Local history – then and now (tree edition)

Alongside the photos of the Great Storm of 1987, my parents also found some old images from 1972. Around that time, an entire new housing estate was being constructed nearby, and the below images make for some nice arboricultural comparisons between then (1972) and now (2016). I have made all images black and white, as the old ones were quite faded, and have also tried to be as accurate as possible with the locations. Also note the old images were taken in summer, whilst these new ones were taken during winter. I shall return to the locations in the summer and get more photos then, if I remember!


Location 1

In 1972, we can see how this street was graced with a large line of sizeable trees.
However, in their place now reside two sycamores (Acer pseudoplatanus). A huge shame!

Location 2

When this image was taken, the housing estate development had not yet reached this area.
Lo and behold, it soon did. And here’s a comparison image to today. Instead of mature trees, we can see some fastigiate hornbeam (Carpinus betulus ‘Fastigiata’) and another tree (I didn’t check for species).

Location 3

In this vista, we can see how some young trees have been planted on the verge to both the left and right hand sides of the road.
Oh, look! More fastigiate hornbeam. Some Acer platanoides can also be seen on the other side of the road.

Location 4

Standing in the same location as the above set of images but looking the other way, we can see some large trees over to the right.
It seems like they were cleared and replaced with yet more fastigiate hornbeam.

Location 5

Some nice scrub can be seen over to the right, and some mature trees within the property in the distance can also be observed.
The scrub has since gone and been replaced with a housing development, though the mature trees still can be found within the grounds of the house. I believe they were horse chestnut (Aesculus hippocastanum), though I confess I didn’t look too closely.
Local history – then and now (tree edition)

The Great Storm of 1987 and the associated arboreal devastation

In the UK we quite arguably have it lucky, in terms of climate. Generally, the weather is relatively calm, and very strong winds are a rarity as a result. However, we do experience erratics, and during 15-16 October of 1987 the Great Storm hit. This storm graced with UK, France, Spain, Belgium, and The Channel Islands with winds peaking at 134mph, and therefore was massivelely destructive. From an arboricultural perspective, a huge number of trees (of which many were mature, and some very famous – such as Sevenoaks, which technically became Oneoak) were uprooted, and some areas were left rather barren afterwards. For a really good set of images showing how trees were impacted, the book entitled In the Wake of the Hurricane is a good book to purchase (they are absurdly cheap on Amazon, though there are different editions for different regions of the UK that were hit by the storm).

Back on topic, whilst my parents were going through some old family photos this morning they came across a series of cards showing how the local area was impacted by the Great Storm. Wonderfully, many had trees in the images, and I have shared them below as I don’t imagine these images are online anywhere.

In this image, what looks like a mature sycamore (Acer pseudoplatanus) has fallen onto a parked car and completely written it off!
Here, a red phone box stands nonchalantly amongst a scene of arboricultural devastation. I imagine these trees were oaks (Quercus robur) within the park alongside.
Outside of a swimming pool (which has now also been demolished), two large trees have fallen and are being cleared by the most smartly-dressed axe-wielding man ever to grace that location.
This scene shows how two conifers (probably x Cuprocyparis leylandii) have uprooted and fallen towards a property. One has actually struck the roof, probably causing some damage.
Another conifer (again most likely x Cuprocyparis leylandii) can be seen to have uprooted. The two children atop the stem give a sense of proportion to the size of this tree.
A much smaller conifer in this image can be seen to have a slight lean. I don’t know whether flying roof panels caused this tree to lean, or whether the wind did.
The line of Arboreus invisibilia in this image were swept away to the point that they were replaced with an advertisement for polo mints.
The Great Storm of 1987 and the associated arboreal devastation

A hazard beam on a willow (Salix sp.)

Here’s an interesting one. We have a willow (suspected Salix alba) that has a bit of a wayward limb, which has developed into a hazard beam after being bent straight (during a loading event that was probably caused by the wind, though maybe even by the limb’s own weight?). Curiously, the hazard beam is not necessarily recent as there is distinct ribbing around the wound area. I’m not sure whether one could say the region is entirely stable in terms of structural integrity, though it doesn’t appear to have split any further along the limb since the inception of the issue.

