Recognition of Tree Haz
RECOGNITION OF TREE HAZARDS Figure 1Standing, dead Engelmann spruce threaten visitors in the vicinity of this toilet facility. These trees should be removed.
Figure 1 Standing, dead Engelmann spruce threaten visitors in the vicinity of this toilet facility. These trees should be removed.
Common tree defects that may indicate potential hazard:
Hazardous trees may be classified into general categories by symptoms. Each category exhibits unique symptoms and each has a given probability for failure. These categories are by no means discrete; a tree may exhibit several types of defects, consequently the probability of failure increases. The following categories are listed roughly in order of importance.

Dead Trees—Snags are the most dangerous type of tree hazard. Once a tree dies, decay organisms begin weakening tree structure. Deterioration occurs most rapidly in the butt portion and root system where moist conditions favor decay. Structural weakening increases with time making older, snags a hazard; however, weathered snags may also add visual quality to a recreation site as well as provide valuable wildlife habitat. A dead tree is a hazard when it threatens a forest visitor with personal injury or would cause damage to personal property or structures if it failed (Figure 1).
Figure 2 Leaning lodgepole pine within a family unit campsite poses a threat to visitor safety. This tree should be removed.
Figure 2 Leaning lodgepole pine within a family unit campsite poses a threat to visitor safety. This tree should be removed. 		Leaning Trees-These trees are a threat only when the lean is the result of structural damage. Trees that lean naturally usually are reinforced by compensatory growth. The greater the lean of damaged trees, the greater the probability of failure during wind gusts or snow loads. In some cases, leaning trees may have aesthetic value; however, if visitor safety is threatened or recreational structures may be damaged, corrective action must be taken (Figure 2).
 

 
Root Injuries-About 76 percent of softwood and 48 percent of hardwood failures in the Rocky Mountain Region occurred in the root system (Table 3). Roots function as an anchor, providing the major resistance to windthrow. Any agent causing root damage increases the chance for failure. Wood--rotting fungi destroy wood fiber in the root system, greatly reducing strength and resistance to windthrow (Figure 3). Visual indications of root injury may not be apparent; however, increment cores usually reveal the presence of root rot. Sporophores (mushrooms or conks) around the base of the tree indicate advanced decay and therefore greater potential for failure (Figure 4).
Figure 3 Aspen infected with root decay fungus, Ganoderma applanatum, failed. Note absence of supporting root system.
Figure 3 Aspen infected with root decay fungus, Ganoderma applanatum, failed. Note absence of supporting root system.
Figure 4 Aspen infected with root and butt decay fungus, Ganoderma applanatum, is highly susceptible to failure and should be removed.
Figure 4 Aspen infected with root and butt decay fungus, Ganoderma applanatum, is highly susceptible to failure and should be removed.
TABLE 3. Location and frequency of reported tree failures in recreational areas listed for softwoods and hardwoods, USDA Forest Service 1965-1980.
Location of Failure Softwood (%) Hardwood (%) Total (%)
Limb 5.3 10.4 5.7
Upper Bole 7.4 17.0 8.4
Lower Bole 5.9 13.1 7.0
Butt 5.6 11.1 6.3
Root 75.8 48.4 72.6
Total 100 100 100
Physical injury to roots can weaken tree structure and provide avenues of entry for root-rotting fungi. Construction activities and vehicular and pedestrian traffic are often responsible for direct injury to roots (Figure 5). The root system may be injured indirectly through soil compaction and fluctuating water tables. Figure 6 Aspen that has failed at Ceratocystis fimbriata canker.
Figure 6 Aspen that has failed at Ceratocystis fimbriata canker.



Figure 7-Thickness of sound wood in outer shell required to maintain 66% of the original strength in trees with heart rot.

