Rigidoporus ulmarius deep within a basal cavity

It’s odd. Twice, within the space of seven days, have I found a poplar (Populus sp.) cavity to be host to some Rigidoporus ulmarius sporophores. The last time it was slightly more overt, though in this case it really was a case of having to look. This poplar is by quite a busy residential junction, and given its poor condition around the base (from Sesia apiformis exit holes, general mechanical damage of roots and the butt, and the fungal decay – perhaps all are associated), one would perhaps reasonably expect for there to be a form of risk management involved with this tree’s future.

Below are some photos that I think give a good indication of the situation, and why the fungal decay was not necessarily entirely discernible from the outside.

Here sits the poplar (Populus x canadensis), next to both some residential properties and a public highway (walkway and roundabout).
On the northern side, we can spot some damage to the base of the tree. A surface root has been injured by mechanical means, and there is a lot of damage to the lower stem.
Peering in closer, the damage becomes more evident. We can also see a small cavity.
Inside that cavity, this little guy is growing. Hiding from sight – the cheek!
On no – wait – there’s two! Both sporophores were very fresh and very moist as a result. My blade cut cleanly through a small tangential cross-section of the larger one. The bracket is perhaps drawing moisture out from the tree to create conditions right for mycelial advance (it is a brown rot, and thus dries the wood).
The smaller of the two sits just below. We can also see the underside of the larger sporophore.
Looking up into the tree (the two brackets are below) is rather interesting. We can see many more sporophores forming of varying sizes, and a lot of decaying wood.
Rigidoporus ulmarius deep within a basal cavity

Citizen science – a novel approach to investigating horse chestnut leaf miner

Many studies undertaken by scientific researchers are – by a sort of default – quite limited, in terms of the study’s spatial scale. Whilst this is not necessarily a bad thing, when the point of the research is to use a specific area (such as a city) as a case study, it can limit the effectiveness of problems that occur across a very large geographical area. One such example would be Cameraria ohridella (horse-chestnut leaf miner) presence within the UK, where it is an invasive pest (having arrived in 2002) progressing across the UK that may aggressively defoliate Aesculus hippocastanum (horse chestnut) and thus cause serious tree health problems.

An example of horse chestnut foliage being attacked by the leaf miner. Source: The Wild Diary.

Because the pest is found in many areas of the UK, understanding its characteristics as a meta-population is important, but tricky (in terms of cost and time) if relying upon the standard method of research. To remedy this issue, a form of science known as ‘citizen science’ can be used, which directly involves members of the public, across very large areas, gathering data and relaying it back to scientific researchers, where the data can then be processed. And would you know it, the study looked at here employs such a form of science, with the aim of understanding: (1) more about how serious leaf damage is from Cameraria ohridella, when comparing leaf damage extent to how long the miner has existed within the local geographical area, and (2) whether parasitism of the leaf miner (by parasitic wasps) is higher in areas where the leaf miner has existed for longer periods of time. The research project was dubbed ‘Conker Tree Science‘ (and is still ongoing, in a slightly different format).

In order to bring in accurate information from members of the public, which could be processed and transformed into conclusions, those aiding with the survey (around 875 people, for the first part) were asked to complete a few basic tasks. The first was to determine an average level of leaf mining damage upon foliage of the horse chestnut, with the aid of a very basic diagram (shown below). Such recording was undertaken during June – September of 2010, and alongside the numerical scoring the surveyors also submitted photos of the foliage inspected. This enabled an experienced individual to determine whether recording was accurate, and if not, bias would be accounted for in the data analyses. No bias was found, so the data gathered by the survey participants was considered to be accurate.

Ranging from 0-4, with 0 being no damage and 4 being major damage, survey participants reported to what extent the horse chestnut they were inspecting was mined.