There are three courses of action here: (1) do nothing, (2) take some weight off the limb, or (3) remove the entire limb. Options 2 and 3 are perhaps more preferable, and from there it simply becomes a case of determining whether removing the entire limb would be undesirable from a long-term retention perspective (given it’d create a large wound on the trunk that a willow would probably struggle to compartmentalise, and removal would also take out a fair portion of the overall crown structure).

Here we can see the willow in its entirety.
Zooming-in onto the hazard beam, we can see how it has grown out as a very strong angle.
In this image, we move closer towards the wound itself. The magnitude of the split really starts to become evident.
Closer still, some ribbing around the area can also be observed.
A hazard beam on a willow (Salix sp.)

Tree spotlight: Corylus colurna

The Turkish hazel is by no means not utilised in the urban environmnent, as I have seen a few examples of streets lined with this species. However, I don’t see it as a tree that is necessarily appreciated as much as it could be, in terms of its utility within the street scene (notably on slightly wider verges). Possessing a great mature form that sees it adopt a rather fastigiated style (at least before the height of maturity), it also seems to tolerate enclosed rooting environments quite effectively. The species also has a really lovely bark texture that gives it some extra amenity value, on top of its rather elegant form. I admit, the leaves are also quite interesting, as is its nut crop. Granted, for those highly allergic to nuts, the presence of this tree may be somewhat disconcerting. A nut crop all over the pavement may also be an issue, in terms of pedestrian safety. Of course, this can easily be remdied by having the street swept whilst the problem is evident.

I had a few planted recently, as a matter of fact. Below are two photos from one of them, showing close-ups of parts of the tree.

The wonderfully-textured bark, upon touch, has a rather cork-like feel.
A shot of the shoot and terminal bud.
Tree spotlight: Corylus colurna

Fruit properties and the location of urban holly cultivars – do they matter to frugivorous birds?

Birds are without doubt the principal means of seed dispersal by animals, and trees that produce fleshy fruits are typically reliant upon such dispersal means to expand their geographical ranges. Ornamental trees that produce fleshy fruits grown within urban locations, which are not necessarily native to the area in which they grow, may therefore also benefit from such a dispersal mechanism and begin to succeed into the surrounding environment. Granted, frugivorous birds within urban environments may behave differently to birds within woodland stands (or other ‘natural’ setting), and not much is known about the foraging preferences of such birds in the urban setting. Despite this, it is considered that the colour of tree fruit is one driver influencing upon attracting birds, because of the good vision of birds (to make up for their poor sense of smell).

In this study, the authors investigate how the preference of birds to the fruits of Ilex aquifolium cultivars (and the standard Ilex aquifolium) growing within urban environments has impacted upon the dispersal of seed, and in turn enabled the species to advance into previously uncharted territory within northern Europe. Because the main dispersal mechanism of the species’ seed is via birds, understanding what fruit characteristics (colour, size, etc) influence avifaunal frugivory is important. By the same token, do birds preferentially select cultivated Ilex aquifolium, or instead opt to consume the fruits of the native non-cultivated species? Similarly, understanding what site qualities attract birds can help to improve understanding of the dissemination of Ilex aquifolium into the surrounding landscape.

Because the natural range of Ilex aquifolium reaches its north-eastern climax in Denmark, the study site was located around the current climax zone (within Greater Copenhagen). Within this area, six sites were selected, of which three were cemeteries and three botanic gardens – all within urban areas. Over the past few decades, this climax zone around had been relatively stable, until more recently (since the turn of the millennia) when it was observed to expand its range by a distance of 100-200km up into Greater Copenhagen. Such a shift, the authors remark, would be down to the more favourable climatic conditions, though would be facilitated by seed dispersal courtesy of frugivorous birds.

To investigate if birds had different preferences with regards to fruit from different cultivars, the following cultivars were used within the study: ‘Bacciflava’ (strong yellow fruits, in clusters of 3-5, and 8mm in diameter), ‘Crinkle Green’ (vividly red fruits, in clusters of 1-3, and of 7-9mm in diameter), and ‘Pyramidalis’ (vividly red fruits, in clusters of 1-4 and of 8-10mm in diameter). The standard Ilex aquifolium fruits also featured, in order to compare cultivars to the standard (and native) holly species. To understand the fruit properties of all four hollies used, samples were taken from the individuals from which branches were sourced for the study, and the maximum diameter of the fruits, the fresh mass of the fruits (pulp:seed ratio), and water content were measured.

