Diagnostic Tools: Sonic Tomography and Static Pull Testing
Two methods of non-invasive tree assessment commonly used to detect decay in the low stem, butt, and roots are sonic tomography and static pull testing. Both of these diagnostic tools work in combination with software to interpret the data collected and compare to other tree characteristics to create a more complete profile of likely modes of failure.
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Sonic tomography detects areas which are likely to have decay by sending sound waves through the wood and measuring their speed between different sensors set around the stem. Tomography is an effective tool for identifying trees with extensive internal damage, but can be less effective in detecting early decay (Dunster and Edmonds, 2023), especially for soft rots like Kretzschmaria which selectively cause deterioration in cellulose (Schwarze, Engels, and Mattheck, 2000, pg. 18). Tomography is portable and can frequently be used from the ground by a technician who may not have climbing expertise. Placement of the sensors and appropriate set-up is tedious, but the overall process is straightforward. The primary limitation of this equipment in detection of decay low in the trunk is that it must completely encircle the tree or the resulting data will be incomplete - therefore it is not practical for assessing the integrity of a tree’s root system. The software provided for processing the data collected often has the option to take a photo of the profile of the tree and calculate wind loading and compare to loss of strength at the point where the tomogram was taken. If the structural capacity of the tree is not sufficient to support the wind load from the crown, mitigation action such as crown reduction can be undertaken to reduce loads or the tree can be removed if the assessment determined that other forms of mitigation will be insufficient. 
Photo of "ArWiLo" software from Rinntech. (Rinntech, n.d.)
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(Top) Example tomogram. (Bottom) Example set up. (Rinntech, n.d.)
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Diagram from A review of tree risk assessment using minimally invasive technologies and two case studies. (Van Wassenaer and Richardson, 2009)
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Static pull testing, on the other hand, assesses likelihood of whole tree failure by putting a load in the direction of prevailing winds and measuring stem deflection and root plate uplift. This testing looks alarming, but once a certain threshold of deflection or deformity is reached, the test can be terminated because sufficient information has been gathered. This level of change is very minimal (about .25°) (Rinn, 2017) and does not cause permanent damage to the tree or pull hard enough to create a risk of failure (Baker and Bunton, 2023).
The load put on the tree to reach the diagnostic threshold (or maximum load the winch is capable of) is then compared to a wind profile analysis to determine the factor of safety that the tree has built into its structure. Most trees have a factor of safety between 1.5 and 2 (that is, they can withstand 150-200% of maximum anticipated wind as estimated by their crown size and local weather patterns) (Baker and Bunton, 2023; Rinn, 2017; Van Wassenaer and Richardson, 2009). This is an excellent method of testing for root and butt rots that a tomograph may not be able to detect because the decay is lower than the device can practically be installed. Static pull testing set up is more complicated than tomography, but in situations where root rot is suspected (such as an area known to have laminated root rot (Phellinus weirii)) it is one of the most effective non-destructive tools for identifying trees which may fail. Setting up a static pull test first requires a tree with a single stem (or two in alignment with the direction of pull) and a site which can accommodate the long ropes and a solid anchor to tension from (Baker and Bunton, 2023). It also requires one technician comfortable with climbing, as well as at least one other person on site to tension the line or watch the test equipment readout. Lastly, the profile of the tree in line with the pull must be visible enough to take a photo to estimate wind loading (Baker and Bunton, 2023). |
Set-up for a static pull test demo with the US Forest Service in an area known to have laminated root rot.
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The Griphoist winch for tensioning the system at the USFS demo.
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The wind profile analysis used in combination with these two tools is an interesting and important part of their functionality. It is a fantastic tool for estimating load during weather events, but also subject to some degree of error due to the three dimensional nature and heterogeneous material properties of trees, as well as the need for line of sight for photographing the entirety of the canopy from the direction of prevailing winds. In general, drawing a polygon around a tree’s crown and estimating wind load for that will be likely to overestimate, and may indirectly cause the removal of trees with sparse growth habits.
More frequently, arborists have been employing photogrammetry - either from their phones or another device from the ground - or from imagery taken by drones programmed to fly around the tree (Komen, presentation; Martin, presentation). Companies such as greehill are offering inventory services using ground-based LiDAR which takes a nearly complete image of every tree the LiDAR system passes by (greehill, n.d.). These have the potential to reduce the error in analyses of wind loading by providing more complete information about the structure of the tree as well as, in the case of an inventory, have the wind load estimation already created at the time when the diagnostic evaluation takes place. Many non-invasive tools are becoming commonplace, and the technology continues to improve. This allows risk managers to have more confidence in their decision-making and more trees to be retained while reducing overall risk exposure.
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Image from greehill's website showing how a tree is represented as a point cloud after LiDAR imaging. (greehill, n.d.)
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This project was built as an assignment for the Tree Risk Management module within the MSc Arboriculture and Urban Forestry course at Myerscough College. All photos and content are my own unless otherwise attributed or cited. All names, addresses, and dates are made up, but accurately reflect season and site conditions at the time of assessment.
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