Insects, diseases, decays, and other forms of tree defect and mortality are important parts of a healthy, functioning forest ecosystem. They play many ecological roles in forests such as altering plant succession and providing wildlife habitat. In forest settings, trees die and fall to the ground where they become recycled. Although dying and falling trees are important in the development of forests, they are not welcome in high use recreation areas where they risk human life and property.

People enjoy recreating in forests, particularly forests with large old trees. These are the very trees that often provide the greatest risk in terms of hazard. Large trees are more likely to contain significant amounts of internal wood decay and other defects. Also, they can cause more damage when they fall. Thus, recreation managers are often faced with an apparent paradox -- how to maintain safety for visitors while providing an aesthetic environment with large trees.

The goal of hazard tree management is to sustain the forest in an aesthetically pleasing condition and at the same time eliminating unacceptable risk to visitors. A hazard tree program can be part of the solution of meeting this challenge. Simply identifying and treating hazard trees is a short-term solution to the problem, however. Careful and wise vegetation management is the long-term key to sustaining a healthy forest condition that will produce a minimum of hazard trees in the future.

Reasons to be concerned about hazardous trees
Moral obligations. Few visitors to recreations areas are experts on trees. Managers cannot assume that visitors will be able to evaluate hazardous trees and avoid them. Visitors will be either oblivious to the possibility of hazard trees or they will assume that managers have eliminated such trees. Thus, managers have some moral obligation to provide a relatively safe environment for visitors.

Legal obligations. Visitors are expected to accept some level of risk when they recreate in forests. But managers that invite visitors into designated recreation areas are liable, to an extent, for their safety. The Federal Tort Act of 1946 holds the federal government liable in the same way as any private party for negligent acts. Having no program to inspect and treat hazard trees in recreation areas would likely be viewed as negligence in a court of law. Also, informing visitors (e.g., by signs) does not eliminate liability. It is the legal responsibility of managers to inspect and correct any unreasonably dangerous conditions in order to minimize the potential for injury or damage to property.

Discussions with attorneys involved in litigation on hazard trees suggest that judgments typically are based on what a reasonable profession would have done. If managers know of hazards, or should have known but failed to make any evaluations, then they will likely be judged as liable in the case of damage. Thus, some form of hazard tree program is necessary to help avoid or win litigation. An adequate hazard tree program consists of trained individuals inspecting trees on a regular basis and follow up mitigation on trees that are judged as the high-risk hazards. Litigation often leads to questions about documenting: 1.) the training level of tree inspectors and 2.) the forms used to demonstrate inspection. Thus, training and documentation are essential components of a hazard tree program.

What is hazard?
Hazard is the exposure to the possibility of loss or harm. With regard to trees, it is the potential that a tree or part of a tree will fail and cause injury or damage property. All standing trees of sufficient size, alive or dead, present some hazard. All trees will eventually come down. But high potential for tree failure by itself does not automatically mean a tree is hazardous. Hazard exists when a tree is within striking distance of an object of any value.

Hazard Tree Evaluation
Hazard increases with: A.) potential for tree failure, B.) potential for striking a target, C.) potential for serious damage of the target, and D.) value of target. These factors are treated below and later used in an example of a quantified hazard rating system.

A. Potential for Failure

Lean. Leaning trees or large branches do not always indicate high potential for failure. Is this a recent change (potential high) or has the tree exhibited growth indicating that it has compensated for lean (potential lower). Is the lean associated with unstable soils or cracks in the tree (potential high).

Root damage, including root disease. Because of cold soils and sometimes excessive moisture, many trees in Alaska have shallow root systems. Their root systems are easily damaged or killed. Trees with damaged or dead roots are more likely to uproot causing the whole tree to fail during windstorms. Trees in forests that are opened up due to thinning or the removal of hazard trees are more susceptible to windthrow. Exposed roots can be wounded in the same manner as tree boles and lead to the same problems of invasion by wood decay fungi. Along streams and areas of road building are likely places to encounter trees with undercut root systems.

