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.
Estimating the likelihood of the tree failure is challenging because of many interacting variables, but can be done with reasonable assurance. Failure potential is estimated by examining a tree, determining defects that contribute to weakening or failure, and estimating the likelihood of failure before the next inspection period.
The location of the defect can be critical factor when determining the hazard level of an individual tree. Tree stems have a zone called the "hot spot" which is presumed to be mechanically weaker than wood above or below it. The hot spot zone occurs from about 4 feet above the groundline up to the lowest living branch. If defects occur within this zone, failure potential is increased.
Courtesy of MN DNR  The "Hot Spot" zone occurs from about 4 feet above ground, up to the first living branch. |
Common tree defects in Alaska trees include: cracks, lean, root damage, top damage, internal decay, scars, dead trees and branches, and insect boring dust at the base of the tree. The unique symptoms and associated probabilities for failure of the defect categories are presented below.
Cracks are the number one hazardous defect because they indicate the tree is already failing. Cracks need to be evaluated very carefully. They may or may 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.
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Hemlock Crack |
Birch Crack |
High Failure Potential:
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Leaning trees or large branches do not always indicate high potential for failure. If the lean is a recent change, the potential for failure is much higher than if the tree is a 'long-term leaner' and exhibits growth that compensates for lean (indicative of a strong root system).
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Tree leaning over a high use trail |
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 wind storms. 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.
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Two Major Types of Root Problems:
More information on rot and butt rot fungi: Common Disease Agents of Alaskan Trees High Failure Potential:
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Uprooting caused by Tomentosus Root Disease (Inonotus tomentosus) |
This defect is 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. Look for cracks or other signs that the fork is beginning to fail.
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Forked and broken top |
Broken top |
High Failure Potential:
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Decay is difficult to observe in some trees and a leader in the cause of tree failure. Hemlocks, cottonwood, birch, and aspen are all very prone to high levels of heart rot, but heart rot is common in all tree species of Alaska. Decaying trees can be prone to failure, but the presence of decaying by itself, does not indicate that a tree is hazardous. Thus, 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. In most cases visible indicators of heart rot can be detected on a tree. Common indicators include: conks and mushrooms, large wounds, broken or dead tops, and cracks.
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High Failure Potential:
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 Internal decay |
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.
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 In most cases, visible indicators can be used to identify
trees with heart rot. The most reliable indicators of decay are conks and mushrooms. Other indicators include large exposed wounds, broken or dead tops, and cracks: |
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| Conks on base of tree
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Large exposed wound
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Broken Top
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Crack
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| More on the common disease agents of Alaska
trees |
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Unless a tree is scared all the way around the circumference of its bole, scaring 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 the extent of decay.
 Scar from beaver damage |
High Failure Potential:
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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.
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High Failure Potential:
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Dead trees are simple to recognize and among the most likely to fail. Dead trees should typically be removed immediately, because once a tree dies, decay organisms invade and structurally weaken the stem. Large limbs and the top often break out of the crown before the entire tree fails. All Alaska trees other than western red cedar and Alaska yellow-cedar decay rapidly after death. Spruce bark beetle killed trees in particular are rapidly invaded by saprots which quickly compromise the structural stability of the tree.
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High Failure Potential:
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Live trees with heartrot may be invaded by carpenter ants, further compromising the structural integrity of a tree. The ants specifically select softened wood to tunnel into for shelter and brood raising, but they do not eat the wood. Presence of carpenter ants can be dectected by piles of boring dust at the base of a tree. Suspect trees should be increment cored to determine sound wood thickness and whether the tree passes the 1/3 Rule.
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 Carpenter ant boring dust at tree base. |
High Failure Potential:
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Additional factors to consider when assessing hazard level include: Site Factors, Tree Age and Size, and Tree Species.
Site factors to consider include exposure to wind, especially prevailing wind during storm seasons, slope, soil conditions, and history of tree failure. 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 diameter ratio (e.g. tall trees with skinny boles) are more susceptible to wind damage.
Recent research in old-growth stands in southeast Alaska, indicates that slope has a strong influence on tree failure direction. In most cases, trees fall downslope. While this seems obvious, it is an important factor in evaluating whether a tree has the potential for striking a target in the event of failure. Targets located upslope of hazardous trees have a lower potential for being struck than those located on the downslope side.
Polar diagram of tree failures for trees with broken boles and uprooted trees. Note that the direction of tree failure was predominantly downslope, indicating that targets upslope have a lower potential of being struck.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 have a characteristic lifespan and the risk of tree failure increases as they reach maturity, For example, paper birch trees less than 50 years old are generally free of stem and root decay and thus have a low failure potential. However, birch over 100 years old routinely have extensive heart rot and root rot. Thus, mature and overmature birch trees constitute a higher failure potential than younger trees. Similar relationships of higher failure potential have also been noted for older conifer trees.
Graph modified from Kimmey 1956.
The amount of decay in trees is highly correlated with tree age. Note how the percentage of trees with any decay increases sharply after trees are about 100 years old. Very few trees over 500 years old are free of decay. Note also the differences by tree species.
Graph modified from Kimmey 1956.
Also, the amount of decay increases sharply in trees over 400 years old. Western hemlock has more decay than Sitka spruce across all tree ages. Western redcedar is even more defective than either hemlock or spruce.
Tree longevity is an important factor to consider both when evaluating tree hazards in developed recreation sites and when selecting locations for new sites. Short-lived species, such as aspen and paper birch, should be avoided unless plans are made to regenerate or replace these species as they reach maturity.
Each tree species has its respective defects that affect the failure potential of the tree or sections of the tree. From this general information, preliminary hazard profiles of Alaska tree species have been developed. For each tree, profiles include their general potential for failure and the specific types of defect or injury that indicate failure potential. As more information becomes available through research or monitoring of failures at recreation sites, the profiles will be refined.
A hazardous situation requires both a defective tree and a potential target. Trees or tree parts that could fall on a potential target need to be inspected.
Needs Inspection Tree could fall on potential target Tree would not reach the potential target
The potential for a tree or tree part to strike a target is determined by evaluating where failed trees will likely land and whether the strike zone is occupied by a target at the time of failure. Variables to consider include:
Is the tree in striking distance of: parking areas, tent pads, picnic area, fire rings, restrooms, children's play areas?
Timing of probable failure and use of the area: Are some targets absent (people) when failure is likely (winter)?
The 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.
The value of a target depends upon the maximum extent of loss in the target is struck by a failed tree. Values are typically expressed in relative terms such as low, medium, and high. Obviously, target value is at a maximum when human life is at risk. Examples of low value targets may be garbage cans, and information boards when people are not present.
High Failure Potential:
