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• Hazard Tree Handbook

Hazard tree management is the evaluation of and reaction to the trees we encounter. To a parent taking a child to a park or camp, hazard tree management is probably not an issue. The parent should be able to assume that the managers have taken the necessary steps to make the park or camp safe for their visit. The same should be true for private parks and recreation areas.

On some occasions, we must make hazard tree management our own responsibility. On a camping trip, for example, we should not pitch our tent under a dead, leaning tree during a storm. Trees near our homes are our own responsibility. We can learn about tree safety and evaluate them ourselves or we can have them evaluated by a professional. In some cases, we may have to contact a neighbor regarding a tree posing a threat from across property lines.

Managers of outdoor use areas with standing trees have a special responsibility - the management of those trees with the safety of visitors and staff as a primary objective.

Trees are an integral part of any outdoor use area, but like any other living thing, they require maintenance. As the number of visits around and under them increases, the chance of an accident increases.

Hazard tree management is needed any time there is a tree within striking distance of a human concern or places people live or visit. In some cases, hazard tree management involves no more than a brief look up into a tree to realize that there is no hazard. In some cases an expert forester or arborist is needed to determine if a certain tree is a threat, for example, to identify hidden defects in a tree where people congregate.

Any time trees and human safety are a concern, hazard tree evaluation and management should take place. Hazard tree management should be conducted in camps and campgrounds, parks, roads and trail corridors, picnic areas, around lodges and cabins, and, of course, around homes. An analysis of the potential for tree failure against potential for damage to persons or property can prevent an accident.

If having trees in the area being evaluated is not important, then a liberal approach to hazard tree management can be taken. Cutting down all the trees is certainly an effective method of ensuring safety. If trees are appreciated and retaining them within the community is important, then even though it is more difficult, a balance can be achieved between living safely with trees and enjoying their aesthetic qualities. In some cases, such as in a young stand or grove of trees, there is not much to be done in providing for safety; there is little threat. In some cases, such as in a mature or over-mature stand, much evaluation and treatment is needed. There is usually a threat in this case.

In most cases where there are trees, usually a certain percentage has defects. The younger the tree or the forest, the less defects. The older the tree or forest, the more defects, and therefore the more hazards.

Storms play a very significant role in affecting hazard trees. Most tree failures occur during storms. Storms do serve as positive task. They "mop up" hazards created by human activity or by other natural events such as previous storms. Weak and hazardous trees are forced to fail, and dead branches are dislodged during a predictable time, the storm event.

Storms also create additional hazards as well as weakened situations that may or may not wait for the next storm. Roots/soil relationships are interfered with. Trees’ boles are cracked, split, or placed under pressure from bending. Limbs are also cracked or split, sometimes broken but being held by a thread. Depending on the severity of the crack, split, lean, or break, a slight breeze may be all that is necessary to cause complete failure. Some of these failures occur during perfectly calm weather. Widowmakers are the most common, but leaning trees are a close second, especially if there is decay in the root systems. If decay is involved, failure is only a question of time. If, for example, a leaning tree has decay in its roots, then the tree continues to grow while the decay in the roots continues to spread. This situation results in more weight being added to a failing root support system. Failure of that tree is simply based on the increasing weight of the tree and the decreasing strength of the roots and, of course, weather.

The National Park Service maintains as its policy that “the saving of human life will take precedence over all other management as the Park Service strives to protect human life and provide for injury-free visits.“

If other parks, camps, and campgrounds adopt this attitude, how will we know when we have achieved our goal of protecting human life and providing for injury-free visits? By compiling a well thought-out plan that includes a close look at each tree that is within striking distance of something we care about or where people are involved. If this is done properly, then hidden defects are the only unknown or unreliable factor. Managing for hidden defects is beyond the scope of any responsible manager. We cannot manage what we cannot see. Accidents will always happen, but through responsible management and removing the accidents-waiting-to-happen, we can minimize the risks.

If we have a tree leaning over our home or a trail and it fails, injuring someone, can we say that we strive to protect human life?

