When Trees Become Dangerous 



Mary Torsello and Florence Peterson 



All trees eventually fail. Fallen trees and 

 branches may block pathways, damage prop- 

 erty and homes, destroy habitat for wildlife, 

 and hurt people. High winds, snow, and ice 

 can contribute to tree failure, but sometimes serious de- 

 fects can lead to failure without the aid of these factors. 

 Trees that fail often have structural defects that, if de- 

 tected and treated, can prevent an impending failure. 



A tree is hazardous when a tree 

 part or an entire tree has the poten- 

 tial to fail and strike a target. If there 

 is no target, a tree in any condition is 

 not considered hazardous. A target 

 can be a person, structure, vehicle, or 

 anything of value that is within strik- 

 ing distance of a potential hazardous 

 tree. It can also be a place where 

 people tend to congregate or stop, 

 such as a picnic table or walkway. 

 Hazard trees are not only a safety is- 

 sue, but also a liability issue. A per- 

 son shows reasonable care when trees 

 on their property are evaluated for 

 hazard potential and corrections are 

 made if needed. 



Evaluating trees for hazards starts 

 with determining the probability of 

 failure. Understanding the interaction 

 of many factors that influence the tree will help predict 

 the probability of failure. Factors that influence a tree's 

 relative hazard potential include: tree species, age, loca- 

 tion and condition of the tree, and the presence and type 

 of structural defects. 



Some species are more prone to specific types of 

 defects. For example, silver maple tends to form weak 

 branch unions when it grows a codominant stem. Since 

 trees have limited life-spans, older trees often require 

 more attention because they have had more time to ac- 

 cumulate injuries and defects. Site history, climate, and 

 soil type play an important role in hazard tree develop- 

 ment. For example, construction can damage and kill 

 tree roots and trees growing in continually wet soils can 

 have shallow root systems. Evaluating overall tree con- 

 dition helps because deteriorating trees have high 

 probabilities of branch failures. 



Identifying structural defects is key to evaluating 

 trees for hazard potential. Defects are visible signs that 

 trees are failing. A tree with a defect is not an auto- 



matic reason to remove all or part of that tree. There is 

 a range of probabilities for failure from low to high. It is 

 the degree of the probability of failure and the prob- 

 ability of striking a target that a professional arborist 

 uses to evaluate a tree for hazard potential. The seven 

 main categories of defects are dead wood, cracks, weak 

 branch unions, decay, cankers, root problems, and poor 

 tree architecture. 



J. revention can be 



union occurs 



suit can increase the 



DEAD WOOD. Dead trees and 

 branches can fail at any time. Hang- 

 ing material is especially dangerous 

 and should be removed immedi- 

 ately. 



CRACKS. Cracks are a physical sepa- 

 ration of the wood. They are very se- 

 rious because they show that the 

 structure of the tree or tree part is 

 already failing. 



WEAK UNIONS. Sometimes branches 

 or codominant stems are not strongly 

 attached. The union has too much 

 bark between the branch and stem 

 or between the two stems. This is 

 known as "included bark" and it 

 looks as if the bark is inrolled at the 

 union. Although a weak branch union 

 is only of moderate concern, if the 

 conjunction with other defects, the re- 

 kelihood of failure. 



DECAY. Decay is the degradation of wood that is the 

 structural frame of the tree. Bacteria and decay fungi in- 

 vade a tree when it is wounded. iVlushrooms or conks 

 are evidence of decay fungi and advanced decay. They 

 can be found growing on root flares, stem, or branches. 

 Advanced decay may lead to a cavity which does not 

 necessarily mean a tree is dangerous. A tree can have 

 decay or a cavity and remain sound or have a low po- 

 tential to fail. The wood surrounding the decay or cavity 

 may have enough strength to support the tree even 

 when weather is severe. Structural soundness depends 

 on the amount of solid wood compared to decayed 

 wood. Also, since the live part of a tree which carries 

 water and nutrients is a thin layer just below the bark, 

 trees and branches can contain decay, yet still retain 

 lush green leaves. Determining the amount of decay 

 that is dangerous can be a difficult task. 



rEBRUARY & MARCH 



