(Arboricultural-styled) 'Fact of the Day'

[Kveldssanger]

Gary, don't think those comments were aimed at anyone. More just me airing my personal thoughts on the issue to nobody whatsoever, but simply to add my view to the conversation.

 

For clarification, when I say globalisation is a scourge I do mean on an ecological sense principally. I cannot think of many ways in which globailsation has been good for ecosystems, as usually it is just us partially correcting our mistakes and looking back on such correction with a smile and thinking "we did good there" (and that's not a 'good' thing). Of course, we can learn more about it via globalisation (internet), but generally only to supplement knowledge on why our mistakes have caused what they have. Even when we genetically sequence things, we do it because we want to recognise how a pathogen works (for example, in order to then cultivate trees to resist the pathogen, to replace the ones that died after we intriduced it - or to anticipate future issues that we will likely be a driver in creating). The buck seems to largely stop at a human mistake.

Edited March 27, 2016 by Kveldssanger

[Kveldssanger]

29/03/16. Fact #182.

 

I don’t think that one can question the anecdotal benefits of walking through a woodland or forest. Away from civilization, it’s a time where one can really begin to relax, let the mind shut off from stressors, and let the eyes take in the wonderful sights. I know, from personal experience, that a woodland walk can bring about a state of calm and ‘inner peace’, and I often find myself exploring for hours on end only to realise I’ve been gone for half a day, am slightly lost, and need to walk home before it gets too dark.

 

Personal journeys aside, there is a growing amount of evidence to state how woodland and forest settings can be used to reduce anxiety, treat (as part of a wider range of things) stress, and maintain a general good state of well-being. In this post, I thought I’d look at a study from Japan that assessed how the practice of ‘Shinrin-yoku’ (‘experiencing the forest’) can impact upon the mental and physical state / well-being of individuals (for this study, this was 12 male university students of 21-23 years of age). The forest in question was an old-growth broadleaved deciduous forest, and the experiment was undertaken in the height of summer.

 

In order to quantify the benefits of Shinrin-yoku, the 12 men were separated into two groups of six. One group spent the first day in the forest and the other in a nearby city (to enable for comparisons to be drawn), and on the second day the two groups swapped locations. All 12 men stayed in the same hotel, which was located around one hour away from either site (by car). Before leaving for each location, they would have breakfast, and upon return they would have dinner. Once at each location, individuals had their day planned so that they would first spend some time in a rest room on the site, and then go on a leisurely but lone walk (a pre-planned route) within the study area. They would then return for lunch at the rest room on the site, before remaining in the rest room to look at the scenery (by themselves). During each day, all individuals had physiological data collected from them, including blood pressure and pulse rate, before and after each activity (at the hotel for breakfast, at the rest room prior to the walk, after the walk, before looking at the scenery, after looking at the scenery, and back at the hotel for dinner). The men were also asked to rate, on 13-point scales, whether they felt comfortable or uncomfortable, and calm or roused (agitated). They were also asked to rank how refreshed they felt. Again, these subjectivity tests were taken six times each day, for all individuals.

 

The results of this study are very interesting, as they show that the 12 men almost always preferred the forest experience in a subjective sense, and their physiological measurements backed this up. Benefits were particularly evident after the men had experienced the forest (after the walk and after looking at the scenery from the rest room), though even prior to going on the walk and observing the scenery there were marked benefits (as if the expectation automatically improved an individual’s perception of well-being, and their physiological condition). Below, we’ll look at specific data sets and what they show.

 

How calm or roused the men felt at each site and at each measurement time. * p<0.05; ** p<0.01

 

In terms of how calm the men were, one can clearly observe how the forest walk had a significant impact upon their state of mind, which was even more significantly impacted following a period where they individually took in the view from the rest room site. Alarmingly, the men actually felt slightly agitated after observing a city vista. However, we can observe how the impacts on the feeling of calm were largely immediate, as by the evening there was no significant difference between the two data sets. However, the fact that those who experienced the forest felt calmer in the evening than those who experienced the city is interesting, as it is the inverse of the morning’s results, and even prior to the walk’s results.