Ilex aquifolium ‘Bacciflava’. Source: University of Richmond.

From the three cultivars and the native species, branches containing fruits were sourced (within Denmark) and placed at the six locations (from December to February). All branches had similar qualities, possessing 10-20 leaves and fruits, and were fixed to identical wooden boards atop a 1.6m wooden stick to expose the fruits to birds exclusively. Five of these boards were set up in open areas at least 5m away from trees over 3m in height, and five were set up under trees and tall shrubs over 3m in height. Over the course of the survey period, the branches were all checked 15 times, and records gathered as to how many of the fruits had been taken from each sample.

Once the survey was completed, it was found that birds (blackbirds and robins) had eaten 2,655 of the 3,404 (78%) fruits across all four branch types. However, the rate at which the fruits were removed varied between branches, with ‘Crinkle Green’ having its fruits removed most abundantly. In terms of the total number of fruits removed from each branch type, the cultivar ‘Bacciflava’ massively reduced the average by having only 35% of its fruits removed. ‘Crinkle Green’ had 94% of its fruits removed, ‘Pyramidalis’ 92%, and the native holly 91%. Therefore, it is evident that birds had a strong preference for red-coloured fruits (fruit of ‘Bacciflava’ was a green-yellow in colour), though there is little evidence to suggest there is any preference as to what fruits were eaten beyond mere red colouration. There was also a marked difference in the rate at which fruit was removed from the different feeding station locations, with branches under trees having their fruit removed at a much higher rate than those in exposed settings.

Ilex aquifolium ‘Pyramidalis’. Source: Helmers.

In light of the results, it can first be noted that birds did eat the fruits from all four branch types and across both feeding station locations (exposed and sheltered). Therefore, there is potential for many Ilex aquifolium cultivars to enable for seed dispersal and the associated expansion in the native range of the species. Granted, red fruits were eaten far more readily, and this suggests that cultivars with red fruits (and the red fruits of the standard Ilex aquifolium) are far more appealing to birds. It must of course be noted that other fruit colours, such as white and orange (which some Ilex aquifolium cultivars have), were not used within this study, and therefore all that can be ascertained is that red fruits are more desirable to birds than green-yellow fruits.

Looking beyond fruit colour, it can be said that smaller fruits are removed more readily by birds. This is because ‘Crinkle Green’, which had 94% of its fruits removed, possessed the smallest fruits. Such a finding does however conflict with the understanding of larger fruits being more desirable to birds (up to a point, when fruits become too large to fit within a bird’s beak), and particularly earlier in the fruiting season of Ilex aquifolium. The pulp:seed ratio was however shown not to be an influencing factor, as ‘Crinkle Green’ in fact had the lowest pulp:seed ratio, whilst the second highest branch type from ‘Pyramidalis’ had the highest ratio. Building upon this, because the nutritional profile of fruits was not measured, it is difficult to make the assertion that the amount of flesh on a fruit is significant in determining bird frugivory. It may very well be that birds seek nutritional fruits, and these fruits may very well have varying pulp:seed ratios.

A robin perched upon the branch of an Ilex aquifolium. Source: Warren Photographic.

In terms of the amount of fruit per branch, because the number of fruits was relatively similar across all branches (10-20 fruits on each branch), there was little data to support claims that the abundance of fruits influences upon bird frugivory. Of course, if entire specimens were studied, then it may  very likely be found that fruit abundance does influence upon frugivorous birds. In fact, other research has shown exactly this, and the authors remark that Ilex aquifolium cultivars that produce more fruits will hasten the species’ expanding range by attracting birds more readily.

The fact that birds also more routinely ate fruits from branches sheltered by the canopy of trees and tall shrubs is also telling. From an evolutionary and habitual perspective, this is not surprising, because birds will utilise the cover to reduce the risk of predation whilst they are foraging for food. An open environment leaves the bird exposed, and therefore if fruits can be obtained in sheltered settings then that is much preferred. Such a preference is actually quite beneficial for Ilex aquifolium, the authors allege, because it is a shade-tolerant species that can readily exist beneath tree canopies. By this token, the habit of birds eating fruits from sheltered hollies in urban areas may enable the species to expand its range by using urban woodland sites (and other sheltered locations) as vectors.

