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Advanced Tree Risk Assessment: Resistance Recording Drills1

Drew McLean, Andrew Koeser, and Gitta Hasing 2

Tree risk assessments can be conducted at various levels depending on a client's needs, the resources available, or the goals of the assessment. The American National Standard (ANSI) A300: Tree Risk Assessment Standard and the International Society of Arboriculture (ISA) Best Management Practices: Tree Risk Assessment define three levels of tree risk assessment:

  • Limited visual (or Level 1)

  • Basic (or Level 2)

  • Advanced (or Level 3)

The level of assessment required should be established with your client before any work begins. If tree conditions cannot be adequately assessed under the initial assessment level carried out, the assessor can recommend the use of a more advanced assessment level to gather the additional information required.

Advanced, or Level 3, assessments provide detailed information about specific tree parts, defects, targets, or site conditions. Tree assessments performed at this level are usually time intensive and more expensive to conduct, as they require specialized tools and/or skills. Advanced assessments typically are reserved for high-value trees or trees of historical importance. Level 3 assessments can include, but are not limited to:

  • aerial crown inspections

  • internal decay inspections

  • subsurface root inspections

  • measuring changes over time (e.g., leans)

  • load testing or pull tests

Limited Visual and Basic Risk Assessments: A Closer Look

Though this series focuses on advanced (Level 3) risk assessment methods, limited visual and basic risk assessments are still important approaches for assessing tree risk in a variety of commercial, municipal, and utility arboriculture applications.

In a limited visual, or Level 1, assessment, the arborist performs a quick visual assessment (walk-by, drive-by, or aerial), typically looking for obvious defects (e.g., dead trees, broken branches, or large cavity openings). This level of assessment usually is reserved for rapid assessments of larger tree populations and is most commonly used for urban forestry and utility rights-of-way management. Limited inspections allow greater numbers of trees to be assessed on a more frequent basis relative to Level 2 and Level 3 assessments. A limited visual risk assessment may not include risk mitigation recommendations. It sometimes is used as an initial screening to help determine which trees need a higher level of inspection.

A basic, or Level 2, assessment is the assessment level used by most arborists when performing tree risk assessments for private clients. The Level 2 assessments consist of a detailed visual inspection of a tree and its surrounding site and a complete analysis of the information gathered. During a basic assessment the arborist will walk completely around the tree looking for defects. from the root crown to the canopy top. Basic risk assessments may include the use of some simple tools (i.e., binoculars, mallet, and probe) to help acquire information about any potential defects. A basic risk assessment also includes an analysis of the information collected during the inspection and a list of possible mitigation options. See Koeser et al. (2013) for more information.

Resistance Recording Drills

Resistance recording drills are specialized pieces of decay detection equipment that may be used as part of an advanced risk assessment. While not required for tree risk assessment work, resistance recording drills have been shown to be effective in helping arborists detect and document internal tree decay in trees (Costello and Quarles 1999; Johnstone et al. 2007). As the drilling needle (bit) passes through the wood of a trunk, branch, or root, the resistance it encounters is recorded. Sharp decreases in the resistance serve as an indication that a pocket of decay or a crack is present. As their name indicates, resistance recording drills provide visual documentation on drilling resistance for final reporting. Measurements are recorded at a 1:1 scale on wax paper or on receipt paper printouts, allowing a trained user to assess the condition of the tree part drilled in a minimally invasive manner. Some drill models record resistance data electronically for visualization and assessment with proprietary software suites.

Commercially available resistance recording drills are currently produced by two companies (Rinntech, St. Charles, IL and IML, Inc., Moultonborough, NH). Both manufacturers offer a range of models which differ primarily in their size and the length of the needle they can accommodate (which in turn determines the maximum drilling depth) and the manner in which resistance is recorded (electronically, manually on wax paper, or printed on paper). Some models offer higher levels of precision than others and are intended for research use. In some instances, these drills can be used to measure the annual growth rings of a tree, as early- and late-wood have differing densities.

Using a Resistance Recording Drill

This article serves as a photographic tutorial of a resistance recording drill and its use as part of an advanced tree risk assessment. While the IML Resistograph® is shown, this simply reflects its availability to the authors and should not be considered an endorsement of the product. The model pictured in Figure 1 is powered by a standard cordless drill. Alternatively, resistance recording drills may feature an integrated motor. In the model depicted, the drill chuck is connected to a series of gear wheels and clutch discs which help control the movement and speed of the drilling needle (Figure 1). The drilling needle passes through a brass sleeve that is pressed against the trunk of the tree during use (Fig.1 and Figure 2). A paper strip with a wax coating (similar to a scratch-off lottery ticket) is inserted at the top of the drill and marked with a metal recording stylus (Figure 1). As the drilling needle moves, the recorder stylus plots the resistance of the drilling needle onto the wax paper for a 1:1 scale output.

