How a Qualified Tree Pathologist Diagnoses Internal Decay Using Resistograph Drilling
A tree can appear entirely healthy from the outside while carrying advanced structural decay within its trunk. The bark is intact, the canopy is full, and to any casual observer – and to many unqualified operators – there is no visible cause for concern. It is precisely this gap between external appearance and internal condition that makes the assessment of structural integrity one of the most technically demanding disciplines in professional arboriculture. Resistograph drilling is one of the most valuable tools available for bridging that gap. In the hands of a qualified tree pathologist, it can reveal with considerable precision what no visual survey, however thorough, is able to confirm: the presence, extent and distribution of internal decay within a living tree.
What Is Internal Decay and Why Is It So Difficult to Detect?
The invisible threat inside a living tree
Wood decay in trees is caused primarily by fungi – organisms that colonise woody tissue and break down either the cellulose and hemicellulose of the cell walls (white rot) or the lignin that gives wood its structural rigidity (brown rot). Both processes compromise the mechanical properties of the affected wood, reducing the load-bearing capacity of the trunk or branch in which they are active. The critical point, and the one that makes internal decay so hazardous, is that fungal colonisation typically progresses from the inside outward. A tree’s outermost layers – the sapwood, the cambium, the bark – may remain functionally intact long after the heartwood at the core has been significantly degraded. The tree continues to grow, continues to produce foliage, and continues to look, from the pavement or the garden fence, like a healthy specimen.
Why visual inspection alone is not enough
A competent visual tree assessment, carried out in accordance with the methodology set out in the Arboricultural Association’s guidance and informed by BS 5837:2012, remains the essential first stage of any tree survey. An experienced arborist conducting a ground-level survey will note crown condition, basal decay indicators, fungal fruiting bodies, cracks, bark anomalies and evidence of previous damage – all of which inform the probability of internal problems. But visual assessment has a ceiling. The absence of visible decay indicators does not confirm structural integrity; it confirms only that structural compromise is not yet expressed externally. Where a visual survey raises concern, or where a tree’s location and consequence of failure demand a higher standard of confidence, advanced diagnostic tools become necessary. This is where resistograph drilling enters the process.
What Is a Resistograph and How Does It Work?
The mechanics of resistance drilling
The resistograph is a device that measures the mechanical resistance encountered by a thin, rotating drill needle as it is driven through wood at a controlled, constant feed rate. The needle – typically between 1.5 and 3 millimetres in diameter – is fine enough to cause negligible harm to the tree when used correctly, and is inserted at a predetermined depth appropriate to the diameter of the stem being assessed. As the needle advances, the instrument records the resistance it encounters at each point along its path. Sound, dense wood offers high resistance. Degraded wood – softened by fungal activity, dried out by brown rot, or hollowed by advanced decay – offers significantly less. The instrument plots this data in real time as a resistance profile: a continuous trace, printed or stored digitally, that maps the internal condition of the wood column along the drill path.
Reading the trace: what the data shows
The resistance profile produced by a resistograph is not a photograph of the interior of a tree – it requires trained interpretation to extract meaningful structural information. In sound wood, the trace shows a consistently high, relatively stable resistance reading throughout the measured depth. A sharp, sustained drop in resistance at any point along the trace indicates a transition to degraded or absent wood – a void, a column of decay, or a region of advanced softening caused by fungal activity. The precise character of the drop matters: a gradual decline can indicate progressive softening through a decay column, while an abrupt fall to near-zero resistance typically suggests a cavity. The position of the anomaly within the cross-section – central, eccentric, or close to the surface – informs the assessment of residual wall thickness, which is the key variable in any biomechanical appraisal of the tree’s structural condition.
The Role of the Qualified Tree Pathologist
Selecting drill positions and interpreting results
The resistograph produces data; the qualified tree pathologist provides the judgement necessary to collect that data meaningfully and interpret it correctly. The selection of drill positions is not arbitrary. A single drill path through a trunk provides a one-dimensional profile along one diameter. Because decay columns are rarely perfectly circular or centrally distributed, a single reading can either overstate or understate the extent of internal compromise. An experienced pathologist will select multiple drill positions – typically at different heights on the stem and at different horizontal angles – to build a three-dimensional picture of the decay pattern. The positions are chosen on the basis of prior visual observations: the location of fungal brackets, the presence of cracks or bulges, the suspected entry points of the original infection, and the structural zones most relevant to the tree’s mechanical loading. Without this contextual reasoning, the data produced by the instrument is of limited diagnostic value.
