(Arboricultural-styled) 'Fact of the Day'

[daltontrees]

16/02/16. Fact #152.

 

The higher likelihood of being killed within a vehicle is interesting, as it suggests that it may very well be street trees that cause deaths.

 

It suggests to me that the only way that most people would be out of buildings in weather likely to cause tree failures is in a car. Cars give people a sense of security, which is all very valid except they are not much protection from falling trees. Plus you can't see what's above you when in a car.

 

Plus in USA a lot of people wouldn't walk the length of themselves before considering taking the car instead. Especially in poor weather.

 

Street trees have less than 50% chance of landing on a street.

[Kveldssanger]

Good points about the car. I'd also be interested to know what type of cars the people were in when they died - were they the 'trailer' ones you get quite often in the US, hatchbacks, minis, etc?

 

As for street trees having a 50% chance, I take it you mean it can either go on to the street or not? If it's on the inside of a bend it has a chance greater than 50%, and the outside of a bend lower than 50% chance. On a crossroads, it could be up to nearer 100%, and maybe even 100% for a roundabout or central island (that US crossroads lack, I guess?).

[Kveldssanger]

22/02/16. Fact #156.

 

There exist four different colonisation strategies of a living tree by wood-decay fungi: heart rot (heartwood-exposed), unspecialised opportunism (sapwood-exposed), specialised opportunism (sapwood-intact), and active pathogenesis (Boddy, 2001; Rayner, 1993; Rayner & Boddy, 1988; Schwarze et al., 2000). Over the coming week or so, I’ll be looking at each strategy. We shall begin with heart rot.

 

Heart rot

 

In simple terms, this strategy involves colonisation of heartwood – via an entry point where heartwood becomes exposed – and subsequent decay of such heartwood (or core) of the host, where parenchyma (living) cells are lacking and conditions are very gaseous (Boddy, 2001; Rayner & Boddy, 1988; Schwarze, 2008). Colonisation is considered to be through heartwood-exposed wound surfaces, or alternatively via exposed heartwood from dead or diseased areas of the tree that are old enough to contain heartwood. Entry via such mechanisms can be divided into two distinct segments: top-rot (colonisation originates at the crown and progresses downwards) and butt-rot (colonisation originates at the root collar-butt interface and works upwards). For butt-rots, colonisation can further be divided, with entry being via root-mycelium contact, or by fungal spores (Rayner & Boddy, 1988). Rarely do butt rots cause hollows more than a few feet up into the trunk (Shigo, 1986).

 

Such strategists are particularly stress-tolerant, predominantly because conditions for decay are initially very unsuitable deep within the heartwood of the host. The lack of oxygen, high levels of carbon dioxide, undesirable moisture levels (particularly if bacterial wetwood is present – this will occur if bacteria are the pioneer invaders of a site, and not fungal pathogens), and abundance of inhibiting compounds (tannins), mean decay will be a very slow process and may take many years to even initiate substantially. Species that adopt this strategy therefore are largely non-combative, very slow with regards to their decay and colonisation of the heartwood, and may well be species-specific; or at least show certain levels of host species-preference (Boddy, 2001; Boddy & Rayner, 1983; Cartwright & Findlay, 1958; Rayner & Boddy, 1988; Shigo, 1986; Weber & Mattheck, 2003). The predominant reason behind such frequently-observed selectivity is suspected to be due to the fact that different species of host possess vastly different characteristics with regards to heartwood formation and properties, and by limiting host preference the fungal species directly reduce their potential fungal competitor range to, in some instance, almost zero (Rayner & Boddy, 1988). Species-specificness ultimately varies between heart rot strategists, therefore; a continuum, of sorts (Rayner & Boddy, 1988). Genus-specific strategists include Fistulina hepatica (Quercus spp.), Phellinus pomaceus (Prunus spp.), and Porodaedalea pini (Pinus spp.), whilst generalist strategists include Armillaria spp. and Heterobasidion annosum. Rayner and Boddy (1988) also note that Laetiporus sulphureus may colonise seemingly unrelated species such as Castanea spp., Quercus spp., Salix spp., and Taxus spp.

 

This oddly-shaped Fistulina hepatica, a heart rot strategist, was found (by me) at the base of a very mature Quercus robur.

