Visual Tree Assessment (VTA) remains the foundation of every tree inspection. However, where the extent of internal decay or structural weakness cannot be determined visually, advanced diagnostic testing can provide objective evidence to support informed tree management decisions.

Thrive Tree Consultancy offers PiCUS Sonic Tomography and Resistograph testing to investigate internal decay, cavities and structural defects within standing trees. These investigations are undertaken independently and objectively, helping clients make proportionate, evidence-based decisions regarding tree retention, management or removal.

What is PiCUS Sonic Tomography (Tree Tomography)?

PiCUS Sonic Tomography is a non-invasive diagnostic technique that uses sound waves to assess the internal condition of a tree stem. Sensors positioned around the trunk measure the speed at which sound travels through the wood, allowing specialist software to produce a cross-sectional image indicating areas of sound timber, decay and cavities.

The system provides valuable insight into the distribution and extent of internal defects that cannot be identified through visual inspection alone.

What is Resistograph Testing?

A Resistograph is a precision micro-drilling instrument used to verify and quantify internal wood condition. A fine drill bit measures drilling resistance as it passes through the stem, identifying sound wood, decay, cavities and changes in wood density.

Resistograph testing is often used alongside PiCUS Sonic Tomography to verify findings and provide additional confidence when assessing the structural integrity of significant trees.

When is advanced decay detection appropriate?

Advanced diagnostic testing may be recommended where:

• Fungal fruiting bodies indicate potential decay.
• Cavities or structural defects require further investigation.
• Internal decay is suspected but cannot be confirmed visually.
• Important trees are being considered for retention or removal.
• Trees are situated adjacent to buildings, highways or areas of frequent public use.
• Objective evidence is required to support arboricultural decision-making.

These techniques are not intended to replace a thorough Visual Tree Assessment, but to complement it where additional information is necessary.

Assessment Throughout the Tree

Whilst PiCUS Sonic Tomography is primarily used to investigate the lower stem and root buttress, typically from ground level to around 3 metres above ground, Resistograph testing offers considerably greater flexibility.

The compact nature of the Resistograph allows targeted investigations to be undertaken throughout the tree, including the main stem, scaffold branches and significant unions. Where defects are located above ground level, Resistograph testing can be carried out in conjunction with an aerial inspection using either rope and harness climbing techniques or a Mobile Elevating Work Platform (MEWP), enabling the internal condition of higher parts of the canopy to be assessed directly.

This combination of aerial inspection and targeted micro-drilling is particularly valuable where visual defects, cavities, fungal colonisation or suspected decay are confined to elevated parts of the crown and cannot be adequately assessed from ground level.

Independent interpretation

Diagnostic equipment does not make decisions. PiCUS and Resistograph results must always be interpreted alongside tree species, age, biomechanics, fungal ecology, site occupancy and the findings of a detailed Visual Tree Assessment. Incorrect interpretation can be as misleading as having no data at all.

At Thrive Tree Consultancy, PiCUS and Resistograph investigations are interpreted within the wider context of a comprehensive arboricultural assessment, ensuring recommendations remain balanced, proportionate and evidence based rather than relying solely upon instrument outputs.

Typical Applications

• Investigation of suspected stem and butt decay
• Assessment of trees with fungal fruiting bodies
• Veteran and notable tree management
• Quantifying residual wall thickness
• Tree risk assessment
• Tree Preservation Order applications
• Planning and development
• Insurance investigations
• Expert witness instructions
• Monitoring the progression of internal decay over time

Every investigation begins with an initial discussion to determine whether advanced diagnostic testing is appropriate. In many cases, a detailed Visual Tree Assessment is sufficient and further investigation is unnecessary. Where PiCUS Sonic Tomography or Resistograph testing is likely to improve the evidence available, an investigation can be designed to answer the specific arboricultural question in a proportionate and cost-effective manner.

Arrange an investigation

If you have concerns regarding the internal condition of a tree, or would like to discuss whether PiCUS Sonic Tomography or Resistograph testing is appropriate, please get in touch. Every investigation begins with an initial discussion to ensure the proposed assessment is proportionate to the question being asked.