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Core Values

The process of learning by our mistakes means there inevitably comes a point where we no longer do things that once seemed the proper thing to do, and, often later, a point when we shudder to think we ever did.  In arboriculture it’s no different.

Many of us in our earlier lives as tree surgeons solemnly dangled buckets of evil black gunk from our toolbelts to paint over the humungous wounds we had just carefully cut as flush to the trunk as we artfully could; drilled massive holes at the bases of water-filled cavities to drain them; and filled basal cavities with expanding foam, or other substances too embarrassing to recall. It seemed the right thing to do at the time, but later research showed it wasn’t, and I guess we sort of quietly hoped the world would forgive, if not totally forget.

When pendulums of good practice swing the other way, however, often something of value gets lost in the process. The realization of the importance of the processes of compartmentalization of injuries by trees, and of the significance of breaching barrier zones and walls, has led to a generally unfavourable view of drilling holes into living trees, and no doubt rightly so. As a result, considerable effort has gone into devising methods of examining trees internally that don’t require invasive drilling.

But what about the humble old increment borer? Does this still have a place in today’s arboricultural practice? Can it provide us with answers to questions that are often critical in arboricultural investigations, but which are less easily discovered by other means? Definitely yes.

For those unfamiliar with the tool, it’s essentially a hollow drill which is screwed manually into a tree trunk or large branch to the required depth, and then the cylindrical core captured within it, which is around 5mm in diameter, is extracted by means of a steel extractor inserted into the hollow section of the drill. The extracted core contains the annual growth rings of the trunk or branch to the depth extracted – in some cases, depending on the tree species, these can be ‘read’ directly in the field, but more often than not, the core requires some cleaning up and preparation back at the home base to enable the annual rings to be seen clearly.

Like any other piece of kit, it takes a bit of practice to use effectively, and some experience to read the resulting core samples with confidence. However, for the information that can be gained, it’s worth the effort.

So what can they tell us? First and most obviously, the age of the tree or branch (normally the tree) being investigated, and although it’s a bit of a cliché, it really is only bettered by cutting the tree down and counting the rings, which isn’t always what one wants to do.  To get the age right, the increment borer ideally needs to be drilled directly towards the trunk centre, and if possible to half the trunk diameter, or a little further. But it doesn’t necessarily matter if one doesn’t hit the exact centre – from the radii of the innermost rings in the core, one can normally estimate where the exact centre would be, and how many rings one has missed.

The beauty about a tree’s annual growth rings is that they don’t lie – not many things (or people) in life one can say that about – so what a core sample provides is an incontrovertible physical record of a tree’s growth. This can be invaluable in some types of investigation – for example, was the tree present when this or that land was sold? or when this or that house was built? or when this old TPO was made? - that sort of thing – and can save many hours of argument over wildly varying calculations or estimates of age or growth.

The second most useful type of information core samples provide is how a tree has been growing. The width of the annual rings from year to year varies in response to climate and the prevailing weather conditions of the year in question, and this is the basis of the science of dendrochronology, the archaeological dating of timbers used in old buildings, ships and other structures. Comparison of the ring growth in a particular timber with ‘reference sets’ of annual ring growth stretching back for centuries allows this to be done with remarkable accuracy. 

But ring growth is also influenced by the tree’s immediate environment, and by its physiological health.  Depending on species and age, the average width of annual rings of semi-mature or mature trees of most large-growing species in normal health will generally be around 4.5mm to 6.5mm. So if one sees a sudden change in annual ring width, say a reduction to only 2mm-3mm, that normally indicates something has happened to impact on the tree’s ability to make normal growth. This can result from the loss of a large part of its leafing canopy, for example as a result of storm damage or from severe pruning or pollarding; but equally it can occur because of adverse changes in its soil environment – the loss of significant roots to excavations on a construction site, or asphyxiation of its root system through soil compaction caused by construction traffic, for example.

In this sort of situation, the growth rings within a core sample can help to confirm what has occurred, when it occurred, and crucially, whether or not the tree is recovering from the event, continuing to suffer from it, or (worse) declining further. This type of information can be particularly useful in cases where, for example, protective measures on construction sites have either failed, or have not been properly implemented, and the questions either of responsibility for the damage, or of what remedial actions may be necessary to assist tree recovery, are at issue between the developer and the local authority.

Not only can the pattern of growth ring response indicate what has happened to a tree itself, but also what has happened to other trees around it. If a tree is growing in conditions where it is being competed with by other trees nearby, such as in a woodland, its growth ring widths will typically be much less than those normal for a tree growing in the open, often by as much as half. If surrounding trees are felled, this will show up in the suddenly wider growth rings of the tree which remains, due to the increase in resources available to it.

In a recent planning case, for example, which involved extensive debate about a sweet chestnut coppice woodland’s past management, core sample analysis of the oak standards in the wood enabled the dates and frequency of the cycle of coppice cutting around them to be demonstrated beyond doubt – as the coppice grew up in successive cycles, the growth rings of the oaks progressively narrowed, and then widened again back to normal ‘open-grown’ levels immediately following each occasion the coppice was cut.

Core sampling has other diagnostic uses as well – dating trunk injuries by counting the annual rings present in ‘woundwood’ tissue around them which has formed since the date of the injury; helping to establish the extent of sound wood remaining around central trunk decay columns (although other tools are available which can establish this less labour-intensively); and assisting diagnosis of disorders through being able to see discolouration or staining of wood tissue. It is particularly useful where waterlogging is suspected, as this is immediately obvious in the characteristic black staining of growth rings formed since the onset of the problem.

But do the holes drilled by an increment borer damage the tree, or make it more susceptible to fungal infection or decay subsequently, or encourage the spread of decay already existing within the tree as a result of breaching compartmental ‘walls’? Research suggests not, or at least not as much as one might think. In a tree of average health, the holes themselves are normally completely occluded by the following season’s growth ring, and results of comprehensive testing have shown that the extent to which any spread of pre-existing decay is encouraged is extremely limited, and little different from that resulting from the considerably smaller diameter (3mm) drilling holes made by other decay detection equipment, notably the Resistograph.

So whilst it’s sensible, on a precautionary basis, not to overdo things and start madly core sampling every tree one sees for no good reason, equally we shouldn’t be as nervous about it as perhaps, in recent years, some practitioners have been inclined to be. In the right circumstances, the value of the information a core sample can yield will normally far outweigh the fractional damage and risk of longer-term harm that will be caused by obtaining it.

In cases relating to tree age, past or recent damage or management, property damage, and even boundary disputes, the evidence in even a single core sample can prove crucial, and often decisive. Still very much a tool to have in the box, then, and one which even in the 21st century, we shouldn’t be scared about taking out and putting to good use.