Cracks. Cracks need to evaluated very carefully. They may or not indicate substantial amounts of internal wood decay. Sometimes, cracks form as a consequence of massive internal decay as the tree buckles in the wind. Cracks near major branch unions can also indicate that trees have begun to fail sometimes without associated heart rot. In other cases, bark cracks are caused by frost or some weather extreme and may or may not indicate decay or immediate failure. Careful evaluation using an increment borer will help indicate extent of any associated decay.

Top damage (dead top, forked top) -- usually fairly simple to recognize. Dead tops can result from various forms of damage, including root disease and soil problems. Forked tops need to be evaluated carefully because they may or may not be hazardous. If the tree forked because its top was broken out or killed, it may have internal wood decay around the fork. One or both sides of forked tops can fail even when no wood decay is present. Look for cracks or other signs that the fork is beginning to fail.

Internal decay. Decay is difficult to observe in some trees and a leader in the cause of tree failure. Hemlocks, cottonwood, poplar, and aspen are all very prone to high levels of heart rot, but heart rot is common in all tree species of Alaska. In most cases, visible indicators of heart rot can be used:

conks (fruiting bodies of the decay fungi)
- some conks of fungi that cause root decay emerge from roots or seem to grow from the ground
- learn to recognize the leading root disease fungi, especially Phaeolus schweinitzii and Inonotus tomentosus
large exposed wounds broken or dead tops cracks

When heart rot is discovered in a tree, it is important to determine its extent. Regarding failure potential, experience suggests using the 1/3 rule:

Trees have a high rate of failure when less than 1/3 of its radius (or diameter) is sound

Thus, if a tree with an 18" diameter (9" radius) has a column of wood decay in its center, then, at a minimum, 3" of sound wood should be present in the outer wood for the tree to pass the 1/3 rule. This rule needs to be modified when an exposed scar is present; an additional 25% sound wood is needed to provide more support.

Scars. Unless a tree is scarred all the way around the circumference of its bole, scarring alone does not usually kill trees. Invasion by decay fungi is often the most serious aspect to a scar. Trees with scars, especially old, large scars should be evaluated very carefully for the extent of internal wood decay. Trees with scars need more sound wood than the 1/3 rule to be considered anything but a high hazard potential. Use an increment borer to determine extent of decay.

Dead branches, sometimes hanging. Depending upon the location of targets, dead branches may or may not present a hazard. Hanging dead branches are likely to fail soon and should be treated immediately if a target is nearby.

Tree dead or dying. Dead trees are simple to recognize and among the most likely to fail. Dead trees should typically be removed immediately. All Alaska trees other than western redcedar and yellow-cedar decay rapidly after death. Large limbs and the top often break out of the crown before the entire tree fails.

Tree age and size. Large trees present a greater hazard because they can strike targets at considerable distance and cause more damage when they fail. Defect, especially heart rot, is highly associated with tree age.

Tree species. As described below, specialists need to build a profile of Alaska tree species that includes their general potential for failure and the specific types of defect or injury that indicate failure potential.

Site factors (e.g., exposure to wind, especially prevailing wind during storm seasons. Trees that have lived most of their lives in an exposed condition are usually well-adapted to wind. A higher potential for failure exists when stands are opened through thinning or other forms of management. Trees with a high height to basal diameter ratio (e.g., tall, skinny boles) are more susceptible to wind damage. Other site factors to consider are slope, soil conditions, and history of tree failure.

B. Potential for striking a target

parking areas tent areas picnic areas
fire rings restrooms children's play area, etc.
consider timing of probable failure and use of these areas
are some targets absent (people) when failure is likely (winter)?

C. Potential for serious damage

Depends upon size of failed portion of tree (e.g., limbs, or size of entire tree in complete tree failure). Consider that some structures far from a hazard tree may not seriously be damaged if the top of a tree strikes, but a structure close to a hazard tree may be demolished if the bole strikes.