Where a member of the public had stated the leaf was at least partly damaged, the researchers compared the location of the tree to how long Cameraria ohridella had been known to exist in the area. Of course, the time of year was factored into account, as leaf mining will progressively worsen as the larvae progress through their instar stages; as will it generally worsen as the summer progresses (there may be more than one life cycle of the pest in one summer). Furthermore, because the presence of the leaf miner may have been under-recorded prior to this study taking place (as people became used to its presence,a nd no longer recorded it), the researchers constructed a model that would predict where the miner should have progressed to in the years following 2002. Therefore, if reports came in from areas where there were no records of the leaf miner, the model would allow for comparisons to be made between mining severity and how long the miner had likely been present for in that locality.

With regards to ascertaining whether parasitism increased with how long the leaf miner had been present within the local environment for, survey participants were also asked to take small cuttings of foliage during early July 2010 and seal those cuttings in a plastic bag for two weeks. After this time, a count for Cameraria ohridella, parasitic wasps, and any other insects (in case of contamination) was undertaken. In order to identify the different organisms, the researchers provided an identification key to all survey participants. Much of this part of the data collection was done by schoolchildren, with the aid of trained researchers. Following counting of all insects, the data was compared to that gathered by experts, in order to check for bias. No significant issues were found with regards to identifying the leaf miner, so the data gathered by the school children was used in the study without being corrected in any manner. For the parasitic wasps however, it was found that the school children failed to fully identify them in some instances (under-estimation).

After collating the results and analysing them, the researchers found that the damage caused by Cameraria ohridella rose for the first three years; at which point, it begun to quickly ‘flatten’ (and thus, damage then remained rather constant, albeit significant). This is shown in the below graph. In this sense, in the fourth year, the leaf miner will most likely be causing maximum damage, and from here-on-in, such maximum damage will routinely occur (of course, it will still vary from year-to-year).

Foliar damage associated with Cameraria ohridella, in relation to how long it had been present in the local area of the UK.

As for whether parasitism of the leaf miner increased over time, the 2,208 cases of reared insects (1,810 of the cases were from school children) showed that the rate of parasitism increased aongside how long the leaf miner had been present in an area for. Consulting previous literature of parasitism on the leaf miner, the researchers suggest that initially pre-pupal stage generalist parasites will use the leaf miner as a host, though after a few years more specialised pupal stage parasites will succeed into the trophic system. However, once specialised parasites do arrive, it is not expected for there to be a continual rise in their population abundance, as research in mainland Europe has shown that parasitic wasp populations plateau after a some years. Therefore, parasitic wasps may not be able to greatly limit the damage caused by Cameraria ohridella.

Such results are certainly interesting, though the manner in which they were obtained is equally so. What we can draw from this is that citizen science certainly has the ability to work, and across a large geographical area; all whilst costing little money and ‘outsourcing’ the time spent on data collection to willing volunteers. This can be good as it engages people with the scientific process, thereby removing it from its pedestal and giving science a form of accessibility. The act of engaging with school children was particularly pleasing to read about, as one never knows whether such research could evoke a greater interest in scientific research for some of those children. Granted, citizen science isn’t a ‘cure-all’ approach, as there are many limiations. The researchers remark that one main limitation in this study was the inability to directly sample wasp parasites, and thus specific species couldn’t be identified. Furthermore, the data is only as good as those collecting it, and because volunteers are unlikely to be amateur gall enthusiasts, data collection must be simple, swift, and succinct. There’s also the need to verify the data after it has been collected, unlike with scientific researchers who will know how to gather data prior to gathering it (and thus eliminating bias, ideally).

Nonetheless, a good study, and hopefully citizen science can be used in the future for other projects!

Source: Pocock, M. & Evans, D. (2014) The success of the horse-chestnut leaf-miner, Cameraria ohridella, in the UK revealed with hypothesis-led citizen science. PLoS One. 9 (1). e86226.

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Citizen science – a novel approach to investigating horse chestnut leaf miner

Two new species of Magnolia discovered

Exciting stuff! Two new species of Magnolia have been found, in the cloud forests of Mexico. Curiously, they had actually been found some years ago in 2010, though at the time the photographer (Roberto Pedraza Ruiz) had identified them as the already-in-existence Magnolia dealbata. After sharing a few with the image-hosting site for flora and fauna Arkive, they sat improperly identified for many years – until recently. A Mexican botanist, by the name of Dr José Antonio Vázquez, who was looking over the images, considered them rather different to what they were meant to look like for that particular Magnolia species, and contacted the photographer. After a request for further photos from the botanist, Roberto returned to the cloud forest and collected many more photos, after which it was found that two new species of Magnolia had been identified – Magnolia rzedowskiana, and Magnolia pedrazae. It seems that the internet can be useful for identifying new tree species, therefore!