 

How comfortable the men felt at each site and at each measurement time. * p<0.05; ** p<0.01

 

With reference to how comfortable the test subjects felt, we can clearly see how the forest experience (even when not on site, but eating breakfast at the hotel before heading out to the forest) made the men feel significantly more comfortable. The forest walk and scenery were particularly significant in raising the level of comfort within the men, whilst the city walk and scenery actually made them feel very uncomfortable. Following each walk and each observance of scenery in both locations, all men were significantly impacted by the experience, and the benefits and adverse impacts of these experiences have already been mentioned and can be seen in the graph above.

 

How refreshed the men felt at each site and at each measurement time. * p<0.05; ** p<0.01

 

Looking at how refreshed the men felt throughout the day, we can again see how the forest experience trumped the city experience. Again, the refreshed state of each individual rose significantly after each experience, and individuals were significantly more refreshed after forest experiences (notably after observing the scenery). We can now see a trend, that observing a forest view from a point of rest is certainly very beneficial, and studies on how hospital recovery time is positively impacted by the view of trees from a window now start to make yet more sense (as mental well-being manifests in the physical form, via blood pressure levels, and on on).

 

The pulse rate of the men at each site and at each measurement time. * p<0.05; ** p<0.01

 

Moving onto physiological impacts of a forest experience, we can see how the pulse rate of the men was, on average, lower before and after each forest experience, and also in the evening following the day’s experience. Only prior to the forest visit was pulse rate higher, and perhaps this was down to a form of excitement and / or apprehension (I am only speculating). Interestingly, the pulse rate of the men was quite markedly lower prior to the walk when they were awaiting a walk in the forest, suggesting that even the prospect of walking in a forest is physically calming, though curiously this doesn’t align fully with the men’s subjective feeling of calm prior to a forest walk. Therefore, we can observe perhaps a slight disparity between perception and actual physiological responses from the individuals to the prospect of a forest walk (perhaps, because walking in a city is more of the norm, they were more nonchalant about such a walk?).

 

The blood pressure of the men at each site and at each measurement time. * p<0.05; ** p<0.01

 

As for blood pressure, we can once again see how it was lowered in both the top (systolic) and bottom (diastolic) readings. Yet again we can see how the forest walk and observance of forest scenery were beneficial for lowering blood pressure, though this was more significant with the sitting down and watching of the scenery. Perhaps this is to be expected, as walking, which is a physical activity, will raise blood pressure (even slightly). Importantly, this benefit progressed into the evening, where evidently the men were still more relaxed, albeit not significantly so.

 

So there we have it. Forests are good both physically and mentally! Please, however, bear in mind this is only a study of 12 men of the age 21-23, so it’s by no means a conclusive study, but it’s a useful indicator of how a larger study might turn out. Further to this, it was done in the height of summer in a broadleaved forest of significant age, so we must also accept the marked benefit may also be seasonal (and younger forests, perhaps also coniferous in nature, may elicit a different response). There may also be differing cultural responses, and therefore replications in other areas of the world would also be interesting to see. I’ll let you look over the graphs again so you can form more conclusions for yourselves, though I hope this has been of use for many who are reading this. For me, it certainly aligns with my own experiences from walking through broadleaved woodlands. And I write all this as I overlook miles of fields and woodlands from my bedroom window, which has also had a very calming impact upon me – that is without doubt.

 

What’s not to love…!

 

Source: Tsunetsugu, Y., Park, B., Ishii, H., Hirano, H., Kagawa, T., & Miyazaki, Y. (2007) Physiological effects of Shinrin-yoku (taking in the atmosphere of the forest) in an old-growth broadleaf forest in Yamagata Prefecture, Japan. Journal of Physiological Anthropology. 26 (2). p135-142.

[Kveldssanger]

30/03/16. Fact #183.

 

Following on from my initial post about using electronic noses to pick up decay within a tree (which I suggest you glance over before reading this), I thought I’d visit further literature on the subject but with more of a refined focus. In this instance, we’ll be looking solely at the ability for electronic noses (specifically the PEN3 electronic nose) to detect decay within the rooting environment of trees. As was elucidated to in my previous post, the PEN3 can pick-up decay from root systems in the field setting, though here we can observe how a slightly earlier study fared in terms of its efficacy upon root samples inoculated in the laboratory and stored beneath different types of soil.

