The utilisation of a device that creates a wound means that the locally-exposed surface can be readily colonised by a pathogen, thereby meaning decay propagation around the wound area and then further beyond is possible – particularly if the tree cannot compartmentalise the wound effectively. If the tree is already weak, which may likely be the reason for such an investigative practice (to determine extent of decay), decay has an even higher probability of either setting in or further propagating. Furthermore, the deeper the wound, the more potentially significant the wound becomes in terms of decay onset and facilitating succession into surrounding areas – small, shallow holes that span no further in than two growth rings are the lowest risk, followed by narrow holes that delve deeper into the wood (Johnstone et al., 2010; van Wessenaer & Richardson, 2009; Watson, 2006; Weber & Mattheck, 2006). Species-specific traits in relation to CODIT do of course dictate how decay may develop (Shigo, 1986; Shigo, 1991), though artificial drilling (or the removal of bark plates) is never beneficial from an energy perspective as the discolouration of wood local to the wound means less wood is available for the tree to store energy, in addition to the potential onset of decay and the consequences incurred thereafter.
It is important to recognise that decay organisms can travel down a bore hole and propagate further within the tree as a result, albeit on a local scale initially. Over-frequent boring can therefore exacerbate instances of decay – particularly as it breaks the natural barriers formed by the tree to stop further decay from occurring (reaction and barrier zones) that may have been laid down in response to pre-existing decay (Lonsdale, 1999; Schubert, 2007; Weber & Mattheck, 2006). Further, the larger the bore hole the greater the risk, on average, of decay onset or propagation (Kersten & Schwarze, 2005). Species that lack true heartwood may be more susceptible to the adverse effects of drilling, because their ‘heartwood’ does not have the extractives that react with oxygen as a mode of defence against pathogenic invasion (Shigo, 1991).
Polyporus squamosus, for example, was found on half of the specimens subjected to increment bore holes during one study. As a fungal species that has a strategy that involves entry via stem wounds, such bore holes may provide conditions for germination of its fungal spores (Kersten & Schwarze, 2005). Such an observation can act as a proxy indicator for similar fungal strategists, that too employ such a tactic for entry. Curiously, the study suggests that certain fungal species might be adversely impacted by the presence of bore holes, given the conditions created as a result of such boring. In this study, Inonotus hispidus was found to have limited radial outgrowth around areas of boring damage sustained by an increment borer (that creates larger holes up to 10mm), whilst not having limited radial outward growth from damage via the IML-Resistograph (that creates much smaller holes). This may be to do with Inonotus hispidus having a preference for low-oxygen, high-carbon dioxide conditions, which other decaying fungi are also adapted to – these conditions are not present when bore holes are larger.
With regards to micro-drills in particular, and for other devices that bore into the wood, shavings created from the drill’s passage through the wood can sometimes be displaced along the bore tunnel, thereby directly aiding with internal propagation and the spread of decay organisms (Axmon et al., 2004; Johnstone et al., 2010; Kersten & Schwarze, 2005). Research has nonetheless concluded that such decay onset from micro-drills is typically short term (8-10 years), after which point compartmentalisation had fully completed. In trees that were tested that had no evidence of decay there was no onset of decay post-drilling (Kersten & Schwarze, 2005; Shigo, 1986). Concerns should however manifest when boring into trees with existing decay, as drilling can, as established, facilitate fungal progression into sound areas of wood.
One must also note that vascular tissue will be damaged during the boring process, which can impact upon hydraulic conductivity and efficiency of the tree’s vascular system by default, in addition to the fact aeration of the xylem and subsequent risk of xylem dysfunction (and subsequent decay onset) will potentially manifest. This is particularly an issue where dulled drill-bits are used – if drilling is necessary, very sharp drill-bits must be utilised. Caution must also be exercised so that, when preparing for drilling, no pulling or twisting movements to remove bark are undertaken, in addition to not drilling when bark is loose in the spring and autumn, and not plugging or dressing the drill holes (Johnstone et al., 2010; Kersten & Schwarze, 2005; Shigo, 1991). Such aforementioned practices can facilitate in the creation of more expansive decay regions, either by providing entry or making site conditions preferable for rapid succession. In addition, bore hole presence can provide conditions for cracks to propagate laterally out from the hole (Shigo, 1986). This can bring about a situation where future failure or weakness can establish and / or cracks can propagate under mechanical stress, caused initially by the presence of the bore hole (Weber & Mattheck, 2003). Ultimately, unless the invasive increment leaves a spindle-shaped hole, the force-flow of the wood grain locally will be interrupted in a manner that detracts from equal stress distribution (Mattheck & Kubler, 1997). The presence of a bore hole (which cannot possibly leave a spindle-shaped tunnel) might therefore impact upon the structural integrity of the tree, given acute stress build-up.