You can read more here on Arkive’s blog.


Two new species of Magnolia discovered

Ganoderma sp. thoroughly colonising a massive Fagus sylvatica pollard

Prior to attending a Forestry Commission event on tree pests and diseases earlier today, I stopped off at Epping Forest to explore for an hour or so. I made a beeline directly to the pollarded beech trees, as they really are truly fascinating to look at. Wonderfully, they’re also riddled with fungal decay, and some of the pollards showcase some quite glorious examples of how fungi can colonise.

In the images I am sharing below, we can observe how Ganoderma sp. (suspected G. australe) has colonised a quite massive Fagus sylvatica pollard, up the entire main trunk and into the confluence of stems. Without question, the fact the pollard has been crown reduced is because of this, as the white rot induced by the mycelium as it degrades the wood structure will progressively weaken the entire stem confluence. Of course, failure is still entirely possible, and one has to just look at other trees in the area to recognise this, though it’s a form of risk management that balances the need to reduce risk but also ensures the tree retains its character. In time, it’ll probably be monolothed.


As we can see, this monstrous beech has had a light crown reduction.
And here’s why! Ganoderma galore.
A closer look at the brackts shown above.
A cross-section shows a rich brown colour indicative of G. australe; at least, this suggests it’s more likely to be that than G. applanatum.
On the other side of the tree, we can see sporophores emanating from many creases / indentations in the stem confluence. Perhaps some reaction wood has been laid down in the areas surrounding, which has accentuated the ‘rippled’ effect of the area.
Panning out but keeping on the same side of the beech, we can see just how plentiful the sporophores are.
Ganoderma sp. thoroughly colonising a massive Fagus sylvatica pollard

Xylella fastidiosa sequence types and host specificity

The pathogen Xylella fastidiosa has been hitting the headlines in the UK recently, because there is growing concern over the risk of it reaching the UK shores and not being caught quickly. This bacteria ultimately causes a leaf scorch that, over time, progresses throughout the crown of its host, and will eventually cause tree mortality. Because it’s not native to the UK, its introduction would potentially be very significant, given no native tree species have any form of resistance to it. Furthermore, as it may attack a huge variety of trees and plants, understanding a little more about its biology is certainly something that should be pursued. With that in mind, I thought I’d look at a study from the District of Colombia, Washington, USA that sought to investigate the population structure of the bacteria in an urban streetscene, with specific focus upon how the bacteria associated with the constituent tree species.

Within the District’s street scape, infected trees of the species Morus alba, Platanus occidentalis, Quercus coccinea, Quercus palustris, Quercus phellos, Quercus rubra, and Ulmus americana had foliar samples taken from their structure, which were then taken into the laboratory and the constituent Xylella fastidiosa sequenced (DNA sequencing). Most samples were taken from trees displaying visible symptoms of infection, though some were also taken from trees, or locations upon an infected tree, where there were no symptoms (asymptomatic). The table below gives a breakdown of exactly what samples were taken from which trees.

The number of samples taken from each tree species.

Following DNA sequencing of all the samples, it was identified that there were five different ‘sequence types’ of the bacteria Xylella fastidiosa. These sequence types were almost exclusively unique to a specific tree genus, though one sequence type (ST-9), whilst found predominantly in oak, was also found in an elm sample (this specific sequence type has been found in elm before, and also sycamore). This near exclusivity of different sequence types occurred almost always, in spite of the fact that many study locations were comprised of infected trees of more than one of the species studied (for example, a site may have contained both mulberry and oak, or elm and sycamore, and so on). Curiously, the elm infected by a type found otherwise only on oak was not near to any oak tree.

Host specificity of different sequences.

