Hazardous Trees

Trees have two components that are necessary for survival: adequate physiological health and structural integrity.  Although poor tree vigor can indicate underlying structural problems, many trees that appear vigorous and healthy can also be structurally unstable. Many times, fungal growths (conks) from wood decay pathogens are visible on the outside of the bark. Only the outer one to three inches of wood around the circumference of the tree is responsible for the upward movement of water and nutrients and the downward movement of carbohydrates and other compounds.  Most of the inner wood of a tree is dead tissue that adds to the structural strength of the tree.  If enough of this wood is decayed, the strength of the tree is reduced.  

Oftentimes, trees can have enough decayed wood to be structurally unstable, yet have enough live tissue around the circumference to maintain vigorous growth.  In general, when 2/3 or more of the trunk, limb, or root diameter is decayed or hollow, the tree is at risk of failure.  The leaves, twigs, and branches of a tree act as a sail that catches wind resulting in tremendous bending and twisting forces on the tree.  When these forces reach a certain limit, the trunk, limb, or root will buckle and crack at the area of decay.  Sophisticated instruments, such as a Resistograph®, can be used to measure the amount of decay inside a tree.  For suspect trees, a qualified arborist should be consulted to provide a detailed hazard evaluation of the tree.

Even though a tree has poor structural integrity and is at risk for failure, it is not necessarily a hazardous tree.  In order for a tree to be hazardous, there must be a target, such as a building or people, within striking distance of the defective portion of the tree.  Removing the target and restricting access to the tree is often an inexpensive and effective treatment for correcting a hazard tree situation.  Without a target, the tree is no longer hazardous.  In other situations where targets cannot be removed, trees with defective branches or small defects in the trunk or roots can be cabled, braced, or pruned to reduce the risk of failure.  Substantial defects in the trunks or roots of large trees usually require removal of the tree to eliminate risk. 

For more information about tree risk, go to:·

https://www.treesaregood.org/Portals/0/TreesAreGood_Recognizing%20Tree%20Risk_0721.pdf