Non-invasive decay detection in trees – are gases the future?

I have previously looked at the pros and cons associated with invasive methods of decay detection, and it is clear that wound creation is not something to be desired – in spite of the benefits of assessing wood qualities through the application of, for example, the Resistograph. Granted, issues mayor may not – stem from such wounds created by invasive instruments, and therefore it cannot be said that using invasive technology is always going to be bad, but surely it would be better if a non-invasive method could be used that has a (generally) good degree of accuracy (such as the PiCUS). Even better would it be, if a system can be developed that may be able to ‘learn’ over time.

Such a system that is being developed that may fit into this category – of being non-invasive and being able to ‘learn’ in time (as the instrument is used and updated by the manufacturer) – is one that utilises an ‘electronic nose’ to measure the levels of volatile organic compounds (VOCs) being emitted by a tree and any constituent wood-decay fungi.

Whilst such a system is still in its infancy, the general gist behind the concept is that all trees, whether healthy or decayed, emit gaseous volatiles. In trees that are ‘suffering’ from decay, a particular mixture of voltailes is emitted. Such a mixture is comprised of volatiles produced by the tree (metabolites, and anti-fungal compounds including terpenes and phenols) and by the wood-decay fungi (metabolites) present within the tree. This very mixture can be recorded by an electronic nose (a sensory device), assuming the concentrations are high enough to be picked up by the instrument. Therefore, by ‘teaching’ an electronic nose to identify particular mixtures of volatiles in the air around a tree (essentially, the instrument will hold an ever-growing databse of gaseous volatiles and mixtures of many), in theory the nose should be able to identify whether the tree is healthy or decayed (and perhaps even to what extent). At present, a few different electronic noses are available, and these include the Aromascan A32S Electronic Nose, the Lybranose 2.1 Electronic Nose, and the PEN3 Electronic Nose.

As mentioned, this technology is still in its very early stages (it is only around 10 years old), and even the authors here acknowledge that this research they are undertaking is pioneering. However, they have found that these electronic noses are able to differentiate between healthy wood samples and those that are decayed, where the wood samples are exposed to ambient conditions. When these wood samples are buried under soil, as would be the case with the rooting system of a tree, the PEN3 Electronic Nose still identified those samples which were decayed, though only after a period where the wood has been decaying for at least 12 months (the authors propose that this is because VOCs take longer to ‘build up’ when a soil substrate separates the source of the emissions and the air).

Of course, as the above research was done in the laboratory setting, it is easy to question whether the ability of the electronic nose to pick-up wood decay also applies in the field environment. Therefore, the authors took the noses out into the field, and looked at whether the instruments could differentiate between healthy and decayed roots of 60 street trees in Milan (species included Acer negundo, Acer pseudoplatanus, and Aesculus hippocastanum). It was concluded, following field experiments, that the PEN3 Electronic Nose could indeed accurately determine whether a tree root was decayed or healthy, and could even differentiate between the host tree species by the mixture of volatiles being emitted.

In the field setting, the electronic nose was placed inside a vacuum cylinder, with a small tube that drew air into the vacuum that the nose could then assess.

In light of this field trial, it is clear that the electronic noses have the potential to be very effective, and both at measuring wood decay in the aerial structure of a tree (above ground) and the rooting structure (below ground). The fact that these noses are able to identify decay that is below the ground (and thus not visible) is particularly interesting, as currently there are no effective means of determining whether a root plate is decaying. Hopefully, as this technology is developed further and more aromatic profiles of VOC mixtures are archived, electronic noses may be able to be very accurate (and immediate) in their determination of wood decay, all whilst being non-invasive. And why stop here? A tree that is stressed for other means will also emit a VOC mixture, and therefore the same electronic noses could be used to diagnose other disorders of unlimited scope. This technology may therefore have huge potential.

Source: Baietto, M., Aquaro, S., Wilson, A., Pozzi, L., & Bassi, D. (2015) The Use of Gas-Sensor Arrays in the Detection of Bole and Root Decays in Living Trees: Development of a New Non-invasive Method of Sampling and Analysis. Sensors & Transducers. 193 (10). p154-160.

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Non-invasive decay detection in trees – are gases the future?

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