This host specificity is important, because it means that a sequence type infected mulberry is highly unlikely to ever be pathogenic towards oak, for example. However, other studies have shown that ST-8, found here only on sycamore, could also be found on oak and elm. Therefore, certain sequence types may have the ability to infect more than one tree genus, though this is certainly not true for all sequence types.

In this sense, a few comments can be made: (1) Monocultures are certainly to be avoided, for those tree species that are susceptible to Xylella fastidiosa. They can swiftly become very extensive inoculum bases for the bacteria as a group, and if they succumb to the infection then an entire swathe of trees can readily be wiped-out. By planting and maintaining healthy populations of an array of tree species of different genera, the impact of Xylella fastidiosa can be reduced (though only in urban areas – outbreaks in woodland settings would be far more impactful, because of a reduced diversity of tree species).

(2) Management of the bacteria can be worked down to the sequence type level, and this may prove to be a double-edged sword in terms of management. In a positive light, because cross-contamination of a sequence type is likely to be quite infrequent, targeted management approaches can be created that are specific to the particular strain of the bacteria and specific to the area of infection. Of course, this is also concerning, because one control method for one sequence type may not necessarily work for another, and for those sequence types that do have more than one potential host genus, adaptations on the genetic level may enable it to branch-out to infect other tree genera as well, or become more adept as infecting peripheral species and genera in its current host range. This latter consideration may be one reason why limiting monocultures is important, as the build-up of the pathogen in a suitable inoculum base may potentially increase its pathogenicity.

Source: Harris, J. & Balci, Y. (2015) Population Structure of the Bacterial Pathogen Xylella fastidiosa among Street Trees in Washington DC. PLoS One. 10 (3). p1-11.

Xylella fastidiosa sequence types and host specificity

Do increased urban temperatures support larger scale insect populations?

Tree defoliators, namely insects, can cause serious damage when outbreaks are severe. In the urban environment, such outbreaks are considered to be more common, because of the urban heat island effect – this is because insects are ectotherms (they require an external heat source). Insects will, in theory, therefore fare much better in such urban environments, compared to nearby rural locations. Additionally, as the vigour of trees will change as a result of warmer urban climates, such alterations may have a positive (or negative) impact upon insect populations. For example, trees that fare less optimally may produce fewer secondary metabolites that dissuade defoliation, and emit fewer herbivore-induced plant volatiles that disrupt herbivory and attract predators and parasites of the insect defoliator. Of course, this has consequences for the host trees, as defoliation outbreaks may thus be more severe, more prolonged, more frequent, and thus more damaging.

In order to add weighting to this statement, the authors of this study investigated how the oak scale insect Parthenolecanium quercifex fared in urban environments across Raleigh, North Carolina, USA, with a specific focus upon how urban temperatures influenced their abundance both directly (attraction to warmer areas, increased fecundity in females) and indirectly (rate of parasitism). Because the scale insect only has one generation per year, where the first instar stage of the insect feeds upon leaf phloem tissue before over-wintering upon the bark and developing through the second instar stage prior to pupating and becoming adults the following spring, it was not expected for increased temperature to improve generational turnover rates, but simply enable females to lay more eggs / lay eggs that have a lower mortality rate. Furthermore, as the scale insect is very similar to an array of other insect species and genera, understanding how urban temperatures influenced its biology could give an indicator as to how other insect species would fare in similar conditions.

What the authors found, following a survey period, was that Parthenolecanium quercifex over-wintering in the second instar stage were 13x more abundant in warm locations than in cool ones (see the below figure). In addition to this, the ovisacs (where a single egg is layed) were 5.5x more abundant when deposited by the very same adult scale insects that were 13x more abundant, and these ovisacs gave way to first instar stage scale insects 7x greater than on trees in cold sites. Generations of scale insect that spent their entire life cycles in hot microclimates were also observed to, when placed in colder microclimates (in a greenhouse), still be found at higher abundances than scale insects that had spent their entire life cycle in cold microclimates. In this sense, scale insects may locally adapt (in a beneficial sense) to hotter locations, and it is suspected that the scale insect has this ability because populations are highly segmented and therefore site-specific adaptations can occur with relative ease (gene flow is ‘locked’ – meta-populations almost don’t exist beyond the level of but a single, or few, trees).

