An Experiment: 4 Methods to Evaluating Tree Stability

In 2008, the German association of certified arborists established a research group to evaluate the tools of measuring tree stability. In addition to the group of certified arborists, there were also independent scientists and equipment manufacturers. There were three objectives to the project, ultimately determining the best tools, methods and analysis for evaluating tree stability.

Sixteen trees were examined in this project, all of which were already planned to be felled due to major defects such as cracking and decay. For each tree, two days of field work were scheduled. Day one included the examination of the trees thoroughly using carious evaluation tools. On day two, the trees underwent a tensile test where the trees were pulled until they fell or fractured. Afterward, samples and data were analyzed to determine the best methods for determining stability.

Drill Resistance Measurement

Drill resistance measurement involves recording the penetration resistance of a drilling needle through wood to generate a profile of wood decay and cavities. Up to 12 drillings per level were completed in order to obtain a three-dimensional image of the truck damage to the tree.

The field test showed that the direct analysis of tree drill resistance tree profiles required a solid understanding of several arborist-related factors. Massive rotting was clearly recognized by all machines and related wall thicknesses were determined in a decisive manner. Lastly, peaks caused by considerable wood degradation were also clearly interpreted.

In some cases, this method could not determine early stages of certain fungal infections. Other measurement methods were deployed to get additional information, like sonic tomography and electric tomography.

Sonic Tomography

Sonic tomographs detect defects in a non-invasive way by generating a two-dimensional map of the sound velocity transmitted across a tree’s section. At similar measurement points, examination of the sonic tomographs produced by different measuring devices revealed similar results.

The test showed that sonic tomography can be used to detect wood structural changes, but is not able to determine factors that cause them (crack, hollow, or moisture). For this, the data generated by drill resistance measurement and electric tomography were used to produce a wood density reference to verify the tomogram measurement.

Electrical Impedance Tomography

This tool analyzes a tree by sending electrical voltage through the investigated trunk zones, providing a two-dimensional map reflecting the corresponding status of electrical impedance, allowing the user to draw conclusions about the tree’s structural integrity.

Wile an electric tomograph usually cannot be used as the sole basis of tree-static assessments, it turned out to be an adequate verification tool of unclear findings given by sonic tomography devices. In combination, these two methods provided findings that enhanced the tree’s overall evaluation.

Static Load Tests

In a tensile test, a tree is exposed to a simulated wind load. The data measured by strain sensors an inclination angle sensors, combined with empirically measured comparative standards, provide information on the trunk’s load-bearing capacity and the tree’s anchorage force in the ground. This results in an evaluation of the tree’s static condition.

The overall analysis put a special focus on the evaluation of the static load test method. In the analysis, this method provided concrete measurement values, allowing for a comprehensible, transparent calculation result. The static load test method came to conclusive results regarding the failure behavior of the trees.

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