Therefore, it can be said that, if an Ilex aquifolium is to be planted within an urban setting then it is to be located in an exposed area, and should not have red fruits. Granted, this is assuming that its northward spread is not to be desired. Furthermore, because the survey sites were only in botanic gardens and cemeteries, there is a failure in recognising how small to medium-sized gardens within the urban and sub-urban setting may impact upon bird frugivory and the subsequent dispersal rate of Ilex aquifolium. Nonetheless, the results are interesting, and there is certainly scope for considering what types of cultivar to plant within an urban environment if succession of the species is of concern. This applies not only to Ilex aquifolium, but across the entire botanical spectrum.

Source: Møller, L., Skou, A., & Kollmann, J. (2012) Dispersal limitation at the expanding range margin of an evergreen tree in urban habitats?. Urban Forestry & Urban Greening. 11 (1). p59-64.

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Fruit properties and the location of urban holly cultivars – do they matter to frugivorous birds?

A frantically-retrenching oak

There are some horse paddocks near to where I live, and in one of the paddocks sits a decent-aged oak tree. Unfortunately, likely because of the ground compaction and grazing pressure beneath its crown, there is very evident crown retrenchment. Unfortunately, the retrenchment seems quite sporadic, with very little order to how the branches are dieing back. Clearly, the oak is still in the process of establishing a new crown at a point where an equilibrium is achieved between energy levels and crown extent. We can also observe a large hollow near the base of the tree, suggesting fungal decay of the heartwood region. Because of the grazing pressure beneath, I dare say that there is perhaps more parasitic fungal activity as well (of course, this is just a guess).

A rather sorry sight, if I am honest. At one point, this oak would have had a glorious crown (as can be seen by its branching structure). The stable right beneath really doesn’t help!
A frantically-retrenching oak

Using antitranspirants to control leaf moisture loss

Antitranspirants are products that, when applied to foliage, reduce the rate of foliar transpiration. Comprised of a wax, plastic, or resin, the application of an antitranspirant leaves a thin, protective film atop the leaf surface (Brent-Jones, 1966; Watson & Himelick, 1997; Watson & Himelick, 2013). The film may last for several weeks, though re-application is necessary if there is a desire to reduce transpiration rates for elongated periods of time. Furthermore, if leaves are still growing then the film will crack and its efficacy will be reduced as a result (Watson & Himelick, 2013), meaning application prior to the leaves being fully-grown is perhaps not wholly effective. In addition to this, as the spray dries to become invisible, there are no means to ascertaining how much has been applied or retained upon the leaf at any given time, and nor is there a way in which the extent of cracking can be ascertained (Brent-Jones, 1966).

When used improperly (applying too much and potentially also at overly-frequent intervals), application of an antitranspirant may be detrimental to plant health. Reductions in root and shoot growth may be observed, in such instances (Lee & Kozlowski, 1974; Watson & Himelick, 1997). However, when used properly, their application can increase growth rate and aid with plant establishment via the retention of more water (Davenport et al., 1972; Steinberg et al., 1990). There is also distinct variability in the efficacy of different products, though not only do the products themselves differ but differing environmental conditions, as well as different species, result in varying levels of efficacy across product ranges and also within the same product (Hipps & Nicoll, 1997; Watson & Himelick, 2013). For instance, on species including the pines (Pinus spp.), application can significantly reduce transpiration (by bolstering the already waxy leaf surface), though can also reduce the rate of photosynthesis very markedly; mainly due to reduced CO2 diffusion (Davenport et al., 1974; del Amor et al., 2010; Kozlowski & Davies, 1975; Watson & Himelick). When applied to some species, certain antitranspirants are also toxic.

Because application of an antitranspirant reduces transpiration, leaf surfaces may also potentially warm up considerably more during warmer periods. This can damage the leaf tissues, and result in dysfunction through injury (Watson & Himelick, 2013), leading to early senescence of leaves that have become damaged (Neumann, 1974). Root and shoot growth may therefore be impacted negatively, in response (Ranney et al., 1989; Wellburn et al., 1974).