Second instar stage scale insect abundance, in different urban locations, per 30.5cm of tree stem.

Conversely, no correlation was identified between urban temperatures and the rate of parasitism upon the scale insect. A total of six parasites were studied, of which none were found to have a significantly increased rate of parasitism when temperatures were higher. In fact, rates were near identical in hot and cold sites, as shown in the graph below. It is suggested that this is because the scale insect’s natural enemies are simply found in less abundance in urban locations, because of the poorer habitat quality. By a similar token, females were not found to lag more eggs on trees in hotter microclimates, and nor was host tree ‘quality’ deemed to impact upon the abundance of scale insects.

The rate of parasitism upon the scale insect Parthenolecanium quercifex. Parasites studied included Coccophagus lycimnia, Encyrtus fuscus, and Eunotus lividus.

It was also mentioned that it is unlikely that stressed trees would be host to more scale insects, because scale insects would probably be found in lesser abundances where water and nutrients are lacking within the tree. Given urban trees typically struggle because of drought and a lack of nutrient availability, it is thus impropbable that tree quality is an influencing factor upon population levels of the scale insect. If it were then, because the scale insect is a sap-sucker that relishes nutrient-rich sap, a tree lacking this (because of drought and poor nutrient availability in the soil) would not be a able to support large numbers. Therefore, the increase in scale insects in hotter microclimates is likely to be independent of tree quality (condition).

To conclude, the authors remark that scale insects are more abundant in urban locations where the microclimate is warmer. As a consequence, if temperatures continue to warm in the urban setting, or become more homogenous (at a higher temperature) across a large urbanised spatial scale, scale insect populations may markedly increase and therefore be potentially very damaging for urban trees. Because urban trees are exposed to so many adverse conditions, an increase in pest activity is certainly not something that will help their case for survival. Such a weakened nature may also leave them exposed to other pests and diseases, which do rely upon weakened hosts to establish in great abundance.

Beyond the urban setting, if temperatures increase in rural locations, scale insects may also become more of an issue there. Granted, such rural locations are home to greater numbers of parasites (natural enemies), and thus an increase in numbers there may perhaps support an increase in parasitoid abundance as well. This is, however, just speculation.

Source: Meineke, E., Dunn, R., Sexton, J., & Frank, S. (2013) Urban warming drives insect pest abundance on street trees. PLoS One. 8 (3). p1-7.

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Do increased urban temperatures support larger scale insect populations?

Rigidoporus ulmarius in a poplar cavity

I have been sharing a lot of stuff to do with wood-decay fungi lately, and thought I’d continue with that theme as it’s certainly a field that we need to understand, as arborists. After all, the presence of a fungal sporophore can dramatically alter how we perceive the tree we are looking at, and how we manage it. Plus, they’re damn cool!

In this instance, I came across a hybrid black poplar (Populus x canadensis) colonised by the fungus Rigidoporus ulmarius. However, unlike in most instances, one of the sporophores was within a fairly large cavity. I have seen this sort of thing before with this fungus (on a horse chestnut), though not with such a large sporophore. Therefore, it just goes to show that, when out inspecting trees, we must also check cavities for signs of fungi (beyond just the showcasing of wood decay).

Perhaps a sporophore within a cavity suggests effective compartmentalisation by the barrier zone, though this doesn’t mean the tree is out of the woods. Decay can still be very extensive, and potentially hazardous, if there is a target zone beneath.

And so, as always, here are a good few pictures to look at over a mug of coffee and some music.