Figure 1. IML Resistograph® model F500-S.
Figure 1.  IML Resistograph® model F500-S.
Credit: Gitta Hasing, UF/IFAS

Figure 2. Close up of the drilling needle and the needle sleeve.
Figure 2.  Close up of the drilling needle and the needle sleeve.
Credit: Gitta Hasing, UF/IFAS

The particular model shown has two sensitivity settings for hardwood or softwood tree species (Figure 3) as well as an electronic unit, located on the rear of the drill attachment casing, which records the drilling data electronically and allows data to be downloaded to a computer.

Figure 3. Close-up of the back of the IML Resistograph®. This unit has two sensitivity settings (for hardwood and softwood) and a data logger for recording resistance data.
Figure 3.  Close-up of the back of the IML Resistograph®. This unit has two sensitivity settings (for hardwood and softwood) and a data logger for recording resistance data.
Credit: Gitta Hasing, UF/IFAS

Prior to measurement, the sensitivity level is adjusted (if applicable) to match the wood type tested (hardwood or softwood). After locating a tree section of concern, the user places the drill flush against the bark and pulls the drill trigger. The resistance recording drill is held as steady as possible while drilling into the tree. (Figure 4). For small trees, the needle length may be sufficient to drill completely through the area of interest. Conversely, large trees may require multiple measurements toward the center of a tree to assess the level of decay present. Once a measurement is complete, the needle is reversed out of the tree and back into the unit. If possible, the needle can be pulled out to help conserve battery power.

Figure 4. Close-up of the IML Resistograph® in use.
Figure 4.  Close-up of the IML Resistograph® in use.
Credit: Gitta Hasing, UF/IFAS

Resistance Recording Drill Output and Interpretations

The resistance recording output shown in Figure 5 is printed on wax paper output at 1:1 scale. Read from right to left, the wax paper has vertical lines scaled in either inches or centimeters (inches shown) and four horizontal lines that represent the relative amplitude from 0 to 100%. Each tree species has a "typical" resistance drilling pattern for a healthy tree and some experience is required to interpret outputs effectively. In the output depicted below, non-decayed wood is associated with the areas of higher relative resistance (Figure 5). As the drilling needle reached decayed wood in a tree, the decrease in wood density resulted in a noticeable drop in resistance on the output printout (Figure 5). Beyond this lower resistance, areas linked to decay often appear a flat areas with few vertical peaks (caused by growth rings).

Figure 5. Close-up of a Laurel oak (Quercus hemisphaerica) slab with a wax paper output laid on top of it. The graph has been enhanced for this photograph.
Figure 5.  Close-up of a Laurel oak (Quercus hemisphaerica) slab with a wax paper output laid on top of it. The graph has been enhanced for this photograph.
Credit: Drew McLean, UF/IFAS

Figure 5 shows a slight increase in drilling resistance as the drill penetrated past the bark and continues into the sapwood of the tree. When the drill bit reached the large cavity a few inches into the slab, the relative resistance on the output profile dropped and leveled off until solid wood was reached again.

Resistance recording drills are just one means of quantifying internal decay in urban trees. Several other decay detection methods are listed and compared in Table 1. With practice, any of these can be used to further inform tree risk assessments beyond what can be noted through a visual inspection.

Additional Resources

Costello, L. and S. Quarles. 1999. "Dectection of wood decay in blue gum and elm: An evaluation of the IML-Resistograph and the portable drill." Journal of Arboriculture 25:311–317.

Johnstone, M., P. K. Ades, G. M. Moore, and I. W. Smith. 2007. "Predicting wood decay in Eucalypts using an expert system and the IML-Resistograph drill." Arboriculture and Urban Forestry 33(2):76–82.

Koeser, A. K., G. Hasing, D, McLean, and R. Northrop. 2013. Tree Risk Assessment Methods: A Comparison of Three Common Evaluation Forms. ENH1226. Gainesville: University of Florida Institute of Food and Agricultural Sciences.

Mattheck, C., K. Bethge, and W. Albrecht. 1997. "How to Read the Results of the Resistograph M." Arboricultural Journal 21:331–346.


Table 1. 

Methods currently used to detect internal decay.


1. This document is ENH1243, one of a series of the Environmental Horticulture Department, UF/IFAS Extension. Original publication date September 2014. Reviewed August 2018. Visit the EDIS website at
2. Drew McLean, biological scientist; Andrew Koeser, assistant professor; and Gitta Hasing, senior biological scientist; Deptartment of Environmental Horticulture, Center for Landscape Conservation and Ecology, UF/IFAS Gulf Coast Research and Education Center, Wimauma, FL 33598.

Publication #ENH1243

Date: 8/16/2018

  • Program Area: Plant Systems
Fact Sheet


  • Andrew Koeser