HAZARD TREE MANAGEMENT PLAN

A good hazard tree management plan should be established by a professional forester or arborist or by the most qualified people the organization has available. In some cases, a professional can train the staff who then can write and manage the plan. The plan should start with goals and objectives, then field evaluations, and succeeding treatments, if necessary. The plan should be written with good records of activities and dates. Follow-up evaluations should be done on a routine basis, at least annually, as well as after storms. A map of the facility is a necessary tool. It can specifically identify and record problem areas. Any tree within striking distance of a use area should be analyzed.

Evaluations

Evaluations in areas with few trees can be done with a single survey. Evaluations in areas with a large amount of trees should be done in two phases. The first phase is to identify level one hazards only. After those are identified and treated, then a second survey is conducted to identify the level two hazards. These trees will require the most consideration.

Decisions have to be made as to whether to remove, modify, monitor, or avoid certain trees. Specific evaluations should be made as to why the tree is less than normal, what the potential damage is, and what action is taken to prevent this potential failure or damage from occurring.

Chapter 5 "Indicators" should be used as a guide for what to look for, and this chapter should be used to prioritize the information gathered. After level one hazards are removed, proper evaluations, considerations, decisions, and planning for the remaining trees can begin. The level three, or normal, healthy trees should be evaluated for whatever outside factors can affect them (e.g., fire, disease, other trees, and damage from people and their various activities). Protective measures for preserving the health and integrity of these trees should be considered.

Considerations

A tree’s age is a very important factor in its probability of being a hazard. During the 1920s and 1930s most of western Washington state was logged for its timber. About a third of this logged area naturally regenerated a species known as red alder. The lifespan of red alder is sixty to eighty years. Today, in the year 2003, there are many camps, campgrounds, parks, homes, trails, and woods functioning among these over-mature, dying, and dead trees, and accidents are happening. Some recreation site staff prudently treat the situation, and some feel that no action is necessary.

It is important to know the age of the trees under consideration and their expected lifespan. As trees approach maturity, branches start to die and fall, disease has had a chance to wreak havoc, and earlier defects are now poised to cause failure.

Adjacent trees can present information about any tree under consideration. Some diseases spread from tree to tree. Insects attacking certain trees can move to others. Activities such as roads, trails, campsite construction, and utilities installation and maintenance, can damage trees - particularly the roots and lower bole. It is almost certain that one pathogen or another will infect any sizeable exposed wound, resulting in a progressively weaker tree.

Potential Hazards

Most trees with the potential to cause injury or damage show signs of that potential. Large trees within striking distance of a structure or human activity should be checked for these signs routinely. If the tree presents signs of defects that indicate a high potential for failure and, therefore, harm, there are three ways to avoid disaster:

1. Modify the tree
2. Remove the tree
3. Avoid the tree during those times when weather factors increase the potential for failure beyond an acceptable level of risk. (This may save a life but offers no protection to a structure).

Damage from tree failure is preventable. In the past when a tree damaged property or hurt or killed a person, it was considered an accident or an act of God. This attitude has changed since we realized that there is a certain level of predictability in tree failure. In most cases, something can be done to prevent the damage or failure.

For example, twenty years ago cars traveled park access roads sporadically, giving failing trees across the road space between cars to fail with a slim chance of striking one. Today there is a steady stream of cars on most days. Failed trees across roads are regularly causing injury to people and damage to cars.

Tree Failure

There are three ways in which a tree fails:

1. Failure of the roots and soil
2. Failure of the bole or stem
3. Failure of the limbs and branches

There are two types of tree failure - complete and incomplete. Complete failures are more common in severe storms when trees are overwhelmed and fall over. From a hazard tree standpoint, a complete failure has done its damage and is no longer dangerous to humans. Incomplete failures occur when a tree leans or loses limbs. An incomplete failure is more dangerous to human beings because a leaning tree or dangling branch can fall on a person or building during a storm. With time, a leaning tree may grow so weak it will fall over without any outside force.

Likewise, there are different kinds of limb failures - a limb can partially break off and hang by a thread, break off and be held up by other limbs, or be severed completely and fall to the ground. In this sense, there is also complete and incomplete limb failure, and again, the incomplete limb failure poses more of a threat to human beings.

Incomplete failure occurs when the tree has been damaged, and is in an unstable situation. Not all incomplete failures result in complete failures. In most cases, partial failure increases the hazard that the tree poses. Its potential to do harm is greater than when the tree was standing in a normal vertical stance. Some trees do regain their balance by growing new limbs, callusing over their wounds, or using new growth to create as much balance as possible under the given circumstances. These recovered trees will be a lesser hazard, but will always be more hazardous than a normal tree.