Figure 7 Thickness of sound wood in outer shell required to maintain 66% of original strength in trees with heart rot (modified from Wagener, 1963). If the amount of sound wood exceeds that established by the line on the graph, the tree can be considered relatively safe from failure.
Figure 5 Lodgepole pine along a well-used trail shows
Figure 5 Lodgepole pine along a well-used trail shows effects of pedestrian traffic on root system.
Trunk Injuries-Approximately 13 percent of softwood and 30 percent of hardwood failures occurred in the trunk (Table 3). The trunk must support the weight of the entire crown and any structural injury increases the chance for failure. Trunk wounds provide entry for wood-rotting fungi that reduce the volume of sound wood and increase the probability of stem breakage. Forked trunks are structurally weaker than single stems. Burls, cankers, and scars do not affect stem strength unless complicated by rots (Figure 6). The thickness of sound wood in the outer shell determines structural strength in trees with rot defect. The minimum standards for safety based on bole diameter are presented in Figure 7. Rots must be considered together with other defects. For example, a leaning tree can tolerate less rot defect than an upright tree.
Root rots are, of course, the most critical and increment cores should be taken in the basal portion of suspect trees (Figure 8, page 8).
Figure 8 Damaged Engelmann spruce root is checked for decay with an increment borer.
Figure 8 Damaged Engelmann spruce root is checked for decay with an increment borer.
Failing branches can cause serious injuries. Spike-topped trees are not dangerous unless they are rotten. During periods of severe wind stress forked tops may fail. Witches' brooms, such as those caused by dwarf mistletoe or rust fungi, are not dangerous unless the brooms are very large or dead.

Insect Activity-In general, the presence of insect activity such as bark beetles may indicate the tree has been weakened by other agents including root diseases. Carpenter ants and wood boring insects may be indicative of butt rot (Figure 10). Therefore, all insect infested trees should be carefully evaluated.

Figure 10 Subalpine fir exhibiting basal wounds and carpenter ant activity. Note sawdust at the base of the tree.
Figure 10 Subalpine fir exhibiting basal wounds and carpenter ant activity. Note sawdust at the base of the tree.


Hazards of major forest types: Each forest type possesses unique characteristics and each component tree species has its respective defects. The major forest types in the Rocky Mountains are presented in terms of their specific hazards.



Crown Defects-Only a small percentage (5.7%) of tree failures occurred in the tree crown (Table 3, page 6). Because of the smaller dimensions of crown components, damage potential is lower than for other parts of the tree. Cottonwood, on the other hand, has a wide spreading crown and large branches. The major hazard of these species, therefore, resides in the upper portion of the tree (Figure 9).

Figure 9 Cottonwood with a defective crown. The defective and dead limbs should be removed to reduce the hazard to the public.
Figure 9 Cottonwood with a defective crown. The defective and dead limbs should be removed to reduce the hazard to the public.
Pinyon-Juniper Type-There have been few reported failures in this forest type. This may be due, in part, because few recreation sites are located in pinyon-juniper. In addition, the hazard is less because of the low physical stature of these trees. Juniper is relatively resistant to wood rots and has few other serious defects. Pinyon, however, is affected by rots and other diseases and should be inspected carefully. Because of the arid environment of these stands, tree cover is at a premium; desirable cover should be retained consistent with safety standards.

Mixed-Conifer Type-Lodgepole pine and aspen account for the majority of the tree failures in the Rocky Mountain Region (Table 1, page 4). Because of the inherent differences in these tree species, they are discussed separately.