 

Because not all fungi will colonise the root system of a tree, the fungal species chosen in this study were Armillaria mellea, Ganoderma lucidum, and Heterbasidion annosum (a shame that Meripilus giganteus was not included). These three species can be considered principal root-rotters. These fungi were, once cultivated and made into mycelial plugs, brought into contact with healthy root samples (1-3cm in diameter and 2-10cm in length) of adult trees of the species Aesculus hippocastanum, Cedrus deodara, Liquidambar styraciflua, Platanus x hispanica, and Quercus robur. Such roots, complete with fungal inoculum, were then buried beneath sourced urban soils (and also ‘professional’ nursery soils), in order to reflect what soils the PEN3 would need to ‘sense’ through if it were to be applied in the ‘field’, and a period of time was given for the mycelium to begin colonising the root samples and. After a year, the samples were ready for testing. Then, following a set of complex processes that only a three-page methodology could ever fully encapsulate, it was time to look at the results (which are equally as complex, so I’ll do my best to simplify them).

 

What the researchers found was that the ability for the PEN3 to detect decay in the roots of different tree species by different wood decay fungi, all whilst under different soil types, was quite good (and more notably for urban soils) – see the below graphs. The nose was able to differentiate between healthy and decayed samples of root tissues with a high degree of variance between the two, meaning that there was little to no scope for ‘confusion’ between healthy and decayed root samples. Similarly, it was able to significantly differentiate between the fungal agents causing the decay in certain instances, and particularly in the cases where the fungi-tree relationship would naturally occur in the ‘field’. For example, the nose could identify (but not significantly differentiate between the two) oak roots colonised by A. mellea and G. lucidum – both are natural pathogens of the species. Granted, this trend wasn’t conslusive, as the nose couldn’t differentiate between healthy and A. mellea-infected samples in horse chestnut. The second set of graphs outlines the data collected with regards to differentiating between the decay from different fungal species.

 

The ability of the PEN3 to discriminate between healthy (green) and infected (blue) tree roots for A. hippocastanum (a), C. deodara (b), L. styracuflua ©, and Q. robur (d).

 

Under the urban soils, the nose worked more effectively, and this is considered to be because soils also emit their own VOCs, and professional nutrient-rich soils will have a ‘stronger’ emission. However, these professional soils still didn’t stop the PEN3 from identifying whether a root was decayed or healthy.

 

How the PEN3 differentiated between decay from different fungi (A. mellea [blue], G. lucidum [red], H. annosum [grey], and healthy [green], on the tree species A. hippocastanum (a), C. deodara (b), L. styraciflua ©, P. x hispanica (d), and Q. robur (e).

 

Evidently, some work is required in ensuring the PEN3 becomes very able to differentiate between the decay from different fungi on different tree species (and under different soils), though even from the graphs above we can see how there is, by-and-large, some degree of differentiation between the readings (as shown by the clusters, which infrequently overlap, but are sometimes close together). The decay from different fungi in Cedrus deodara appears to be particularly well differentiated, whilst decay in Aesculus hippocastanum is less so. Of course, it’s mainly a calibration and sensory game, and if the PEN3 can be accentuated in sensory ability then there’s certainly scope for this to be a very effective field diagnostic test for decay of root systems, and all whilst not having to damage the tree in any manner. Compiled with the more recent results of the PEN3 being able to work in the ‘field’, it’s an interesting piece of technology to keep an eye on.

 

Source: Baietto, M., Pozzi, L., Wilson, A., & Bassi, D. (2013) Evaluation of a portable MOS electronic nose to detect root rots in shade tree species. Computers and Electronics in Agriculture. 96 (1). p117-125.

[Gary Prentice]

29/03/16. Fact #182.

 

I don’t think that one can question the anecdotal benefits of walking through a woodland or forest. Away from civilization, it’s a time where one can really begin to relax, let the mind shut off from stressors, and let the eyes take in the wonderful sights.

 

 

Perhaps this is to be expected, as walking, which is a physical activity, will raise blood pressure (even slightly). Importantly, this benefit progressed into the evening, where evidently the men were still more relaxed, albeit not significantly so.

 

 

 

What’s not to love…!

 

Source: Tsunetsugu, Y., Park, B., Ishii, H., Hirano, H., Kagawa, T., & Miyazaki, Y. (2007) Physiological effects of Shinrin-yoku (taking in the atmosphere of the forest) in an old-growth broadleaf forest in Yamagata Prefecture, Japan. Journal of Physiological Anthropology. 26 (2). p135-142.

