What is the resistograph?
The Resistograph is a precision instrument that is considered the industry standard for wood decay detection around the world. It works by drilling a very small (3 mm) hole into a tree and producing a graph that quantifies the resistance of the wood. TreeLogic is the only business in Israel to offer assessments using the Resistograph, combining scientific data with visual assessments when evaluating the structural integrity of a tree.
The graph can be kept on record for future use. A more detailed description of the instrument can be found here. And a video of how the machine works can be found here.
When do we use the resistograph?
t/R Ratio
Before using the resistograph we must determine whether there are signs of decay within the trunk that warrant sampling. This is done visually using the VTA-method. Additionally, we take the diameter and height of the tree in order to determine the minimum amount of sound wood (t) required for the tree’s safety.
The formula we use as a benchmark is the t/R ratio. This formula was developed by Claus Mattheck, a German engineer who specializes in tree dynamics. If the value of t (sound wood)/R (radius) ≤ 0.3, then the tree poses a risk. Determining the minimum t provides us with the minimum drilling depth required for an informed decision.
Additionally, we check the H (height)/D (Diameter) ratio, as shown below:
Some examples
Species: Casuarina equisetifolia
Diameter (DBH): 34 cm
Height: 13 m
H/D ratio: 38
Min. Required t: 5.1 cm
Actual t at: 9 cm, 10 cm, 13 cm, 9 cm
4 drillings were done on this tree. The amount of drillings depend on the diameter and condition of the tree.
This appears to be a bad defect. However, once we examined it with the resistograph, we found the amount of sound wood to be more than adequate for this tree. .
What is next for this tree?
Decay was found at a depth of 9 cm. It is crucial that this tree gets re-examined again within a year in order to determine the decay’s progress. If the progress is fast, it could be a reason to remove the tree even if the t/R ratio is still within reasonable limits. An aggressive decay in combination with a weakened tree is cause for removal of the tree. If the progress is slow, measures can be taken to increase the health of the tree and the tree could live with the decay for many years. Additional monitoring and its frequency is to be determined during the second assessment.
The crown of this Ficus is thin with dead branches in the tips, indicating root problems
Species: Ficus microcarpa
Diameter (DBH): 68 cm
Height: 11 m
H/D ratio: 16
Min. Required t: 10.2 cm
Actual T at: 0 cm, 7 cm, 4 cm, 5 cm
The trunk appears to be healthy on the outside. However, once we drilled into the tree, we found the amount of sound wood to be inadequate.
Fruiting bodies of the fungus Ganoderma lucidum were found on the roots and trunk.
The fruiting body of Ganoderma lucidum.
Multiple drillings are shown here taken at different heights on the trunk.
When drilling this tree it was clear that the trunk was badly rotten on the inside. The trunk appeared to be sound looking from the outside, but in fact there was little sound wood left.
The tree is dangerous.
What is next for this tree?
This tree must be removed immediately. The lack of sound wood in combination with the Ganoderma lucidum fruiting bodies on the roots are more than adequate proof that this tree is at high risk of failing.
Guidelines for Resistograph sampling
Knowledge of tree biology, physiology, mycology and visual symptoms are required in order to sample with the resistograph, and it should only be used after a VTA concludes that further investigation is necessary. Using any invasive instrument without good reason harms the tree needlessly and should be discouraged, even if the invasion is minimal.
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