 

In spite of their largely non-combative ability, both with regards to colonisation of wood and competition against other fungi, their intricate specialisms that have optimised them for heartwood decay enable them to create large individual territories amongst the expansive heartwood extent within their host. Mycoparasites (fungal parasites that predate upon other fungi) may however be a potentially limiting factor, in certain instances, where such fungal parasites establish within the decaying wood zone(s) and attack the wood-decaying fungi present – as may fungal viruses (Badalyan et al., 2004; Boddy, 2000; Boddy & Rayner, 1983; Shigo, 1986).

 

Research by Highley et al. (1983) also suggests that the lack of difference in performance under low oxygen and high carbon dioxide regime levels for heart rot strategists means they may have evolved to become so specialised by adapting to species-specific heartwood traits (pH, volatiles, extractives, etc) – such as with Laetiporus sulphureus and its ability to tolerate tannin-rich and acetic acid-rich wood, which correlates with the low pH of Quercus spp. heartwood, and its high tannin levels (Hintikka 1969, Hintikka 1971, Rayner & Boddy, 1988).

 

In this image (taken by me) a mature Pinus nigra, with major storm damage upon its stem, has been colonised by the heart rot strategist Porodaedalea pini.

 

Heart rot is typically non-fatal for trees (at least, in the direct sense – the tree may die as a result failure induced by the decay), in the sense that it is considered to be more economically destructive to foresters than it is the health and longevity of the tree (Rayner, 1993, Rayner & Boddy, 1988). Because such strategists largely lack the ability to invade intact sapwood, their extent is confined to the heartwood of the host, thereby enabling the tree to continue in its metabolic pursuits without marked hindrance. However, death can be caused when heartrot strategists that are able to attack sapwood (through suspected active pathogenesis – Phellinus pomaceus), for the purpose of creating fruiting bodies (on sites where exposed heartwood does not exist) and for means of continued colonisation (Mattheck et al., 2015; Rayner & Boddy, 1988), do so extensively – to the point that the stem may be girdled, or the sapwood significantly damaged. Such a means of sapwood attack is through the development of a canker, initiated by the creation of a thick mycelial pad, which serves to force bark outwards and thus enable for an exit point (Rayner & Boddy, 1988).

 

References

 

Badalyan, S., Innocenti, G., & Garibyan, N. (2004) Interactions between xylotrophic mushrooms and mycoparasitic fungi in dual-culture experiments. Phytopathologia Mediterranea. 43 (1). p44-48.

 

Boddy, L. (2000) Interspecific combative interactions between wood-decaying basidiomycetes. FEMS Microbiology Ecology. 31 (3). p185-194.

 

Boddy, L. (2001) Fungal community ecology and wood decomposition processes in angiosperms: from standing tree to complete decay of coarse woody debris. Ecological Bulletins. 49 (1). p43-56.

 

Boddy, L. & Rayner, A.. (1983) Origins of decay in living deciduous trees: the role of moisture content and a re-appraisal of the expanded concept of tree decay. New Phytologist. 94 (4). p623-641.

 

Cartwright, K. & Findlay, W. (1958) Decay of Timber and its Prevention. 2nd ed. London: HMSO.

 

Highley, T., Bar-Lev, S., Kirk, T., & Larsen, M. (1983) Influence of O2 and CO2 on wood decay by heartrot and saprot fungi. Phytopathology. 73 (4). p630-633.

 

Hintikka, V. (1969) Acetic acid tolerance in wood – the litter decomposing Hymenomycetes. Karstenia. 10 (1). p177-183.

 

Hintikka, V. (1971) Tolerance of some wood decomposing basidiomycetes to aromatic compounds related to lignin degradation. Karstenia. 12 (1). p46-52.

 

Mattheck C., Bethge, K., & Weber, K. (2015) The Body Language of Trees: Encyclopedia of Visual Tree Assessment. Germany: Karlsruhe Institute of Technology.

 

Rayner, A. (1993) New avenues for understanding processes of tree decay. Arboricultural Journal. 17 (2). p171-189.

 

Rayner, A. & Boddy, L. (1988) Fungal Decomposition of Wood: It’s Ecology and Biology. UK: John Wiley & Sons.

 

Schwarze, F., Engels, J., & Mattheck, C. (2000) Fungal Strategies of Wood Decay in Trees. UK: Springer.

 

Shigo, A. (1986) A New Tree Biology. USA: Shigo and Trees Associates.

 

Weber, K. & Mattheck, C. (2003) Manual of Wood Decays in Trees. UK: The Arboricultural Association.

[Kveldssanger]

23/02/16. Fact #157.