Examples:

1. A branch breaks off and is hanging by a few fibers
2. A tree loses its battle with the wind only partially and leans at an angle over a home
3. Wind bends or lightning cracks or splits the bole of a tree and leaves it upright
4. The tree is pushed over just enough to break some roots, remaining upright and possibly leaning.

Predicting whether a specific tree will fail is difficult because there are many variables. Some variables allow us to be able to predict, with a high level of accuracy, trees that have the potential to fail. There are minor variables that, when combined, will cause a tree to fail, as well as single significant factors that can spell failure. Most trees challenged by a 100-mile per hour wind have a high potential for failure. Add wet soil or defective roots, and the potential for failure goes even higher.

Treatment

Treating hazardous trees involves choices. The first choice is whether to treat the hazard or to remove the threatened activity. For example, a trail passes under a tree with large limbs attached to a defective bole. The tree is deemed a level one hazard, because the potential for limb or bole failure is very high. But the tree has significant meaning to the organization. The choices are to either remove the tree or to reroute the trail beyond the striking distance of the tree. A tree can be honored from a short distance.

Other than removing a tree in its entirety, the remaining choices are pruning to remove certain branches or limbs or topping to remove the uppermost portion of the tree. The reasons for pruning are to remove dead, diseased, or otherwise hazardous material and to reduce the sail. Topping can reduce the striking distance as well as reducing the sail.

Monitoring and pruning on a regular basis to control limb growth eliminates additional hazard. Monitoring at least every three years (preferably less) and re-pruning before the new growth expands to unmanageable sizes can prevent large limb growth and have the added advantage of sculpting and shaping the tree. Topping can be used when a large tree is within striking distance of a human concern. The tree can be topped short so it will not make human contact if it fails. With this in mind, pruning and topping can be used as a valuable tool in the safe management of trees, especially in those cases where the tree has significant social and aesthetic value. The problem with pruning and topping is that, similar to mechanical damage, they expose wounds for pathogens to infect. Once a tree receives any sort of treatment such as pruning, monitoring becomes imperative. We should not expect that if a tree has defects that are treated with a pruning program that our concern ends there.

Logically, pathogenic activity can be controlled by physically blocking the pathogens from the tree’s wound. There are no known proven treatments for blocking or preventing pathogens from infecting trees’ wounds. Paints, and some chemicals, work on occasion. Perhaps instead of attempting to control the whole situation we should aid the tree’s natural processes. But using chemical applications to stave off pathogens may also interfere with the tree’s natural processes.

Certain trees use pitch over a wound as a physical barrier to invasive spores; artificially treating a wound should not interfere with this process. It is logical, then, that duplicating or aiding the tree’s natural efforts over a pruning or other wound would help the tree in its fight against infection. The downside is that if the pathogen infects the wound first, then a barrier on the wound will seal it and the infectious medium. Even if one pathogen infects a wound before a barrier is applied then the barrier will still block out other pathogens, and if a pathogen infects a wound, adding a barrier or not will not change the fact that the tree was infected. While this may not stop all pathogenic invasions, it is a big step in prevention. Furthermore, if limbs can be kept from growing large, then a certain amount of rotting bole can be tolerated. Re-pruning can also remove infected areas of trees years after the initial pruning or topping.

If the goal is to keep as many existing trees as possible, and pruning and topping enable us to do this, they should be considered appropriate methods of hazard tree control. Topping with regular monitoring and re-pruning can turn a hazardous tree into an aesthetically pleasing tree that is relatively safe.

Pruning can be used:

1. To open a tree crown, allowing wind to pass through the tree and reducing the sail effect of the crown;
2. To balance a tree’s structure. If a tree has been damaged, pruning can be used to re-create a new balance in the tree’s structure. If two trees have been growing close together, there will be very little foliage in the area between them. If one of them is removed, the remaining tree will have a one-sided crown. Pruning could re-create the balance.
3. To reduce the striking distance of a tree.

Pruning and topping of trees duplicates natural functions. A good percentage of trees in a natural environment receive similar treatment from weather.