Such products are nonetheless useful for regulating water loss post-transplanting (Berkowitz & Rabin, 1988), and may be particularly effective during the spring months (Harris & Bassuk, 1995). However, the reliance on such products should not replace good transplanting practice (Watson & Himelick, 1997), and care should be used when applying any type of antitranspirant. It is preferable to apply an antitranspirant only lightly, for an overall net benefit in response to an application (Watson & Himelick, 2013).

Granted, it should be noted however that once water availability within the soil reaches the lowest critical threshold, even the application of an antitranspirant is of no aid (Steinberg et al., 1990). It may therefore be wise to combine such application, in times of significant drought, with irrigation.


Berkowitz, G. & Rabin, J. (1988) Antitranspirant associated abscisic acid effects on the water relations and yield of transplanted bell peppers. Plant Physiology. 86 (2). p329-331.

Brent-Jones, E. (1966) Some aspects of moving semi-mature trees. Arboricultural Association Journal. 1 (3). p71-76.

Davenport, D., Fisher, M., & Hagan, R. (1972) Some counteractive effects of antitranspirants. Plant Physiology. 49 (5). p722-724.

del Amor, F., Cuadra-Crespo, P., Walker, D., Cámara, J., & Madrid, R. (2010) Effect of foliar application of antitranspirant on photosynthesis and water relations of pepper plants under different levels of CO 2 and water stress. Journal of Plant Physiology. 167 (15). p1232-1238.

Harris, J. & Bassuk, N. (1995) Effects of defoliation and antitranspirant treatment on transplant response of scarlett oak, green ash and Turkish hazelnut. Journal of Arboriculture. 21 (1). p33-33.

Hipps, N. & Nicoll, F. (1997) Preconditioning Trees to Improve Outplanting Performance. In Claridge, J. (ed.) Research for Amenity Trees No. 6: Arboricultural Practice – Present and Future. UK: HMSO.

Kozlowski, T. & Davies, W. (1975) Control of water balance in transplanting trees. Journal of Arboriculture. 1 (1). p1-10.

Lee, K. & Kozlowski, T. (1974) Effects of silicone antitranspirant on woody plants. Plant and Soil. 40 (3). p493-510.

Neumann, P. (1974) Senescence of attached bean leaves accelerated by sprays of silicone oil antitranspirants. Plant Physiology. 53 (4). p638-640.

Ranney, T., Bassuk, N., & Whitlow, T. (1989) Effect of Transplanting Practices on Growth and Water Relations of’ ‘Colt’ Cherry Trees During Reestablishment. Journal of Environmental Horticulture. 7 (1). p41-45.

Steinberg, S., McFarland, M., & Worthington, J. (1990) Antitranspirant reduces water use by peach trees following harvest. Journal of the American Society for Horticultural Science. 115 (1). p20-24.

Watson, G. & Himelick, E. (1997) Principles and Practice of Planting Trees and Shrubs. USA: International Society of Arboriculture.

Watson, G. & Himelick, E. (2013) The Practical Science of Planting Trees. USA: International Society of Arboriculture.

Wellburn, A., Ogunkanmi, A., Fenton, R., & Mansfield, T. (1974) All-trans-farnesol: a naturally occurring antitranspirant?. Planta. 120 (3). p255-263.

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Using antitranspirants to control leaf moisture loss

Inonotus hispidus on Sorbus – a winter follow-up

Back last summer, I came across these two sporophores up in the lower crown of a Sorbus intermedia. Today, I re-visited the site, and as expected they are still there – albeit inactive and blackened. I imagine that, in the coming year or two, they will drop from the tree and persist on the ground beneath. In fact, this is key for Inonotus hispidus, as one should always inspect the ground beneath the tree in order to see if fruiting bodies of this species can be found. Normally, an inspection of the ground beneath a mature ash tree would be wise, though of course the fungus can colonise other species as well (such as Sorbus).

Here we can see the two brackets up near the lower crown.
A close-up reveals the blackened appearance, though we can still see a slight orange undertone.
Inonotus hispidus on Sorbus – a winter follow-up

Tree and shrub roots in sewer pipes

In the urban environment, conflict below the ground exists between tree roots and services – namely, sewer pipes. Because tree roots will grow up a moisture gradient, and pipes both contain moisture within and may collect condensation on the outer surface, tree roots are drawn to the pipes and, in many cases, are able to intrude into the pipe network (through small openings; usually at junctions, and when the pipes are old and made of clay). Such intrusion obviously causes issues, with regards to the blockage of the pipes, and their subsequent fixing. In response to this, the aim of the authors in this study was to assess what urban tree and shrub species’ roots were found within pipes, whether different species and cultivars had different rates at which their roots were able to enter into the pipes, and if the material the pipe was made of impacted upon the rate of root intrusion.