 

Such strategists have their spores colonise the (now dysfunctional) sapwood after a wound exposes what would otherwise have been functional sapwood. Such strategists are adapted to a wood environment with high oxygen content and (initially) high moisture levels (Boddy, 2001; Rayner, 1993; Rayner & Boddy, 1988; Schwarze et al., 2000). Cankers may also facilitate the establishment for spores of such strategists, in particular instances (Shigo, 1986).

 

Sapwood-exposed strategists are typically rapid in colonisation rate, though do not begin to cause decay until the wood dries out (Schwarze et al, 2000); by this point, non-decay-causing organisms have likely already begun to colonise, and may in fact aid with fungal succession and wood degradation (Rayner, 1993; Schwarze et al., 2000; Shigo, 1991). As such, there is a (brief) ‘latent’ period in between infection and decay. During this delay, the fungus will take advantage of readily-available food sources such as sugars, using the energy to fuel its rapid colonisation habit. Such rapid establishment means the tree does not have enough time to properly compartmentalise the attack around the wounded area (Rayner & Boddy, 1988; Schwarze et al., 2000). Some of these strategists also deploy offensive mechanisms to further damage the tree, such as via the secretion of toxins to kill or damage parenchyma cells.

 

The decay column that manifests following fungal establishment is largely axial in spread, progressing vertically from the wound site with – at least initially – little radial spread. The decayed area will be surrounded by a discoloured margin, where the tree has deposited tyloses, suberin, and phenols, in an attempt to compartmentalise the decay process by shutting down and clogging its vascular tissues (Boddy, 2001; Dujesiefken & Liese, 2015; Shigo, 1991; Weber & Mattheck, 2003). Discolouration and decay extent both vary depending upon the species of fungus and the tree species’ characteristics (intrinsic), as well as the environment in which the host tree resides (extrinsic) (Rayner, 1993; Rayner & Boddy, 1988).

 

On this Aesculus x carnea, which has suffered major windthrow within its crown that thus exposed large tracts of sapwood, Polyporus squamosus has colonised.

 

In certain instances, numerous unspecialised strategists may colonise a tree in different regions, surrounding either the same wound or, if the tree has many wounds, various ones (Boddy, 2000). This can lead to intricate patterns of decay and barrier zones between each different hyphal network, at times with barriers being visibly breached on numerous occasions. Ultimately, it is critical that invading pathogens create and retain their own zones within the wood structure, protecting the hyphal network from both tree defence mechanisms, mycoparasites, and other invading pathogens (Boddy & Rayner, 1983; Shain, 1979; Shigo, 1986).

 

Furthermore, such strategists possess a wide range of ‘sub-colonisation’ strategies, varying from the ruderal (saprophytic) mold fungi (Hyphomycetes) to the combative (parasitic) Basidiomycetes (Schwarze et al., 2000). Ruderal strategists do not typically degrade wood but merely discolour it, though may initiate decay that may, as already suggested, initiate succession by higher-tier Basidiomycetes of the same site. This is because ruderal strategists tend to enter early, colonise, and exit, before conditions become undesirable (due to lowering nutrient availability, competition from other decay organisms, desiccation of substrate, etc). They are largely non-selective with regards to species preference (Boddy, 2001).

 

As a partial aside, unspecialised opportunists will also attack incredibly young seedlings. Seedlings, until a certain age (species-specific, in part, though also driven by environmental conditions – may be from 5 days to 2+ weeks), lack the ‘mature’ tissue and resistance to pathogens that established ones have (this occurs when pectin begins to convert to calcium pectate within cell walls). This means seedlings are susceptible to unspecialised opportunists, particularly those within the soil. Depending upon the extent of soil-based inoculum, seedlings may in fact be killed before they even emerge from the soil (high inoculum potential). If the inoculum base is lower, seedlings may instead be killed post-emergence. In such instances, where localised humidity is high due to an abundance of seedlings creating a humid micro-climate and high rainfall (or artificial watering), fungal mycelium may spread across the surface from hypocotyl to hypocotyl – such rapid spread is aided by better aeration when compared to soil aeration (Garrett, 1970). Such a concept is termed ‘damping-off’ disease.

 

References

 

Boddy, L. (2000) Interspecific combative interactions between wood-decaying basidiomycetes. FEMS Microbiology Ecology. 31 (3). p185-194.

 

Boddy, L. (2001) Fungal community ecology and wood decomposition processes in angiosperms: from standing tree to complete decay of coarse woody debris. Ecological Bulletins. 49 (1). p43-56.

