback, reduced height growth, basal resinosis, exposed decay in butt or roots.
  5. Also based on indicators (wound, bird cavity, etc.), you may need to increment core or drill the tree to assess internal stem decay. At a minimum you should have 1/3 of radius left sound (sound shell). Use DBH/2 to get radius, not the core.
  6. Calculate the rating. In some cases, you may feel the rating does not accurately reflect the hazard of the tree. Tree inspection and hazard rating are a combination of science and art: do what experience and sound judgment tell you is right, but make notes about what you do.


Acceptable risk

Managers must decide what level of risk they can accept. In other words, what hazard rating level and above should be treated? This decision can be made before the inspection (which means trees at and above that level can be marked for treatment during inspection).

Making the decision after inspection, however, means a more informed decision can be made. The manager will know how many trees there are at each hazard rating, the species, the size of the trees, and what kinds of defects are involved.

Approaches to Treatment/Mitigation

Tree removal is overwhelmingly the most common approach and will likely remain so, but sometimes it is worth considering other options. These possibilities should be considered in some cases:

  1. Pruning can take care of large dead branches and tops, though it is expensive, requiring a tree climber or bucket.
  2. Topping. This means cutting off the live crown or more, leaving a partial snag that is short enough that it will no longer reach the target or it will likely 'melt' in place rather than fall over. An experienced tree climber/sawyer is needed, but leaving residual stem provides:
    1. tree-like, vertical structure where it is needed because of lack of trees
    2. may provide esthetics of old-growth forest appearance
    3. target for those who must practice their hatchet-throwing and chopping
    4. wildlife habitat
  3. Removing target. This means closing certain campsites, trails, roads, moving structures, and in some cases entire sites. These measures are often necessary in the short term when serious hazards are discovered and other treatment is not immediate. But sometimes they are reasonable long-term solutions. For instance:
    1. Campground capacity greatly exceeds demand anyway.
    2. Very large trees that have great esthetic, wildlife, or even historical value.
    3. Resources are simply not available for active forms of treatment.

Finally, a pitch to consider another nontraditional practice. Normally hazard trees, after felling, are removed from the site. Certainly most of them must be removed to clear roads, etc. However, consider leaving some large logs on the site in positions where they will not interfere with designated sites and traffic. If the trees are being sold, cull logs can be used. Benefits:

  1. Give campers targets for chopping with hatchets and such.
  2. Break up excess traffic, allowing ground vegetation to recover.
  3. Provide protected sites along logs where tree seedlings can establish without being trampled and, where necessary, some protection from the sun.
  4. Some consider it an esthetic benefit. Many campers prefer sites with large logs, using them for benches.
  5. Over the long term, decaying logs greatly benefit soil properties by adding organic matter, improving moisture holding capacity and countering the negative effects of soil compaction.



Site Development

Existing Trees, New Construction

It is becoming more common that existing trees are left, to the extent possible, when new developments, homes or other buildings are constructed. Unfortunately, this is often not done well. Here is what often happens:

Here is what should happen:


Here is an excellent primer from the U.S. Forest Service on why and exactly how to prune trees:How-to logo Prune Trees (also available in web page format if you are pdf-challenged). Please read it before you go near a tree with a pruning saw!

Pruning is best done when trees are young to develop good form. The goals are to:

If there are no other concerns, winter, especially late winter, (or dormant season) is usually chosen for pruning for several reasons:


  1. Trees and shrubs that flower in early spring, and whose flowers are desired, should be pruned immediately after flowering; flower buds will then form on new growth.
  2. Maples and birches bleed a lot in the spring, so late summer is a better time for pruning. The wounds will harden off so that they won't bleed in the following spring.
  3. Christmas trees are pruned in summer after shoots extend but before buds are formed. This stimulates lateral buds to form, making the foliage denser.
  4. Dead branches can be pruned at any time, but you still need to follow proper pruning procedures to avoid tree damage

Vegetation Management

We know that the trees that make our developed landscapes so beautiful will, at some point in the future, either die, fail, or be removed because we can no longer tolerate the risk that they present. There is no better time than now to plan for replacement of those trees. If woody vegetation and the benefits that it provides are to be maintained indefinitely, it is important to manage the vegetation, preferably based on a vegetation management plan.