A CCTV image of tree roots blocking a pipe entirely. Source: South Gippsland Water.

Underground sewage pipes were (prior to the study, from 1970-2007) inspected – via CCTV cameras – in the Swedish cities of Malmö and Skövde, and the number of root intrusions along pipe lengths were established and mapped as a result (a total of 2,180 intrusions along 33.7km of pipe). From these records, the authors located survey sites. These surveys also noted what the pipe was made of (concrete or PVC), the pipe’s date of construction, and the pipe dimensions. In relation to the plants featured in the study, a 2008 survey of the tree and shrub populations in both cities led to 4,107 individuals being identified within a 20m radius from the pipes where intrusions had been located. Data from earlier inventories enabled the authors to expand the research to 14,552 trees and shrubs.

Once the tree and shrub locations had been plotted against locations of root intrusions into pipes, the trees were segmented into 186 different genera, species, and cultivars. As this was considered a vast range, the authors sought to narrow-down the list to a more manageable number. This was achieved by selecting only those trees and shrubs within 10m of an intrusion point, where no other vegetation was found within the original 20m distance; though if two individuals of the same species were found within a 20m distance from an intrusion point then the nearest tree of the two would feature as part of the survey.

After the final tree and shrub list (comprising of 2,4,21 individuals from 52 different species and cultivars) was drawn up and the data relating to pipe intrusions analysed, it was found that roots of both broadleaved and coniferous species were able to intrude into pipes. Curiously, it was the PVC pipes (0.661) that had a greater rate of intrusion than concrete ones (0.080) per joint (one length in between two joins). The below table outlines species included within the study, and the rate of root intrusion. Evidently, there are many tree species featured, and this was after many had already been filtered-out of the study scope.

Data captured within the survey for each tree species (or genus) relating to the rate at which roots intruded into pipes.

From the results gathered, the authors remark that the rate at which Malus floribunda entered into pipes was very significant. Compared to previous studies, and even compared to other species of Malus, this species could be considered very able in terms of root intrusion. In fact, so great was its ability to enter into pipes that it trumped willows at a rate of 3:1. At a broader level however, what this study shows is that many tree and shrub species have similar rates of intrusion into sewer pipes, which therefore suggests that discrimination against particular species may not necessarily be wholly justified.

Building on the above comment, we can observe that Tilia cordata and some species of Ulmus also intrude into pipes quite significantly, and even more so than the Salix species observed during the study. Not surprisingly however, Populus canadensis was found to have roots within pipes more often than most other species surveyed (asides from Malus floribunda). Granted, other Populus species were almost half as likely to enter pipes, in light of this survey data. Therefore, it’s not simply a case of observing particular genera that infiltrate regularly into pipes, but particular species within a genus. This seems constant throughout, though Acer spp., Malus spp., Populus spp., and Ulmus spp. most effectively demonstrate this.

Interestingly, the lower than expected rate of root intrusion by Populus spp. and Salix spp., the authors allege, is because previous studies have obtained results with a disproportionately high number of thse two genera across the survey sites. As a result, it may simply have been a case that, because these two genera featured so heavily, their roots were more often found in pipes than other genera and species were. If populations of each tree species were normalised, it may have been found that other tree genera and species intruded into pipes more frequently, potentially. This is, in fact, what this study shows, as it was the mean number of root intrusions per pipe joint that were calculated (instead of just the number of intrusions).

Granted, this study was not without its limitations. The authors even acknowledge this, and state that tree and shrub species present on a site is not the only determining factor to do with intrusion rate. Soil properties, what pipes are made out of, their age, condition, and the distance to the nearest pipe from a tree are also almost certainly to influence upon infiltration rate.

This study is nonetheless serious food for thought, and consideration should of course be given as to what species are to be planted in areas near to pipes. Hopefully, this research will aid with the decision-making process (or, perhaps, the opposite!).