 

Boddy, L. & Rayner, A.. (1983) Origins of decay in living deciduous trees: the role of moisture content and a re-appraisal of the expanded concept of tree decay. New Phytologist. 94 (4). p623-641.

 

Dujesiefken, D. & Liese, W. (2015) The CODIT Principle: Implications for Best Practices. USA: International Society of Arboriculture.

 

Garrett, S. (1970) Pathogenic Root-Infecting Fungi. USA: Cambridge University Press.

 

Mattheck C., Bethge, K., & Weber, K. (2015) The Body Language of Trees: Encyclopedia of Visual Tree Assessment. Germany: Karlsruhe Institute of Technology.

 

Rayner, A. (1993) New avenues for understanding processes of tree decay. Arboricultural Journal. 17 (2). p171-189.

 

Rayner, A. & Boddy, L. (1988) Fungal Decomposition of Wood: It’s Ecology and Biology. UK: John Wiley & Sons.

 

Schwarze, F., Engels, J., & Mattheck, C. (2000) Fungal Strategies of Wood Decay in Trees. UK: Springer.

 

Shain, L. (1979) Dynamic responses of differentiated sapwood to injury and infection. Phytopathology. 69 (10). p1143-1147.

 

Shigo, A. (1986) A New Tree Biology. USA: Shigo and Trees Associates.

 

Shigo, A. (1991) Modern Arboriculture. USA: Shigo and Trees Associates.

 

Weber, K. & Mattheck, C. (2003) Manual of Wood Decays in Trees. UK: The Arboricultural Association.

[David Humphries]

.........

This oddly-shaped Fistulina hepatica, a heart rot strategist, was found (by me) at the base of a very mature Quercus robur........

 

Fine piece of homework here Chris. possibly a tad over enthusiastic in terms of the Level 4, but I get why you do it

 

Your Fistulina image is more than likely the anamorph (asexual) stage of F. hepatica known as Confistulina.

 

Not readily found in the texts

 

Here's a link to an older paper.....

 

http://www.cbs.knaw.nl/images/ResearchGroups/Publications/1983Stalpers0001.pdf

 

 

here's an image of one we found on sweet chestnut at Whippendell Woods near Watford a few years ago

 

.

[Kveldssanger]

That's for the kind words, David! Means a great deal. I always find it better not just to 'scrape by' with anything - I just do what I want to do until I get bored or figure I need to stop.

 

As for Confistulina, that's really awesome! I have seen some similar-looking fruiting bodies though much smaller - perhaps they would also be the asexual stage? I had a quick look on Wikipedia and am I right in thinking that the anamorph stage is not currently accepted as a means of classifying a species taxonomically (as in, it's seen that only one 'over-riding' teleomorphic name, in this case F. hepatica, would be used to cover all stages of the fungus' life)?

 

Interesting that the authors suggest it is a new genus, however. I admit I got lost after the first page - too many words I don't know!

[Kveldssanger]

24/02/16. Fact #158.

 

This strategy sees fungi colonise the sapwood in a living tree by taking advantage of the tree’s physiological stress due, for example, to root dysfunction or drought conditions (Boddy, 2001; Parfitt et al., 2010; Rayner, 1993; Rayner & Boddy, 1988; Schwarze, 2008). Development will be in apparently intact, yet dysfunctional sapwood of areas of the tree which remain uninjured, though decay onset will be timed with desirable conditions within the tree (induced by stress). Single genotypes will usually manifest with distinct speed (up to a few metres per year), and spread extensively, using the xylem as a vector, throughout underlying sections of bark, forming vast decay columns. This therefore entails that such decay fungi are present extensively within the tree (yet not in an overt manner) within its functional sapwood, prior to attack. Onset of decay is likely not observed until the tree suffers localised xylem dysfunction, given the high water content of functional sapwood is undesirable for fungal decay (Baum et al., 2003; Boddy, 2001).

 

This mature Betula pendula, whilst still alive, has been aggressively colonised by Piptoporus betulinus, following physiological stress.

 

Latent invasion may also in fact result from the development and subsequent assimilation of separate fungal mycelia, under the conditions associated with dysfunction. The spores may have spread widely within the sap stream over long periods, initiating only later (following stress in the host) their mycelial development, with subsequent ‘assimilation’ of the many establishing mycelium networks as they coalesce. The consequent decay associated with the assimilation and the host’s inability to defend against the widespread attack by the host may ultimately be very significant (Boddy & Rayner, 1983; Parfitt et al., 2010). Research also suggests that even once sapwood does become dysfunctional, presence of decay may not become overt. Decay may not even begin whatsoever. Further, as many fungal species latently exist within specific hosts, particular conditions may only trigger the onset of decay by one, or a portion of, the fungal species present (Parfitt et al., 2010).