 

It might be interesting to find some comparison studies after dark:biggrin: I'm surprised at a number of people I know or who I've met in the past who are uncomfortable or admit they avoid woods and forests of a night.

 

A deep-seated inherent primeval instinct? I don't know, personally I'm more at ease in a wood than some urban areas at night:001_huh:

[DeltaBravo]

Very useful thread - just found this. Great stuff!

[Kveldssanger]

Very useful thread - just found this. Great stuff!

 

You're very welcome.

 

It might be interesting to find some comparison studies after dark:biggrin: I'm surprised at a number of people I know or who I've met in the past who are uncomfortable or admit they avoid woods and forests of a night.

 

A deep-seated inherent primeval instinct? I don't know, personally I'm more at ease in a wood than some urban areas at night:001_huh:

 

For me, it would depend on the woodland. I'm always more alert at night if out and about - probably an in-built defensive mechanism we all have, to some degree.

[Kveldssanger]

01/04/16. Fact #184.

 

In the UK, the plane tree (Platanus x hispanica) has been, and perhaps still is, a very popular choice of tree for urban landscapes. In London, the recent i-Tree report suggested that up to 4% of Inner London’s tree population of 1,587,000 trees are plane (meaning 63,480 are plane trees), though in terms of leaf area is provides as much as 8.9%. In this sense, it’s evidently a tree that is usually found to be quite large, and this landscape dominance makes it a very prominent feature as well as quite a common one (for a larger tree, where feasible planting space is automatically more limited). However, the ability to clonally propagate plane, in addition to the report’s findings that 21% of London’s trees are clones, we can assume that at least 13,331 planes are genetically not unique (perhaps more, as many planes are assumed to the ‘Pyramidalis’ cultivar, though other cultivars may also be found and thus raise the total level of ‘clonal-ness’ amongst plane meta-populations). This presents problems, as such a lack of genetic diversity means pests and diseases can readily sweep through clonal stands, as there is no inherent level of variable resistance in the population. Beyond London, they also feature quite prominantly in some of the new towns, and particularly along main vehicle routes and within urban parks.

 

Introductory spiel aside, my remarks on pest and disease are quite pertinent to this post, which will look at a recent article published by Tubby & Perez-Sierra in the Arboricultural Journal, which looks into the current status of plane and those pests and diseases that may soon put the species at risk, within the UK. For those that get the print journal (like myself), then you may have already read this, though it’s got some great information and thus is a good one to share with an audience far beyond those who get (or have access to) the journal.

 

Arguably the most prominent aggressor of plane in recent years is what we know as Massaria (Splanchnonema platani). Despite this fungus being considered a “weak pathogen”, where it will ‘aid’ (in an adverse sense) with twig abscission and cause small cankers upon minor branches, it has been increasingly observed, across central and western Europe, to cause decay and dieback within larger-diameter branches at the upper branch crotch and along the upper side of the branch, and notably in the lower crown where branch diameter is between 100-200mm. The dieback has the potential to cause very wide decay strips, with potentially around 30% of the entirety of a branch’s circumference being damaged by the fungus (which induces a soft rot). Such damage will induce symptoms of decline within the affected branch (which may be highly discernible if the branch is large and bears a lot of foliage), as will cambial dieback create lesions that are evident from above the crotch (a pinkish-orange colour). Failure can occur in as little as three months following on from the onset of decay. Of course, as most plane trees are present within the urban environment in the south east of England (and are generally large in size), which incidentally is also the most densely-populated area of the UK, there is an evident risk to public safety. For this reason (amongst others), the LTOA (London Tree Officers Association) published guidance quite recently on the subject.

 

Cross-sections of the decay associated with Massaria. Source: LTOA.

 

Beyond Massaria, which is indeed present within the UK, we can also observe how plane canker (Ceratocystis platani) may be a very possible threat in the near future. Whilst it is a fungus not present within the UK right now, courtesy of transportation over from the US during WWII, it is indeed present on the continent, and ‘uses’ humans as its primary vector across the landscape. In both urban and forest-borne planes, in countries such as France and Switzerland, this fungus causes marked xylem staining (extending up to as much as 2-2.5m a year), thereby induces wilting, and may even cause the death of its host plane tree as a result. Because its spores may remain ‘active’ for long periods of time before finding a host to infect (usually following wounding), chainsaws (and other equipment), boots, and even the transporation of soil may enable it to spread over vast distances. For this reason, infected trees are felled and disposed of on site, and all equipment used is thoroughly cleaned (or replaced) afterwards. Such a rigorous sanitation measure is, unfortunately, necessary, and therefore its emergence in the UK could be very damaging and costly.