Source: Östberg, J., Martinsson, M., Stål, Ö., & Fransson, A. (2012) Risk of root intrusion by tree and shrub species into sewer pipes in Swedish urban areas. Urban Forestry & Urban Greening. 11 (1). p65-71.

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Tree and shrub roots in sewer pipes

Fungal colonisation strategies Pt. IV: Active pathogenesis

Colonisation via active pathogenesis involves direct penetration of the host by the fungal pathogen, largely through the roots though also via air. The establishment of sufficient inoculum base (such as a dead stump or infected root) is critical for successful active pathogenesis, given the aggressive nature of active pathogenesis (Boddy & Rayner, 1983; Lonsdale, 1999; Schwarze et al., 2000; Shigo, 1986). Fungal species of this strategy can employ tactics that infect both healthy individuals and stressed individuals, depending both upon the species of fungus and the context of the site.

Active pathogenesis can be broken down into three categories: ectotrophic root infection, wound infection, and canker production (Boddy, 2001). Establishment is via the production of pectinolytic enzymes that destroy pit membranes and advance the spread of desiccated zones, or through wholly combative behaviour where parenchyma cells are destroyed completely during – or more likely in advance of – colonisation. The latter is achieved by the creation of superficial, predominantly non-assimilative mycelium (such as soil rhizomorphs with Armillaria spp.) that grows over the surface of roots, inducing dysfunction and cell death. In killing such cambial tissues, the fungi can colonise without significant hindrance (Garrett, 1970; Rayner, 1993; Rayner & Boddy, 1988).

Such strategists may also utilise pre-existing stress within the tree, caused for instance by defoliating insects or pathogenic disease, as a means of entry as a secondary pathogen. As energy must be used by the tree to combat the damage induced by such damaging agents, there is less energy available for additional defensive processes beyond that of combating the damaging agent. Active pathogenesis strategists can utilise this situation to their advantage, as it may mean that (when focussing on the rooting system of a tree) the boundaries between woody and non-woody roots do form form root periderms, become ‘corky’, or become suberised, leading to soil-borne fungal pathogens (such as Armillaria spp.) beginning their attack (Shigo, 1986). In some cases, such secondary infections can be so rapid that they are mis-identified as the primary causal agent.

An English oak (Quercus robur) that has been aggressively colonised (on all sides) by Armillaria mellea.

The well-known pathogens Heterobasidion annosum and (as ascertained) Armillaria mellea are classified as active pathogens, with colonisation of the sapwood being preceded by mycelial development in the bark. This leads directly to the death of the cambium within the region, and enables subsequent colonisation (Boddy & Rayner, 1983; Fox, 2000; Lonsdale, 1999; Schwarze et al., 2000). The preceding development in bark is a result of spores being washed into the soil, via rhizomorphs, or by direct contact with roots of a separate but infected host (Wargo & Shaw, 1985). Interestingly, Armillaria spp. will by-and-large colonise via soil rhizomorphs. This may be because the spores of the genus are suspected to have to pass through the guts of insects associated with the fungus, before they can successfully germinate (Shigo, 1986). Therefore, much like tree seeds, fungal spores may have an ‘activation’ process equivalent to stratification, fire, digestion, or otherwise – if the means of activation is not present, then the spores will not germinate.

As briefly touched upon above, infection can (perhaps only rarely) occur in branches when, under humid conditions, the fungus produces highly water-resistant bridges between branches that come within close proximity to one another. In the UK the fungus Hymenochaete corrugata, which is considered largely a specialised opportunist of Corylus avellana, establishes within the canopy and then spreads further by bridging from colonised canopy space to healthy branches of different hazel specimens (Ainsworth & Rayner, 1990).

Fungi that employ active pathogenesis as a means of colonisation may also rely initially upon the aforementioned colonisation strategies (heart rot, specialised opportunism, and unspecialised opportunism) in order to establish an inoculum base from which they can invade healthy sapwood. Stereum gausapatum is an example of this upon Quercus spp., where it is considered to exercise all four strategies to varying extents (Rayner, 1993).


Ainsworth, A., & Rayner, A. D. (1990) Aerial mycelial transfer by Hymenochaete corrugata between stems of hazel and other trees. Mycological Research. 94 (2). p263-266.

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Fungal colonisation strategies Pt. IV: Active pathogenesis