 

Additionally, such strategists have a high degree of selectivity with regards to their host site and / or species, with branch junctions being a principal location for decay onset (Boddy, 2001; Rayner, 1993). This is perhaps due to the lower side of the branch junction being an inherent weak point within the tree, because the site has low energy reserves – particularly when the branch attached to the parent branch or trunk is dying (Shigo, 1986). An example of a specialised opportunist’s strategy is therefore the entering into a dying branch with sapwood dysfunction, likely induced by the inability to compete with its neighbours for light, waiting at the junction of the dying branch until the spores are incorporated into the heartwood via secondary thickening, and then establishing and beginning the attack (Baum et al., 2003; Chapela & Boddy, 1988a; Chapela & Boddy, 1988b; Oses et al., 2008). Such a colonisation trait can be described as endophytic – this is where a species resides within the host with no adverse impact upon the host until conditions are right for attack (Baum et al., 2003). Such a scenario may even be beneficial in terms of facilitating the “natural pruning” of limbs that become dysfunctional as tree canopies expand (Rayner, 1993).

 

Under some conditions, certain specialised opportunists may also be able to colonise via active pathogenesis (Rayner, 1993).

 

References

 

Baum, S., Sieber, T., Schwarze, F., & Fink, S. (2003) Latent infections of Fomes fomentarius in the xylem of European beech (Fagus sylvatica). Mycological Progress. 2 (2). p141-148.

 

Boddy, L. (2001) Fungal community ecology and wood decomposition processes in angiosperms: from standing tree to complete decay of coarse woody debris. Ecological Bulletins. 49 (1). p43-56.

 

Boddy, L. & Rayner, A.. (1983) Origins of decay in living deciduous trees: the role of moisture content and a re-appraisal of the expanded concept of tree decay. New Phytologist. 94 (4). p623-641.

 

Chapela, I. & Boddy, L. (1988a) Fungal colonization of attached beech branches. I. Early stages of development of fungal communities. New Phytologist. 110 (1). p39-45.

 

Chapela, I. & Boddy, L. (1988b) Fungal colonization of attached beech branches. II. Spatial and temporal organisation of communities arising from latent invaders in bark and functional sapwood, under different moisture regimes. New Phytologist. 110 (1). p45-57.

 

Oses, R., Valenzuela, S., Freer, J., Sanfuentes, E., & Rodriguez, J. (2008) Fungal endophytes in xylem of healthy Chilean trees and their possible role in early wood decay. Fungal Diversity. 33 (1). p77-86.

 

Parfitt, D., Hunt, J., Dockrell, D., Rogers, H., & Boddy, L. (2010) Do all trees carry the seeds of their own destruction? PCR reveals numerous wood decay fungi latently present in sapwood of a wide range of angiosperm trees. Fungal Ecology. 3 (4). p338-346.

 

Rayner, A. (1993) New avenues for understanding processes of tree decay. Arboricultural Journal. 17 (2). p171-189.

 

Rayner, A. & Boddy, L. (1988) Fungal Decomposition of Wood: It’s Ecology and Biology. UK: John Wiley & Sons.

 

Schwarze, F. (2008) Diagnosis and Prognosis of the Development of Wood Decay in Urban Trees. Australia: ENSPEC.

 

Shigo, A. (1986) A New Tree Biology. USA: Shigo and Trees Associates.

[Kveldssanger]

24/02/16. Fact #159.

 

I thought I'd write more about this. I touched upon it in an earlier fact, though read it again and have wrote about it all in more detail. Awesome paper!

 

It goes without saying that the world of mycorrhizal fungi is so vast and complex that we’re only just beginning to scratch away at the surface of understanding, though this doesn’t mean we haven’t made some very interesting developments over recent decades. For example, we know that trees may use mycorrhizal networks to ‘trade’ resources across a single species and across differing species, as do we know that they will ‘communicate’ to signal neighbours about upcoming defoliation events by insects. However, whilst we understand the concept, we aren’t necessarily in possession of an arsenal of data that can really begin to demonstrate the intricacies of mycorrhizal networks. I feel that this study is one that begins to establish knowledge of such intricacies, and therefore I hope that you find this as brilliant and mind-boggling as I did when I first read it a few months back (and also hopoe I am making sense with what I write!).