D. Value of target

Depends upon the maximum extent of loss if the target is struck by a failed tree. Consider that target value is at a maximum when human life is at risk. Examples of low value targets may include garbage cans, and intepretive signs when people are not present.

Rating Method
All hazard tree programs need to have some method of objectively evaluating hazard. A quantified hazard rating system is recommended. Below is a rating method that has been developed in Region 6 of the USFS. Other systems can be used as long as they combine values of the two necessary components: potential for tree failure and potential and seriousness of damage in the case of a tree failure. Trees are not considered hazardous and need not be evaluated if they are not within striking distance of a target. This can be determined by measuring the height of the tree and distance from the base of the tree to any potential target. Systematic evaluations should be made on a regular basis. Some experts suggest evaluations be made at least annually.

A. Potential for tree failure

1 = Very low failure potential. Sound trees that lack indicators of failure that are not leaning or not exposed to wind or snow load.

2 = Low failure potential. Trees with only minor defects, including internal decay that does not approach or exceed the 1/3 rind thickness rule and are not leaning or not exposed to wind or snow load.

3 = Medium failure potential. Trees with moderate defects (e.g., at or near the threshold of acceptable rind thickness) or that are growing in shallow soil or exposed to a high water table, or highly defective trees in areas well-sheltered from weather and wind extremes; or highly defective trees in areas exposed to weather extremes (e.g., heavy snow loads) only in the off-season.

4 = High failure potential. Highly defective trees in unsheltered areas; trees with root anchor age limited by erosion; dead trees; trees with obvious root disease

B. Potential for Damage (this factor combines the last three factors mentioned above: potential for striking a target, potential for damaging a target, and target value)

1 = No damage. Target impact will only involve very small tree or parts of tree; or there is no chance tree will cause damage when impact on target.

2 = Minor damage. Failure of only small tree or parts of tree; damage is likely to occur when target is not occupied; target value is low

3 = Medium damage. Failure involves small trees or medium-sized parts of trees; impacts will likely occur in areas with targets; impacts will be direct; and damage will likely be moderate; target value is moderate.

4 = Extensive damage. Failure involves medium to large tree parts or entire trees; impacts will be direct in areas with targets; target value is high; damage to property will likely be severe; or serious personal injury or death is the likely result.

Tree hazard rating score = Potential for Failure + Potential for Damage

This would yield a score from 2 to 8 for each tree

The next step is to evaluate the tree rating scores for a recreation area, which can then be helpful to prioritize which trees need treatment and in which order when resources are not available for treating all higher risk trees.

2-5 Low 6 Moderate 7 High 8 Very High
Hazard Rating Score Treatment Priority

Tools used in hazard tree evaluations

Increment borer Binoculars Sounding axe Tree diameter tape
Measuring tape Clinometer Data forms Tree tags

Corrective actions for hazardous trees
Move the target.
Moving the target to a new location or to a safe distance is a good option when the value of the tree is high. A thorough examination of trees in the target's new location is then necessary.

Remove part of the tree (pruning)
When branches or multiple tops are the main form of failure potential, then pruning can be an effective method of action. Pruning can reduce failure potential and maintain the tree. If done correctly, pruning can improve the health of the tree. Improper pruning can lead to an architecture that has a high probability of failure.

Top the tree(remove its top)
Removing the top of the tree is the best option under some circumstances. It may reduce the height of the tree to the point that the tree can no longer reach a target if the tree fails. Topping the tree can greatly reduce the weight high in the tree which may curtail the potential for failure. Topping a tree can produce a more natural appearing structure than a stump, particularly if the cut top has a jagged shape. Topped trees often have considerable wildlife use. Topped trees should be carefully evaluated into the future to monitor for increased failure potential.