Vegetation management, broader in scope than hazard tree management, helps to ensure that we will enjoy the many benefits of trees and reduced hazard problems far into the future. Ensuring the establishment of young trees to eventually replace the old, planning for placement and species of trees, protection of trees from injury and long-term disease and insect problems are some of the issues considered in vegetation management planning. A vegetation management plan should take into account current conditions of the vegetation (including canopy, regeneration, shrubs, etc.), management objectives such as aesthetics and screening, potential pathways of stand development in the future, current and potential development of insects and diseases that may affect those pathways, and alternative approaches to meeting the objectives in the future.

Tree Failure Reporting

Reporting of tree failures and analysis of the resulting data can provide useful information on species, types of failures, defects and indicators associated with failure, etc. It is part of a good hazard tree management program.

An organization at some level must provide the service of receiving reports, incorporating them into a database, and analyzing the data. Until 2005, the best known and most successful reporting system was the California Tree Failure Report Program. Building on this model, the U.S. Forest Service, in cooperation with some leaders of the arboricultural community, have initiated the International Tree Failure Database. It serves all sectors and regions of the U.S. and Canada now, with expansion to other countries planned.

To submit failure reports, users must be trained and receive a username and password. Reports will be available to the public.

Legal issues

Hi! I'm not a lawyer, though I've talked to a few of them. This information is provided for general education but has not been checked by a lawyer and should not be considered legal advice, nor should it be relied upon in making management decisions.

Traditionally, you have three levels of responsibility to people who visit your property:

  1. Trespasser: enters without special consent or privilege, without payment or license. Not necessarily illegal. We owe them lowest duty of care, essentially just to not set traps.
  2. Licensee: Has permission or consent, express or implied, to go on land for own purpose. Applies in general to visitors to public land.
  3. Invitee: Someone who pays a fee or is otherwise specifically invited for recreation or business purposes. Highest duty of care, must assure reasonable care has been used to prepare premises and make them safe for user. Must inspect and then remove or warn of known hazards, generally exercising reasonable care to protect user.

Now the standard for invitee pretty much applies to all visitors. Now most states are using a "prudent-man" test for the duty of reasonable care owed to people who enter. What would a prudent person do?

Property owners have a general duty to visitors to use reasonable care to keep the premises safe and to guard or warn the visitor from any hidden danger or defect that is discoverable using due care (such as inspection by qualified hazard tree inspectors) and presents a reasonably foreseeable risk of harm.


If owners do not comply with their duty, they may be liable for resulting damages. Lawsuits up to $60,000,000 for serious injury or death are not uncommon. A property owner is negligent and liable if a claimant can prove that:

  1. The owner had a legal duty to the claimant; and
  2. The owner breached his duty; and
  3. The owner's breach was the legal cause of the claimant's injury or property damage.

The Federal government has no sovereign immunity that exempts it from such lawsuits. The Federal Tort Claims Act (28 U.S.C 1346(b), 2671 et seq.) waives sovereign immunity. Federal employees are not individually liable except when acting outside the scope of their employment according to the Federal Employees Liability Reform and Tort Compensation Act of 1988 (the Westfall Act).

In a court case involving tree failure, two fundamental questions come up:

  1. Was the manager negligent? Was there an inspection system? Was it prudent and performed to customary standards? Was it documented?
  2. Was this particular tree inspected? Documented? Could the cause for failure have been detected in a reasonable and prudent inspection?

"Act of God" is not as good a defense as you might think. The same questions will be asked, and if there was detectable defect and no/poor inspection, the manager may still be held liable.

Some related books from amazon.com:

Funny picture "I bet Owl would be able to figure this out!"

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Last modified 22 Feb, 2015