Invasive techniques also require human operation for large portions of the process, and thus the process is open to human error more so than in computerised techniques – i.e. a radial core sample may not be a true radial core sample and thus a second sample may need to be taken, and if a sample is being taken and assessed for decay via feel, smell, etc, then it is open to more subjectivity and prognosis might be incorrect. Computerisation brings about objectivity and improved accuracy, which is critical when looking to determine internal wood decay (Nicolotti & Miglietta, 1998). Human error can simply lead to unnecessary (further) wounding, which will have adverse impacts upon the health of the tree that, given the reasons behind invasive tests, will likely already be under stress. Shigo (1986) even suggests that, if boring is necessary, the operative should practice on a fallen log first.
The financial cost is generally relatively acceptable for many invasive devices, though particular devices may cost tens of thousands of pounds. However, the modest cost does not make up for the need for operators to be properly trained and knowledgeable in the interpretation of the results from the device. If operators are not skilled in the device they are using, not only may the tree be falsely-diagnosed but, as previously mentioned, additional invasive tests may need to be undertaken to obtain the required readings. Additionally, notably for drills, the only decay that will be detected is along the bore tunnel – decay pockets could be missed by a fraction of an inch and thereby avoid detection (Nicolotti & Miglietta, 1998). A skilled operator will be more likely to successfully bore into the correct areas after fewer attempts.
Weighing up the cost versus the benefits really doesn’t paint such invasive techniques as ultimately good for health of the tree, in light of the aforementioned points. However, the ease of use of many invasive methods, when compared alongside accuracy (critical for the safety levels of trees in urban areas, in particular), cost, and reduced implications to tree health with their appropriate (not excessive) use, can justify their utilisation without significant concern (Johnstone et al., 2010). Of course, for specimen trees or trees that are otherwise deemed critically important, non-invasive (or minimally-invasive – the PICUS Sonic Tomograph) means of decay-detection might be preferable – thermal imaging may be the chosen option in such a scenario (Catena & Catena, 2008).
References
Axmon, J., Hansson, M., & Sörnmo, L. (2004) Experimental study on the possibility of detecting internal decay in standing Picea abies by blind impact response analysis. Forestry. 77 (3). p179-192.
Catena, A. & Catena, G. (2008) Overview of Thermal Imaging for Tree Assessment. Arboricultural Journal. 30 (4). p259-270.
Johnstone, D., Moore, G., Tausz, M., & Nokolas, M. (2010) The Measurement of Wood Decay in Landscape Trees. Arboriculture & Urban Forestry. 36 (3). p121-127.
Kersten, W. & Schwarze, F. (2005) Development of Decay in the Sapwood of Trees Wounded by the Use of Decay-Detecting Devices. Arboricultural Journal. 28 (3). p165-181.
Lonsdale, D. (1999) Principles of Tree Hazard Assessment and Management (Research for Amenity Trees 7). London: HMSO.
Mattheck, C. & Kubler, H. (1997) Wood – The Internal Optimization of Trees. USA: Springer.
Nicolotti, G. & Miglietta, P. (1998) Using High-Technology Instruments to Assess Defects in Trees. Journal of Arboriculture. 24 (6). p297-302.
Schubert, S. (2007) Acousto-Ultrasound Assessment of Inner Wood-Decay in Standing Trees: Possibilities and Limitations. PhD Dissertation (Diss. ETH Nr. 17126). Swiss Federal Institute of Technology, Zürich.
Shigo, A. (1986) A New Tree Biology. USA: Shigo and Trees Associates.
Shigo, A. (1991) Modern Arboriculture. USA: Shigo and Trees Associates.
van Wessenaer, P. & Richardson, M. (2009) A Review of Tree Risk Assessment Using Minimally Invasive Technologies and Two Case Studies. Arboricultural Journal. 32 (4). p275-292.
Watson, B. (2006) Trees: their use, management, cultivation, and biology. India: The Crowood Press.
Weber, K. & Mattheck, C. (2003) Manual of Wood Decays in Trees. UK: The Arboricultural Association.
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Arboriculture 
To me this article brings up the all important point that we need to stop testing for “extent of decay” and concentrate on “existence of structurally sound wood.”
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I suspect to some degree the two are synonymous with one another, despite their different meanings. Perhaps they should not be used so interchangeably. Granted, I make a clear effort to try and not mix the two, though still find myself referring to attempting to determine decay extent instead of ascertaining wood properties. Of course, ‘sound wood’ is also an interesting point on its own, as stress distributions on a tree make different areas of the wood structure differ in their cellular and structural properties, and therefore ‘soundness’ is not necessarily going to be uniform across an entire tree. I guess it’s a case of how deep down the rabbit hole you go, then!
I hope you found the article of use, too. 🙂
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[…] assessing wood qualities through the application of, for example, the Resistograph. Granted, issues may – or may not – stem from such wounds created by invasive instruments, and therefore it […]
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