Evaluating Internal Tree Decay in Mature Trees in Macomb County

Mature trees provide shade, landscape structure, and long-term value to properties throughout Macomb County, Michigan. When concerns arise about internal decay, trunk cavities, storm damage, or structural weakness, the question often becomes whether the tree is still safe or if tree removal is necessary. 

So, is it possible to determine if a tree is hollow? If there is a hollow area in a tree how can you tell how large it is? Does a hollow area in a tree automatically create a hazardous condition?

Visual inspections provide valuable information, but not all decay is visible from the outside. Trees are highly compartmentalized organisms, meaning significant internal decay can exist while the outer structure still appears intact. This is where advanced diagnostic tools such as the Resistograph become essential.

Resistograph testing allows arborists to evaluate the internal condition of a tree without causing significant damage. Instead of relying solely on visible symptoms, this technology provides measurable data about wood density and structural integrity.

What Is a Resistograph

A Resistograph is a diagnostic tool used to measure wood resistance within a tree. It works by drilling with a very small diameter drill bit, so small it’s referred to as a needle, into the trunk or major limb while recording the resistance encountered as the needle penetrates the wood.

Healthy, solid wood provides consistent resistance. Decayed, hollow, or compromised wood offers significantly less resistance. The tool records this information in a graph format, allowing the arborist to identify areas of concern.

The hole created by a Resistograph is extremely small, typically comparable to the diameter of a finishing nail, and does not create significant damage to the tree.

Why Internal Decay Is Difficult to Judge Visually

A tree may appear healthy externally while internal wood fibers are deteriorating. Conversely, a tree with a visible cavity may still have sufficient solid wood to remain structurally stable.

In Macomb County, older oak trees, maple trees, beech trees, and pine trees frequently develop internal decay due to storm injury, pruning wounds, construction damage, or fungal infection. Determining whether that decay compromises structural integrity requires more than a surface assessment.

Without diagnostic testing, property owners are often left with uncertainty. The Resistograph reduces that uncertainty by providing objective data.


The graph reading from the Resistograph shows if the tree trunk is hollow and the size of the hollow area in a tree trunk. | Ryan Crosby, Owen Tree Service

Common Situations Where Resistograph Testing Is Recommended

•  Trees with visible cavities at the base or along the trunk.

•  Large trees located near homes, driveways, or high-traffic areas.

•  Trees with a history of storm damage.

•  Suspected internal decay from fungal pathogens.

•  Trees being evaluated for preservation versus removal.

•  High-value specimen trees where removal would significantly impact the landscape.

In many cases, Resistograph readings help determine whether a tree can be retained safely with monitoring and corrective pruning rather than removed prematurely.

How the Testing Process Works

Resistograph testing begins with a full visual assessment. Areas of suspected decay are identified based on bark irregularities, fungal fruiting bodies, open tree cavities, seams, or structural lean.

The needle is then drilled in at carefully selected locations. As it penetrates the wood, resistance data is recorded and displayed in a graph. Peaks indicate dense, solid wood. Drops indicate areas of reduced density, decay or the presence of a cavity.

Multiple readings may be taken at different heights or angles to build a clearer picture of the tree’s internal condition. The entire process is minimally invasive and typically completed within a short period of time.

Resistograph readings from a large Norway maple limb. | Trisha Boyd, Owen Tree Service

Understanding the Results

Resistograph data must be interpreted properly. A small area of decay does not automatically mean a tree is unsafe. Structural stability depends on the proportion of solid wood remaining relative to the diameter of the trunk.

In some cases, decay may be localized and surrounded by strong compartmentalized tissue. In other situations, decay may be extensive enough to significantly reduce load-bearing capacity.

An ISA Certified Arborist evaluates the data alongside species characteristics, site conditions, canopy weight distribution, and risk targets to determine the appropriate course of action.

Resistograph Testing and Risk Management

Tree risk management is not simply about identifying decay. It involves evaluating the likelihood of failure and the potential consequences if failure occurs.

For example, a tree with moderate internal decay located in an open field presents different risk considerations than the same tree positioned over a home or parking area.

Resistograph testing provides clarity when making these decisions. It supports informed choices rather than reactive tree removal based on uncertainty.

Preservation Versus Removal

One of the most valuable aspects of Resistograph testing is its ability to support 
tree preservation when appropriate.

Without diagnostic data, many trees are removed out of caution. In some cases, Resistograph testing has revealed that sufficient structural wood remains, allowing the tree to be retained with monitoring, cabling, or selective pruning.

Conversely, testing can confirm when removal is the safest option, preventing unexpected failure.

This balanced approach helps property owners make confident decisions.

Species Commonly Evaluated in Macomb County

Resistograph testing is frequently used on large white oak trees, red oak trees, silver maple trees, Norway maple trees, and mature pine trees. These species often reach substantial size and may develop internal decay as they age.

Because these trees contribute significant value to landscapes, accurate structural evaluation is essential before making tree removal decisions.


The Resistograph can be used to determine if there is a crack in a tree trunk and how large it is. | Ryan Crosby, Owen Tree Service

Limitations of Resistograph Testing

While the Resistograph is a powerful diagnostic tool, it is not a cure or treatment. It provides information about internal wood density but does not eliminate decay.

Testing also evaluates specific points within the tree. Although multiple readings improve accuracy, no tool can assess every inch of internal wood.

For this reason, Resistograph results are integrated with visual inspection, site analysis, and professional judgment.

The Role of Professional Assessment

Using a Resistograph effectively requires training and experience. Interpreting resistance graphs correctly and understanding how they relate to structural integrity is critical.

At Owen Tree Service, Our ISA Certified Arborists utilize diagnostic tools such as the Resistograph to provide data-driven recommendations. The goal is to protect property owners while preserving healthy trees whenever possible.

By combining technology with field experience, we can provide accurate evaluations rather than relying solely on visual estimation.

Conclusion

Internal tree decay is not always visible from the outside. Relying on appearance alone can lead to unnecessary removal or overlooked risk.

Resistograph testing offers a minimally invasive method for evaluating wood density and structural integrity in mature trees. By providing measurable data, it helps determine whether a tree can be preserved safely or requires removal.

In Macomb County, where mature trees contribute significant environmental and property value, informed decision-making is essential. The Resistograph allows property owners to move forward with clarity; balancing safety, and preservation in a responsible and professional manner.

If you have a tree with a visible cavity and you’re wondering if it is safe to keep the tree, Resistograph testing can help with your decision. To set up Resistograph testing for your tree send us a message or call us today at 800-724-6680.