By Drs. Christopher J. Luley and A. D. Ali

Tree risk analysis is the science and art of assessing probabilities. Risk analysis is a better term to use than hazard analysis. Risk analysis is preferred because the focus is truly on risk (and therefore the probabilities) of trees failing and targets or people or property being struck (figure 1).

     Understanding these tree failure and target probabilities is critical for doing tree risk analysis. Total risk from a tree is the product of these risk factors. For example, if there is 1/50 chance of stem failure and 1/100 chance of the target being struck, there is a 1/5000 chance of an accident occurring.

     We like to categorize the tree risk from defects into broad groups because the risk probabilities are mostly unknown. We use serious, high, moderate and low risk to give clients an idea of the relative amount of risk present from a defect. Remember, even healthy trees fail. Never tell a client a tree is safe. There is some risk involved with most trees. In most cases, the risk grows as trees grow larger.

     Properly conducting tree risk analysis requires careful evaluation of all factors that may increase the risk of an accident occurring (figure 2). The International Society of Arboriculture's "Tree Hazard Evaluation Form" provides a good template to collect critical information needed to make informed decisions on risky trees.

     Site and soil conditions, crown exposure, past maintenance or construction, and tree species can greatly influence risk analysis. Be careful, be systematic and make sure to document all observations when involved in assessing risky trees. Our general approach to risk analysis is demonstrated in a couple of case histories later in this article.

     All arborists should know the important defects that most commonly lead to tree or branch failures in their local tree species. Common defects that lead to most tree failures are presented in the sidebar.

     Some tree evaluations will require special tools such as the Resistograph (figure 3). Other risk evaluations will call for root crown excavations or extensive use of a bucket truck. These tools and techniques should be used selectively depending on the defects and targets present. Once the risk is identified, arboricultural practices that reduce risk can then be decided upon. Risk reduction can take many forms. The most common are cabling, rigid bracing, tree-to-tree guying, crown thinning and reduction, target relocation or tree removal. These are topics within themselves. All arborists doing risk analysis should be familiar with the potential application of these risk reduction options.

The Resistograph
The Resistograph is a valuable new tool to arboriculture that can be used to assess decay in trees. Here's how the Resistograph works. A portable drill is used to power a small bit (300 to 500 mm long depending on the model and 3 mm diameter head) into a tree in locations that are suspected to contain decay or other defects. As the drill bit enters the tree, mechanical resistance to the bit is recorded on a paper chart. The chart also shows how far the bit has traveled into the sapwood or heartwood. After a drilling is completed, the soundness of wood where the drilling was performed can be evaluated from the Resistograph chart. Areas of advanced decay, cavities, internal cracks and even growth rings on softwood species show up as low points or low resistance on the Resistograph chart (figure 4). The concept behind the Resistograph is simple. However, the tool offers a number of advantages. First, a documented record is produced of all drill evaluations in a tree. In the past, arborists were forced to rely upon visual observations and other subjective techniques such as hand drilling. These techniques were difficult to repeat and document. The Resistograph removes much of the guess work associated with these methods and records the data on an easy to understand chart. Second, the Resistograph allows direct measurements of the amount of sound wood and decay in a tree. These data can be used to determine stem strength loss and, therefore, the relative potential for failure. Research has shown that if t/R (t = radius of sound wood and R is the radius of the tree minus the bark) is less than 0.3, then the likelihood of stem failure increases substantially. The Resistograph can provide the measurements to make these calculations. Several models (F300, F400, F500) are available from IML, Inc., Marietta, GA (888) 514-8851, with prices starting at $3,400.

     The Resistograph is useful to explore or confirm if decay is associated with other defects. Probing behind branches with included bark or into seams that may have decay associated with them is a good example. Information that is gained by using the Resistograph is only as good as the selection of locations in a tree that is drilled. How the data are interpreted and used once all information has been collected is even more critical.

     As with any tool, the results are related to the skill of the operator. There are only a few studies assessing the reliability of the Resistograph in picking up decay in a tree. It appears from those studies that initial decay may be missed by the tool but that advanced decay will be identified. Also, knowing species drilling profiles from sound wood will be important in assessing charts obtained from any tree species.

     The Resistograph is a valuable tool when conducting tree risk analysis. Arborists are often called on to make difficult judgements on the safety of trees. If these judgements are made inaccurately, the safety of clients or their property could be at stake. The Resistograph is another tool that will help provide the best information to make these difficult assessments.

Case History One:
The suspect tree was a champion American Elm that graced the grounds of a public library (figure 5). The tree was over two metres in diameter and was estimated to be nearly 340 years old. The recent failure of a large scaffold branch prompted the managing agency to inquire about the structural integrity of the rest of the tree. Obviously, there was considerable public focus on a historic tree of this stature.

     The tree was evaluated using a bucket truck and Resistograph, and all observations and information were recorded on an ISA evaluation form. A main concern of the evaluation was the target. The tree was growing in an open area but was attractive to people in the area because of its size and historical significance. Most important was a large 125 cm diameter scaffold that extended out over an adjacent apartment complex.

     The tree was evaluated from the base to the upper reaches of the bucket truck. The greatest concern was with the multiple attachments of three large scaffold branches to the main trunk. Cavities were present indicating extensive decay.

     Each scaffold was closely inspected and drilled with the Resistograph. Drillings showed more than 50 per cent strength loss in two of the three scaffolds. An active crack had developed between two of the scaffolds indicating that failure of another leader was imminent. Even the extensive cabling present in the tree was not holding back the forces of nature.

     The removal recommendation was hard to make. However, the advanced decay, stem strength loss and active crack could not be treated. The target would be struck with 100 per cent probability when the branch failed. It was a sad day for everyone involved.

Case History Two:
The tree was a 107 cm diameter at breast height white oak growing next to a building and a parking lot. Damage to the tree obviously occurred during construction, but the tree had responded to crown restoration efforts and fertilization. A trunk cavity was developing as a result of a construction wound. The obvious question was: How much decay was in the trunk? Resistograph drillings showed that the decay was fairly limited to the observable visible cavity. The white oak had compartmentalized the decay quite well. It was time to complete the rest of the ISA Hazard Evaluation form and hopefully give the tree the okay.

     As we inspected the rest of the tree, however, a slight opening at the base of the tree was noticed. This suggested that root or butt rot problems existed. A brief root collar excavation, where the soil was pulled away from the major flare roots, was started (figure 6). The roots and butt of the tree were sounded with a mallet to detect decay. It didn't sound good.

     Drilling with the Resistograph showed advanced decay in over 60 per cent of the roots and butt. The decay, which followed the wounding during construction, was moving primarily in large flare roots and butt of the tree. Root rot can work in this way, acting much like decay in the main stem. As with stem decay, the crown condition of the tree does not always indicate structural integrity of stems or roots.

     Root crown excavation is often needed to determine hidden decay problems in the root and butt area. Be sure that you have assessed the potential for hidden decay before you walk away from the tree. Based on the excavation and Resistograph results, a decision was made to remove the tree.

Dr. Chris J. Luley is Technical Advisor, Northeast Region, The Davey Tree Expert Company. He may be contacted at cjluley@davey.com.

Dr. A. D. Ali is Technical Advisor, Southern and Southeast Regions, The Davey Tree Expert Company. He may be contacted at adali@davey.com.