Remove the tree
Tree removal is often necessary when the target cannot be moved and when topping the tree does not adequately reduce hazard potential. Care should be taken during tree falling to avoid wounding neighboring trees. If the stumps are viewed as unsightly, they can be treated with a stump grinder which reduces their height to about ground level. Careful analysis of the hazard potential should be made before recommending tree removal; credibility is lost when a specialist suggests removal because of internal defect but the tree is found to be largely sound when it is felled. The use of an increment borer in several locations of the tree's bole is often the best method of determining how much sound wood exists in a tree.

Cabling and pole supports
Using cable supports on leaning or defective trees or poles for supporting large lateral limbs is usually not applicable for large conifers and would be used only as a last resort.

Closing the recreation area
Temporarily or permanently closing off a recreation area is another option, especially if the area is concentrated with severe hazard trees and another more suitable site can be found.

Placement of new recreation sites
A thorough hazard tree evaluation should be conducted before new recreation sites are established. Some forest stands are far better suited for the development of new recreation sites than others. The stage of forest development should be considered -- younger forests generally have fewer decay problems than older forests. Also, vegetation can be manipulated more flexibly in some forest ecosystems or at some forest development stages. In the development of new recreation sites, potential targets can be established away from large valuable trees or in areas of forest stands that have relatively few defect problems.

Minimizing future hazard trees

  • Educate visitors about the importance of not wounding trees in recreation areas.
  • Minimize bole wounding and damage to roots systems during different management activities such as tree removal.
  • Maintain trees in a vigorous condition; consider watering, fertilizing.
  • Develop a vegetation plan that directs efforts of maintaining and enhancing tree health.
  • Tree Failure Reporting
    Using a reporting process on tree failure is an essential way to learn about the effectiveness of hazard evaluations. This involves documenting some details on how a tree failed, what defect factors where present, and how the tree had been evaluated during the most recent inspection. If most trees that fail are in a high risk category then you know that the hazard evaluation is effective. If trees with low scores for failure potential are failing, then adjustments must be made in evaluating hazard.

    Developing hazard profiles for the tree species of Alaska
    Reporting tree failure can also be an effective way to build information on the characteristics of each of our Alaska tree species that may make them hazardous. For example, we might determine that the failure rate is so high for large cottonwood trees that management plans should describe methods for discourage against them and favor tree species with few problems. Or by another example, we might learn that spruce trees killed by the spruce beetle are not a threat to fall the first year, but limbs and the tops begin to fall two or more years after death.

    In compiling ideas and text for this training material, considerable use was made of the book Long Range Planning for developed recreation sites in the Pacific Northwest. I wish to thank the authors, Bob Harvey and Paul Hessburg, for considerably advancing the thought process on this topic. People interested in this topic are encouraged to get this book and read it cover-to-cover.

    Additional reading

    Harvey, R.D.; Hessburg, P.F. 1992. Long range planning for developed recreation sites in the Pacific Northwest. USDA Forest Service, Portland, OR. FPM-TP 039-92. 120p.

    Johnson, D.W. 1981. Tree hazards: recognition and reduction in recreation areas. USDA Forest Service, Rocky Mountain Region. Lakewood, CO. Technical Report R2-1.17p.

    Paine, L.A. 1971. Accident hazard evaluation and control decisions on forest recreation sites. USDA Forest Service, Pacific Southwest Forest and Range Experiment Station, Berkeley, CA. Research Paper PSW-68. 10p.

    Wagener, W.W. 1963. Judging hazard from native trees in California recreation areas; a guide for professional foresters. USDA Forest Service Research paper PSW-P1. 29p.

    For more information contact:

    Forest Health Protection
    2770 Sherwood Lane, Suite 2A
    Juneau, Alaska 99801-8545
    Fax: 907-586-7848
    Dustin Wittwer, Biological Technician
    Phone: 907-586-7971

    Forest Health Protection
    3301 C Street, Suite 522
    Anchorage, Alaska 99503-3956
    Fax: 907-743-9479
    Ed Holsten, Entomologist
    Phone: 907-743-9453