Here we can see the poplar tree, sitting between a petrol station and the back garden fence of a row of houses. A target zone is therefore evident.
Looking at the base of the poplar, we can see a cavity. Most probably, this cavity formed after a stem failed and subsequently decayed. That may be why a new stem has grown out at such an odd angle, as it grew adventitiously from the site of failure. Of course, this is pure speculation on my part.
Peering within the cavity, we can spot quite a large sporophore that spreads, if we look at it as a clock face, from 9-3. Also note what looks like two exit holes of the poplar hornet moth (Sesia apiformis), just above the cavity. Perhaps a relationship between the two – does the moth prefer to exit in such decayed areas?
Closer still, here. I managed to break a bit off with my car keys, as I left my blade in the car.
Here’s the cross-section. We can see a few years’ worth of growth (or a few growth spurts, maybe not one per annum). Quite a large tube layer for the fungus, though perhaps the cramped setting caused that.
On the smaller stem, we can spot another sporophore around 2m up (and just to its lower left, another sporophore beginning to form from between a crack in the bark).
Upon closer viewing, we recognise the intricacies of the sporophore.
And a cross-section reveals the typical flesh and tube layer arrangement and colouration of Rigidoporus ulmarius.
Rigidoporus ulmarius in a poplar cavity

Surface roots in the urban setting

Some tree species, such as the cherries (Prunus spp.), will naturally have a higher propensity of forming a shallow rooting system, and in urban settings this can be particularly damaging. Not only can surface roots act as trip hazards themselves, but they can lift paving slaps (that act also as trip hazards), damage low structures, and generally be somewhat of a nuisance for some (not all) people – notably highway engineers.

Granted, this problem is largely self-inflicted – we plant tree species in the wrong places, don’t give them adequate soil volumes free of compaction and complete with nutrients and moisture, and then bemoan the developing situation of surface roots as roots both go in search of resources and look to anchor themselves into the only ground they can penetrate through. Considering many roads and pavements are laid upon highly compacted ground, because this ground provides the stability required, can we reasonably expect for trees to not create the problems they do? I don’t think so.

Anyway, below is a good example of surface roots of a cherry, and in this case we have a double-whammy of paving slabs and highly-compacted ground. The result is somewhat of an aggravation for those nearby, I would expect. But if the tree comes out, no trees exist. Is that better or worse than having surface roots? Only those living nearby can answer that.

I am sure these are lovely in the spring through to summer, given there are no other trees about in the vicinity.
Order from chaos? Not here, unfortunately.
I dread to think how awful the ground is beneath, if these roots have stretched out so far along the surface. The same is happening on the other side of the stem, but to a lesser degree.
Surface roots in the urban setting

Competition for resources in a dead birch

More often that not, we’ll only see one fungal species overtly present within a tree (by the presence of sporophores). However, at times, we may see two (as is the case in this example). Of course, this doesn’t mean these are the only species present, as the mycelium of other fungi may be spreading within the wood substrate but simply not producing a sporophore, or spores may even be ‘latent’. But then, I suppose, it’s a case of how deep down the rabbit hole do you go?

Staying well-and-truly out of such a rabbit hole, I am sharing below a really nice example of how the stem of a dead Betula pendula (silver birch) is host to both Piptoporus betulinus (birch polypore) and Daedaleopsis confragosa (blushing bracket). They occupy different portions of the stem, with the former placing high up and the latter low down, though I very well imagine there is, or soon will be, marked competition between the two species’ mycelial networks for resources (the birch isn’t markedly decayed, so it may have only died recently). After all, fungi are very territorial, and certainly don’t like to share.

Who will win here (?), and I wonder whether the birch polypore has the advantage because it’s a specialised opportunist (it is present in the sapwood system of its host prior to stress / death), though also because it has gravity on its side. Will gravitational forces make it harder for the mycelium of the blushing bracket to grow upwards, when compared to the mycelium of the birch poypore growing downwards? Just a theory…

At the bottom of the stem we can see many fruiting bodies of the blushing bracket, and further up a few of the birch polypore. Somewhere in the middle there will be a competition for resources, either going on currently, or soon to be taking place.
A closer look at some of the blushing brackets. Note some white marks on the bark in places, suggesting perhaps old bracket locations.
Closer still, in this image.
Looking up, and taking care not to scorch my retinas, I snapped this shot of the underside of one of the birch polypore sporophores. This was the largest one – I could count three more, with one above and two on the other side of the stem just a little further down. This fungus’ spores would have latently present prior to the death of the birch, because the fungus is a specialised opportunist. Perhaps this will give it the advantage over the blushing bracket?
Competition for resources in a dead birch

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?