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Tree Risk Assessment - A Case Study

Updated: Nov 21, 2019

Written by: Certified Arborist August Hoppe WI-0477A

One of the more common questions people ask us is “Is my big tree safe.” Our certified arborists are often called into situations to determine if a tree is a liability. Experience, knowledge and judgement are called upon to make accurate assessments if a tree is safe and sound, or if disaster is looming around the corner. Hoppe Tree Service not only can provide visual assessments, but we have tools to help us look inside trees to determine just how much integrity and solid wood there is inside a limb, stem or trunk. There have been cases where the outside appearance points to removal, yet after an accurate assessment with our “Resistograph” or “Tree Check” tools the trees actually were in much better shape than appeared on the outside. This case study is an example of where outside appearance was deceiving and deserved further examination.

 Hoppe Tree Service received a call regarding a large silver poplar (Poplulus alba) that had suddenly dropped a large 12” diameter limb.  The homeowner was rightly concerned that the tree was unsafe and wanted to find out if the tree should be removed or not.

Board Certified Master Arborist, and Tree Risk Qualified Arborist Fred Hoppe went to the property to inspect the tree.The first thing Fred noticed and pointed out to the homeowner was the large artist’s conk mushroom (Ganoderma applantum)located on main union of the multi stem, codominant trunk tree. Mushrooms like this often can point to large amounts of decay and can be a warning sign that a tree is losing structural integrity. Fred Hoppe used a rubber mallet to hit the trunk and detected a decayed sounding area in the immediate area of the mushroom. This large stem was over 20” in diameter and was growing directly over the house and driveway.

So far, things were not looking too good for this tree. A sudden large branch drop and a decayed section right in the main limb junction of a tree are definitely not positive signs. However, Fred did notice the healthy appearance of the foliage and the good growth the tree had been putting on over the years and felt that this tree deserved further examination.

A sonic wood assessment instrument, called Tree Check, was then used by Fred.  This device sends a sound wave through the wood and measures the time for the wave to travel from the sending point to the receiving point. If a crack, cavity, or decay is present, the sound travels around the defect, increasing the transmission time, compared to a tree with no defects. The results calculated from two separate points found transmission times 40% higher than comparable defect-free wood of the same species. This result was not surprising, and indicating presence of decay, but the Tree Check tool does not measure the extent of weakness. This test alone does not offer conclusive evidence.

For further evidence, Fred Hoppe ran a final test using a Resistorgraph. This tool is uniquely designed drill that accurately measures the soundness of wood and plots it on a graph as it travels through the stem. Four areas of the stem near the visible defect were tested. Results indicated at least 10 inches of solid wood on all four sites surrounding the fungus. The drilling also clearly showed a well-defined response growth from the tree along the perimeter of the decayed sounding area which was starting to compartmentalize the decayed area.

Conclusion: After 3 separate testing methods: rubber mallet, Tree Check sound wave tester, and Resistograph, it was determined that with some proper care that it was entirely reasonable to keep this tree standing. Recommendations included weight reduction pruning on extended large limbs in the canopy, installation of a cable support system and plant health care treatments to invigorate the tree.

Case Update: Three years later, the limb was larger and the tree had experienced no more failures. The tree is still vigorously growing and more visible response growth is occurring, further compartmentalizing the decayed area.

- Originally posted 12 Oct 2017

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