 

Remaining with fungal decayers, Fomitiporia punctata (syn. Phellinus punctatus) also has the potential to infect planes within the UK. On the continent it may indeed use the plane as a host species, and when it does it creates rather non-distinct sporophores (brown ‘splodges’) that can easily be missed. The fungus causes a very significant white rot of the sapwood and (depending upon host species, false-)heartwood region of its host, and therefore in the urban environment such decay may prove to be significant in terms of the level of risk posed by the host tree.

 

F. punctata on Quercus sp. Source: NHGS.

 

Similarly, whilst Inonotus hispidus may already be found colonising plane within the UK, a species of the same genus known as Inonotus rickii may be another potential fungal pathogen. Native to the tropics, it is currently in Italy, where it induces a white rot upon its host and may colonise, like Inonotus hispidus, through open wounds. The authors do note, however, that climatic differences between the UK and Italy may mean its emergence in the UK may not be for a fair few years, though it is likely to succeed at some point (and particularly in urban areas where it is warmer, courtesy of the urban heat island effect – and this is where most plane trees reside). Other wood-decay fungi that may use the plane as a host include the resident Ganoderma species (G. australe, G. lipsiense, and G. resinaceum), and also Perenniporia fraxinea.

 

A type of powdery mildew, with the scientific name of Erysiphe platani, is a further potential pathogen of plane. In fact, it was found in the UK in 1983, though does not appear to be at all significant at this moment in time (if it is even still here – it may have been eradicated?), and is instead far more significant as a pathogen in countries such as Spain. Much like other powdery mildews, it affects the foliage and causes leaf deformation, though may also impact upon the amenity value of infected plane trees. If, like oak mildew, there are also marked impacts to the health of the tree over the long-term, then this powdery mildew of plane may have adverse consequences in that regard as well.

 

Sticking with the leaf, a more evident problem for plane trees is anthracnose (Apiognomonia veneta). Peronal observation of this leaf pathogen (that over-winters on twigs, where it may blight them during milder years) is that it can be very readily observed in larger plane trees, where it can cause extensive leaf dieback (to the point that members of the public are concerned enough to report it). In younger trees, the entire foliage crown may prematurely be lost, and a second one formed. Such a loss of foliage (in plane trees of all sizes) has an adverse impact upon tree energy levels, and like Cameraria ohridella in horse chestnut, is far more than an amenity problem.

 

Again upon the leaf, though this time a pest and not a pathogen, the sycamore lace bug (Corythuca ciliata), since 2006, has been a UK problem. Adults will feed on the underside of plane leaves, which will cause the upper surface of the leaf to become minutely-dotted in white. In time, as feeding continues, the leaf may eventually be evidently chlorotic or bronzed, after which time the leaves will prematurely abscise. Again, this is far more than an amenity issue, and heavy infestations of this lace bug can play a role in the death of a plane tree (particularly when combined with other plane pests and pathogens).

 

The lace bug’s foliar symptoms. Source: SDIS.

 

From the source article, it is certainly evident that there are many looming (and some already present) threats to the plane tree in the UK. Whilst the plane doesn’t necessarily have as much direct ecological value as many other tree species found in the UK (notably native ones), it does have a substantial impact upon airborne pollutants, and its hardy nature means it can thrive where other trees species may suffer beyond belief. Therefore, if it does begin to succumb more readily to pathogens discussed here, then there may be problems with regards to its longevity and abundance. As many of our plane trees are urban-based, will risk management be a principal driver behind its decline? Only time will tell, though as always, protecting and safeguarding ecosystems from invasive pests and pathogens is absolutely critical and, as an island, the UK is (theoretically) greatly-poised to control the entry of these organisms. Of course, I say theoretically, as a control program is only as effective as its weakest link.

 

Source: Tubby, K & Perez-Sierra, A. (2015) Pests and pathogen threats to plane (Platanus) in Britain. Arboricultural Journal. 37 (2). p85-98.

[David Humphries]

..........Remaining with fungal decayers, Fomitiporia punctata (syn. Phellinus punctatus) also has the potential to infect planes within the UK.

 

 

No potential about it.........