Pines-Areas with predominant pine cover are commonly used for campgrounds and picnic areas. From the standpoint of potential tree hazards, the major difference between lodgepole pine and ponderosa pine is that lodgepole pine has very thin bark which is easily damaged resulting in increased susceptibility to decay. Dwarf mistletoe is the major disease of pines in the Rocky Mountains. Large witches'-brooms should be removed to eliminate this hazard and improve tree vigor. Wood-rotting fungi are common in pines. Indicators of rot include basal fire scars, unusual swellings, swollen or punky knots, and sporophores (Figure 11). However, because of the dry climate in this Region, most wood-rotting fungi rarely form easily-visible sporophores. Burls and cankers commonly occur on pines, but do not constitute a hazard unless they are so extensive as to weaken stem structure or are complicated by rot. Increment cores should be taken to determine the amount of sound wood in trees exhibiting signs of decay.
Figure 11 Conks of the decay fungus Phellinus (Fomes) pini on this Engelmann spruce indicate extensive decay.
Figure 11 Conks of the decay fungus Phellinus (Fomes) pini on this Engelmann spruce indicate extensive decay.		 Figure 12 Conks (arrows) of the decay fungus Phellinus tremulae (=Fomes igniarius) on this aspen indicate extensive decay.
Figure 12 Conks (arrows) of the decay fungus Phellinus tremulae (=Fomes igniarius) on this aspen indicate extensive decay.
Aspen-Aspen stands usually contain many defective trees. Sporophores (Figure 12) usually indicate decay that extends 5-6 feet above and below the conk. Aspen, because of their fragile bark, are especially susceptible to trunk injuries. Trees in developed recreation sites are often injured by visitors; such injuries often lead to infection by canker producing fungi (Figures 13, 14, page 10). Cankers do notnot weaken weaken trees structurally unless they are large or are infected by decay fungi. Increment cores maybe necessary to define the amount of defect. However, cores should be taken only when necessary, as they produce wounds which may provide infection sites for canker and decay fungi. Also, cores taken from trees with internal decay provide new points from which existing decay can move into unaffected tissues formed since the decay process was initiated in the tree.
Figure 13 Aspen mortality in the vicinity of a family unit campsite. Development of recreation sites in aspen is discouraged due to the susceptibility of aspen to injury.
Figure 13 Aspen mortality in the vicinity of a family unit campsite. Development of recreation sites in aspen is discouraged due to the susceptibility of aspen to injury.		 Spruce-Fir Type-A significant number of tree failures are reported in this forest type. Rot commonly occurs in overmature spruce and true fir. Subalpine fir is particularly susceptible to decay fungi and the frequency and extent of rot increases markedly with age. Trunk wounds (Figure 15),

Figure 15 Large basal wounds are often infected by decay fungi.
Figure 15 Large basal wounds are often infected by decay fungi.
Figure 14 Wounds inflicted by recreationists are often infected by canker causing fungi.
Figure 14 Wounds inflicted by recreationists are often infected by canker causing fungi.		 punky knots, frost cracks, and broken tops often indicate decay in spruce and fir; whereas, burls and cankers do not. Sporophores, when present, indicate advanced decay (Figure 16). When a defect is suspected, increment cores should be taken to confirm the presence of rot.
Figure 16 Conk of the decay fungus, Echinodontium tinctorium, on this white fir indicates extensive decay.
Figure 16 Conk of the decay fungus, Echinodontium tinctorium, on this white fir indicates extensive decay.
Spruce and fir usually are not windfirm because of shallow root systems. Therefore any damage to the roots will increase the probability of windthrow. Rust brooms, unless large, are not a serious hazard. Both spruce and true fir are relatively tolerant of trunk damage, but once damage occurs they are very susceptible to decay. Establishment of developed recreation sites should be discouraged in old growth spruce-fir stands because of increased occurrence and severity of decay with age.
Riparian recreation sites.
Forested sites along water courses and lakes are favorite recreation sites. Blue spruce and cottonwood are the most common species in this setting. The main defect of cottonwood is large dead or rotten branches (Figure 17).

Figure 17 Failure of thiscottonwood resulted inextensive property loss and personal injury to this recreationist
Figure 17 Failure of thiscottonwood resulted inextensive property loss and personal injury to this recreationist		 Figure 18 Open wounds and unhealed branch stubs (arrows) indicate extensive decay in these cottonwood.
Figure 18 Open wounds and unhealed branch stubs (arrows) indicate extensive decay in these cottonwood.
Large trees are sometimes rotten and the amount of sound wood should be measured on increment cores. Slime flux (foul-smelling and unsightly bleeding from wounds) and wetwood should not be confused with woodrotting fungi (Figure 18). These indicators are common in hardwoods and usually do not indicate decay. Many river bottom trees are not windfirm because of the high water table and coarse soil structure.
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Figure 19 Recreation areas should be divided into hazard risk zones which determine the intensity of evaluation.
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