Certain species of trees such as western hemlock do not respond well to treatments such as pruning, while some species such as Douglas fir immediately start spewing pitch over wounds. Species that do not respond well to pruning are poor candidates for retention where people are present.

In some cases, pruning and topping only postpones the inevitable that the tree is going to die or will sooner or later increase its failure potential so as to achieve a level one rating.

A consideration should be given to immediately planting young trees of a species well suited to the vicinity and the use of the area. Encourage the kind of care that results in a healthy tree and consider this a replacement for the defective and hazardous tree that had received treatment.

Good forest management should be an integral part of any hazard tree treatment program. The primary goals of most forest management plans are to grow healthy, thrifty trees and also to remove the defective, diseased trees, replacing them with healthy, vigorous ones. A hazard tree plan has similar goals. It promotes healthy trees; removes the defective, hazardous trees; and replaces them with healthy, non-threatening trees. Balance can be reached between the aesthetic value of trees and the potential damage or injury that they can cause. The dilemma for managing trees in an outdoor use area usually involves the role of trees in the presentation of the area versus the responsibility a facility has regarding safety. Invited guests have the right to expect that the organization has made the facility safe for their visit.

It is not always necessary to remove all defective trees, but it is always necessary to monitor them, as defective trees do survive storms. But as the value of the concern to be protected increases, the level of treatment should also increase.

Hazard Tree Management

Hazard tree management is needed any time a tree is within striking distance of a human concern, or places people live, visit or travel. From simply guessing if a tree along a road might fall onto a passing car to logically assessing the pathogenic activity inside a tree, if human safety is a concern, then hazard tree management should be conducted.

In an established recreation site such as a park or camp, a methodical system for analyzing the risk from trees should be established. The system should be simple and easy to understand. There is no need for a hazard tree evaluation and rating system to be complicated; the tree either has a problem or it does not. If a tree has a rotting bole, in a hazard tree sense, it is not important which pathogen caused the rot; the only consideration is under what circumstances will this tree fail, and if the tree or part of the tree fails, what are the potential damages. The particular pathogen is a definite consideration in a secondary sense, to determine if the disease will or will not spread to other trees and what part of the tree will be affected in the future.

A dead tree leaning over a well-traveled road or trail can fail at any time, and when it fails it may do significant harm. This tree should receive the highest level of hazard tree rating. A live tree with a hollow cavity in its bole standing along the same road can be spared from the cutter’s saw if the road or trail can be avoided during a storm capable of causing its failure. This tree may receive a low level hazard rating.

In any given stand of trees there is usually a certain percentage of defective trees. Trees fail as an ordinary function in forests, but they usually fail during storms, unless they have surpassed a certain threshold of defects. Once trees have surpassed this threshold then their failure can occur during ordinary weather; in some cases they even fail on perfectly calm days.

The goal of any hazard tree rating system should be to determine a threshold of defects in trees that increase their chances of failure to an unacceptable level and treat those trees. The other trees may be retained and managed. To manage hazard trees that do not pose an immediate risk is to treat, monitor, and conduct follow-up treatment when necessary. A part of this plan may include avoidance during storms severe enough to cause the tree’s failure.

THE HAZARD TREE RATING SYSTEM

The goal of the hazard rating system is twofold: first, to determine which trees are a threat to human concerns; and second, to determine the level of treatment, if any, needed for those trees which are determined to be a threat. This information is also sufficient for determining whether immediate treatment is necessary or if the tree should be monitored.

All trees within striking distance of human activity should be evaluated; there is less need to evaluate trees that do not have the potential to damage human beings or their concerns.

Even though any tree can fail given the right circumstances, certain trees are considered a threat to human concerns because of their location and/or their structural integrity. Trees that are considered a threat to human concerns include those trees that have a higher than normal potential for failure; a three tier system is used to assess the risk.

Hazard Level 1 – TREES WHOSE FAILURE COULD OCCUR IN ORDINARY WEATHER OR A MILD STORM.

Examples:

• Any tree or part of that tree that is dead or decayed.
• Severely leaning tree.
• Slightly leaning tree that is defective.
• Tree with weak, defective or diseased root system.
• Mushroom-like growth (conks) associated with the tree.

Level one hazard trees should be removed or modified so that the hazards are corrected or improved. A dead tree should be removed or avoided; dead branches over an activity site should be removed, and the tree should be monitored.

Activity within striking distance of the tree should be conducted with caution until the tree is removed or upgraded to a level two hazard tree.

Hazard Level 2 – TREES THAT WILL PROBABLY SURVIVE A MILD STORM, BUT ARE LIKELY TO FAIL DURING A SEVERE STORM.

Examples:

• Slightly leaning trees that are top heavy.
• Trees with defective branches.
• Trees with slight root system damage or decay.
• Trees with forks, crooks, spike branches, and multiple stems.
• Trees with cavities and holes.

All level two hazard trees should be monitored routinely. An initial evaluation should record the tree’s present condition and serve as a basis of comparison for all future inspections.

Hazard Level 3 – NORMAL TREES. THERE ARE NO DEFECTS, AND THEY ARE GENERALLY SAFE. HOWEVER, CAUTION IS APPROPRIATE UNDER SEVERE STORM CONDITIONS.

Under severe storm conditions, all trees have the potential for failure. A large, defect-free tree with normal failure potential should be treated as a level one hazard during a severe storm if it is within striking distance of humans or human concerns. The perfectly healthy tree can fail due to storm severity and should be respected as a high risk during severe storms.

Helpful Hints

Most individuals can without training read signs in trees that indicate danger.

• Look at your trees routinely. If there is a threat, it is usually obvious.
• Become aware of local wind history and pattern.
• Analyze trees in areas visited that are large enough to inflict harm.
• Mushroom-like growth or insects on trees may be an indication that the tree is not well.
• Large, longitudinal cracks on the outer surface of trees indicate damage or instability.
• Avoid widowmakers. A small branch falling from a height of fifty feet can maim or kill.
• Disturbance in a tree’s form may be an indication of an unsound tree.
• Avoid leaning trees, particularly during storms.
• Have your trees of major concern evaluated by a professional.
• If you prune or top a tree, monitor it on a regular basis.
• Take responsibility for your own safety.

THE EFFECTS OF STORMS

Storms are responsible for most of the negative impacts on trees. They break branches; they force trees to lean; they bend them until they break; they disturb root systems; they break out tops. This causes trees to fail, consequently damaging other trees on their way to the ground.

Human activities are also responsible for some of the negative impacts on trees. We drive spikes and drill holes, we girdle boles, we disturb root systems, and we change the natural alignment of stands of trees.

For the most part, human activities set up the potential so that storms can have a more remarkable effect on trees. In some cases tree failure is simply an overwhelming force from a storm causing a perfectly healthy tree to fail. For the most part, our activities, previous storms, pathogens, and mechanical damage establish a tree’s weakened situation, and the storm completes the job.

Consequently, a prudent tree owner or manager conducts a tree damage assessment after a storm. This assessment should be carried out after windstorms, ice storms, snowstorms, and even heavy rains or floods. Something as simple as a broken branch above a walkway is worth looking for.

What to look for:

• Leaning trees
• Hanging branches
• Broken branches
• Cracks in trees
• Disturbance in the soil around trees.

If a tree does not fail during a storm, it can use that experience to strengthen itself, given that the damage was not insurmountable and there is sufficient recovery time. A tree can recover from bending that does not cause a split, crack, or break. It can also recover from losing its bond with the soil as long as significant lean has not occurred. But any injury that exposes wood fiber to pathogens or any severe lean is a reason for evaluation and possible treatment.

Recreation site managers must consider both the aesthetic qualities that the trees offer their guests as well as the safety of those guests that the same trees may threaten. Hazardous trees should not be about liability; the consideration should be safety.

The skills and the knowledge are available to identify and to correct hazardous tree situations. Identify areas of concern anywhere that people may visit and trees are present. The areas should include trails, roads, structures, and gathering places. Designate a “safe area.“ If a severe storm develops suddenly, guests should have a safe refuge from falling trees or flying branches. A structure capable of holding the ordinary amount of guests with no hazardous trees within striking distance would be ideal. The dining hall has served some recreation sites very well after ensuring that no threat from failed trees exists around the structure.

If trees exist on a recreational site, the potential for failure exists. Therefore the potential for injury also exists.

All visitors should be informed of the potential hazards and how to avoid them. Furthermore, they should be informed as to what to do or where to go if a sudden storm develops with sufficient intensity to challenge the trees’ stability.