 

Phellinus punctatus (now - Fomitiporia punctata) - David Humphries’s Fungi Directory - Arbtalk.co.uk | Discussion Forum for Arborists

 

 

 

 

 

Found that to be a particularly interesting article in the journal

 

 

 

.

[Kveldssanger]

Nice! Thanks for the addition.

 

Indeed, it was a good one. Really easy to read as well, unlike many journal articles.

[Kveldssanger]

03/04/16. Fact #185.

 

When a tree is damaged by a herbivore (such as an insect), it will generally secrete volatile gases known as herbivore-induced plant volatiles (HIPVs). These volatiles may serve a couple of purposes, with the two principal ones being to deter further defoliation by the insect, and to attract predators and parasitoids of the defoliator in an attempt to reduce defoliation severity (by increasing defoliator mortality). Of course, beyond the release of HIPVs, phenols and other compounds may also be deposited within the leaf structure, and foliar re-growth may contain greater concentrations of herbivore-dissuading compounds. Such additional aspects of defence against herbivory are however beyond the scope of this post, which will focus specifically upon how HIPVs fare in terms of attracting predation of an insect defoliator by birds.

 

The authors of this study, to assess whether HIPVs did increase predation (by birds, upon caterpillars), used small 1.5m tall apple trees (Malus sylvestris), and ‘infected’ certain individuals with the winter moth (Operophtera brumata). Following infection of certain individuals, the research sought to determine whether great tits (Parus major) opted to frequent the apple trees where the defoliating caterpillars were still present, where they had been removed, or where both the damaged leaves and caterpillars had been removed. An aim of recognising whether the great tits responded most optimally to chemical (HIPVs), visual (birds can see the caterpillars and damged foliage), or a mixture of both cues, was also investigated. To ensure the test had reduced bias, only a sensible number of caterpillars were placed upon particular apple trees (a number that would mimic a naturally-occurring defoliation event), and the great tits used for the study were naive (they had been bred in captivity, and thus not exposed to the caterpillars on apple trees before). Specific means of data capture for each research aim can be read in the journal article linked at the end of this post.

 

The winter moth. Source: Bramblejungle.

 

Following on from the study period, it was observed that great tits visited infested trees with a higher significance than those uninfested by caterpillars – both in terms of the trees they would visit first, and the trees they would visit over a longer time period. Interestingly, it mattered not whether the infested trees still had the caterpillars on them, or even had the damaged leaves retained after HIPV emission but prior to visitation by birds (meaning that birds were not drawn to the trees because they could see the larvae or damaged leaves). The volatiles emitted by damaged leaves were also significantly different to those emitted by in-tact leaves (though both leaf types did emit volatile mixtures). For example, α-Farnesene, a chemical compound found in apple pomes and associated with herbivore attraction (so the fruits are eaten and seeds dispersed), was emitted at much higher levels following foliar herbivory by the caterpillars, and therefore may be a principal reason for why birds were attracted to the tree (though likely not the only reason). In summation, chemical cues emitted from damaged leaves (HIPVs) can be seen as a significant factor in determining visitation by birds.

 

The great tit. Source: Wikimedia Commons.

 

However, the fact that the remnant structure of defoliated leaves visually differed to in-tact leaves (principally in the level of reflectance of light), visual cues may perhaps play somewhat of a role, though this role is unlikely to be major as light reflectance may vary for other reasons as well (including overall light availability).

 

To conclude, what this study shows is that chemical cues can be considered as important factors when it comes to both the defence of the tree, and the locating of food by insectivorous birds (the two are perhaps associated with one another, to quite a marked degree). This is important, as it ensures that a tree does not succumb to massive defoliation events, though it does rely upon the defoliator being palatable by any visiting birds. For this reason, invasive insect species may (or may not), whilst inducing HIPV emission, not be predated upon as much as a native insect defoliator (at least, to begin with). Granted, native insect species with high levels of toxins (such as the caterpillar of the cinnabar moth Tyria jacobaeae, which accumulates toxic alkaloids from consuming its host plant ragwort Senecio jacobaea) that build up over instar stages, may also reduce predation levels by birds.

 

Source: Amo, L., Jansen, J., Dam, N., Dicke, M., & Visser, M. (2013) Birds exploit herbivore‐induced plant volatiles to locate herbivorous prey. Ecology Letters. 16 (11). p1348-1355.