Risk tree assessment

This fall Wisconsin DNR Urban Forestry program hosted the inaugural Wisconsin Community Tree Management Institute (WCTMI) graduate workshop. It was held over two days in mid-October at the Green Lake Conference Center. The workshop included presentations and mini-workshops on many topics. One of the topics was risk tree assessment, presented by Dan Traas.

Risk tree assessment is a very important part of maintaining community forests. It is the evaluation of trees and their condition to determine the amount of risk associated with the individual tree. Risk tree assessment is important for homeowners and municipalities to complete because it informs, and hopefully prevents, people of potential liabilities. For instance, if there is a row of trees along a street where people park their car, a risk tree assessment should be done to determine which trees present a higher risk, such as dead branches falling citizens or their cars.

During Dan’s presentation at CTMI he discussed the three categories of risk that a tree can fall under. The categories are associated with the time before a tree needs to be removed or re-assessed. The first category is five year removal. A tree in this category is not destined to removal, but should continue to be assessed for better or worsening conditions. A tree in the five year removal category should have a management plan associated with it. The next category is one year risk. This category represents trees that will be removed, but do not need to be removed immediately, it allows for more time to manage the tree and develop a strategy for removal. The final category a risk tree can fall under is high risk/immediate removal. Trees in this category can often be affected by storm damage. If a municipality or homeowner is unable to have the tree removed immediately, but knows it is high risk, they should not encourage others to use those areas. For example, in parks, move tables so they are not near high risk trees and avoid mowing in areas around high risk trees.

Assessing trees to understand which of the three categories they fall under can be difficult at times. While a tree may look decent on the outside, the inside of the tree may tell a different story. In one of the CTMI outdoor activities, Dan took the attendees around the property and, as a group, evaluated risk trees. An evaluation should include the species of tree being evaluated, it’s DBH (diameter at breast height), a live crown ratio, relative age and site type. The evaluation should also rank the various parts of the tree on a scale (e.g. one to five): crown, stem, root system and basal, and tree health overall. These evaluations will lead to a management plan for the tree: failure potential, target to remove and risk abatement. Include as much information as possible in these assessments for personal use and any potential future legal situations.

There are many ways to go about evaluating a tree. At CTMI, Dan and the attendees used the main, and cheapest, methods of evaluation, a rubber mallet and visual evaluation. They also used a drill with a very small drill bit. The rubber mallet helps detect decay by producing different sounds when hitting decayed wood versus solid wood. By drilling into a tree, with a small drill bit, a person can feel the change in force needed to apply when drilling through decayed versus solid wood. Visual assessment is also important. It is generally easy to see if the crown on a tree is dead or if there is a large branch that is high risk and needs to be removed.

There are also many more high-tech and more expensive detection tools available that allow people to get a look at what is going on under the bark. An article in the October 2016 issue of “Tree Care Industry” mentions the mallet, but also references ultrasonic technology, stress wave timers, radar and electrical resistance. These forms of technology use various wave forms to measure how long it takes the wave to get from one point on the tree stem to another, generally a cross section. The waves travel faster through sound wood than through air. These measurements allow for a computer generated image of the cross section of the tree that indicates areas that are decayed or different densities. There are also trainings available to learn how to use these tools and how to assess a trees risk. ISA offers a course to become TRAQ (tree risk assessment qualification) certified.

WCTMI introduced many people to risk tree assessments and provided them with knowledge and the framework to complete assessments on their own with confidence. Dan Traas showed how simple risk tree assessment can be; he showed that it is something anyone can do and that there is no excuse for not completing a task that could prevent liability. CTMI continues to offer valuable information and presentations on topics that attendees encounter every day. For more information on WCTMI contact your Regional Urban Forestry Coordinator and watch for future articles in the Urban Forestry News newsfeed.


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