Interpreting the resulting traces requires familiarity with the decay patterns associated with specific fungal species, an understanding of the CODIT model and how trees compartmentalise infection, and experience in relating resistograph profiles to the structural behaviour of wood under load. The Mattheck and Breloer residual wall thickness formula – which suggests that a tree with a ratio of residual wall thickness to overall stem radius below approximately 0.3 carries significantly elevated risk of stem failure – is one widely applied reference point in this analysis, though it is a starting position for assessment rather than a definitive threshold applied in isolation.
Combining resistograph data with other diagnostic tools
Resistograph drilling is most valuable when it forms part of an integrated diagnostic approach rather than a standalone test. In practice, a qualified tree pathologist working on a complex case in North London – assessing, for example, a large veteran oak in Forty Hall or a mature London plane in a conservation area in Enfield Town – will typically combine resistograph data with several other sources of information. Sonic tomography, which maps internal structure by measuring the velocity of sound waves transmitted through the cross-section of a stem, can provide a two-dimensional image of the internal condition across the full diameter without drilling. Used alongside resistograph traces, it allows the pathologist to cross-reference findings and identify the spatial distribution of decay with considerably greater confidence. Fractometer testing – which measures the breaking angle of a small wood sample extracted during drilling – can provide additional data on the mechanical properties of the wood at the affected site. Fungal identification, whether through visual recognition of fruiting bodies or laboratory analysis of tissue samples, informs the prognosis: different fungal species degrade wood at different rates and in different patterns, and knowing which organism is driving the decay matters for predicting how the condition will progress.
What Happens After the Diagnosis?
From data to decision
A resistograph assessment does not produce a binary verdict of safe or unsafe. It produces a body of evidence that a qualified pathologist uses to assign the tree to a risk category and to formulate a management recommendation. That recommendation might be that the tree presents no significant structural concern beyond normal monitoring intervals. It might be that a programme of regular reassessment is warranted, with retesting at defined intervals to track the progression of decay. It might be that specific structural interventions – the installation of a dynamic bracing system, the removal of a particularly affected co-dominant stem, or a targeted crown reduction to reduce the wind load on a compromised trunk – are advisable. In cases where the ratio of sound to degraded wood falls below acceptable thresholds, and where the consequences of failure are significant, the recommendation may be for removal. What distinguishes a properly conducted resistograph assessment from a rough judgement call is the quality and traceability of the evidence on which that recommendation rests.
The Enfield context: why accurate diagnosis matters here
The urban forest of the London Borough of Enfield presents a particular set of circumstances that make advanced diagnostic assessment not merely desirable but frequently essential. The borough contains some of the most mature tree stock in Outer London – veteran trees within ancient parkland at sites such as Trent Park and the Enfield Chase remnants, large street trees along residential corridors in Palmers Green, Winchmore Hill and Edmonton, and privately owned trees in gardens whose scale reflects generations of uninterrupted growth. Many of these trees carry Tree Preservation Orders. Others sit within conservation areas where the presumption is strongly in favour of retention. Removing a significant tree on the basis of a suspected but unconfirmed structural problem is not straightforward legally, and nor should it be. Conversely, retaining a tree that is carrying advanced internal decay in a location where failure would have serious consequences for people or property is not an acceptable position either. The resistograph, properly deployed and correctly interpreted by a qualified pathologist, provides the evidential foundation that allows decisions about trees in these settings to be made on the basis of what is actually inside them – not on the basis of what can or cannot be seen from the outside.
Conclusion
The resistograph is not a magic instrument, and no reputable pathologist would present it as one. It is a precision diagnostic tool that, in experienced hands, substantially closes the gap between what a tree shows on its surface and what is happening within its structure. In a borough like Enfield, where the stakes attached to tree management decisions – legal, financial, ecological and human – are considerable, the difference between a professional diagnosis and an educated guess is not a small one. Accurate internal assessment is the basis on which sound tree management is built, and it is one of the clearest illustrations of why the qualifications and experience of the person assessing a tree matter as much as the equipment they are carrying.