 

The focus of this study was a group of 67 Douglas fir (Pseudotsuga menziesii) of varying ages (courtesy of natural regeneration) in an area of 30m x 30m, and on the manner in which the ectomycorrhizal network of the species Rhizopogon vesiculosus and Rhizopogon vinicolor impacted upon the connectedness of Douglas fir individuals. By a similar token, it looked at the population structure of the two fungal species, across the study site.

 

In order to obtain data required to draw conclusions from the study aims, samples of soil were taken from four sides of each tree within the plot area (usually within the drip line, though if canopy cover was lacking then obviously not so). This enabled for the authors to theoretically obtain fibrous tree roots from each individual, and analyse the tree roots not only to identify the Douglas fir they were from, but to determine whether the two Rhizopogon species were present within both the root cambium and soil environment and, if so, of what genet they were from. The map below shows the plot area, and the sample locations. From each sample location, arrows are drawn to show what tree’s roots were found at each sample site, and from what Rhizopogon species (and genet) the roots were associated with. If we take, for instance, the upper-most blue-shaded area indicating a Rhizopogon vesiculosus genet, we can see how the genet is connected to many different trees across the site. These trees were thus deemed to be ‘connected’, as they shared an ectomycorrhizal network.

 

R. vesiculosus genets can be seen in the blue-shaded areas, and R. vinicolor genets in pink-shaded areas. The coloured lines around the shaded areas represent different genets of the two species, and the black dots within are the sample sites. The lines from the black dots show the links to the Douglas firs, which can be seen as the green star-like shapes (larger ones signify larger trees, and there are a total of four ‘cohorts’ marked by different-sized shapes). The arrow marks the Douglas fir most connected to other trees.

 

In light of the results obtained, which are shown above (visually), the authors identified a total of 56 Douglas firs that were connected with other trees in (largely) the plot area, via the ectomycorrhizal networks created by the two Rhizopogon species (and one fungal genet connected 19 trees!). 45 of the trees were inside the plot area, though a further 11 were outside (and this is why some are plotted outside the sample area). Within the site, 27 ectomycorrhizal genets were also found, of which 14 were from R. vesiculosus and 13 were from R. visicolor. 18 of the genets, 9 from each species, were found to connect at least two trees together. More associations were more frequently found amongst the larger and older individuals, most probably because they had been there longer and their larger rooting environments enabled them to assume more associations with ecotmycorrhizal genets. The table below provides a more detailed breakdown of the genets found and their associated with the different cohorts of Douglas fir.

 

A table showing the data obtained from the study.

 

In terms of the population structure of the mycorrhizae and its impact upon the Douglas firs, the authors found that two trees over 43m apart shared a connection via only two different ectomycorrhizal genets. Their connectedness had to span over more than one genet, as the maximum distance one genet (R. vesiculosus) spanned was around 20m. The ability of R. vesiculosus to span greater spatial distances may also be the reason behind why it was found to connect (10.2), on average, more trees per genet than R. vinicolor (4.4). The most connected tree (at 94 years of age), marked with the arrow in the first image, was considered to be ‘central’ to the overall ectomycorrhizal network, and had a relationship with 11 different ectomycorrhizal genets and 47 other Douglas fir.

 

A total of 62% of the Douglas fir from Cohort 1 and Cohort 2 were also found to be connected with trees from Cohort 3 and Cohort 4. This means that these younger specimens shared associations with the same fungal genets that older specimens were connected to, which the authors found interesting as it suggested that the fungal species surveyed had Douglas fir hosts that would ensure longevity of its existence within the landscape (as if the fungus has anticipated that, by only colonising older specimens, it could itself cease to exist when its old hosts all die out – succession-planning, if you will). Furthermore, it enables the younger specimens to share an already established inoculum base, from which carbon and water can be provided by the older specimens to aid with establishment. Beneath, a further image shows associations between individual Douglas fir studied during the research.

 

Showing how individual Douglas fir were linked to other individuals, the coloured circles vary in size depending upon tree DBH and colour (from yellow [young] up to dark green [old]) depending upon age. Thicker lines between individuals shows a greater degree ot connection, associated with how many different ectomycorrhizal genets linked them.

 

What we need to be aware of here is that this study was done over a tiny fragment of Douglas fir forest, and therefore if the associations were extrapolated out over an entire landscape, the connected nature of individuals to others would be absolutely incredible. Not only this, but because two trees were found to be connected at over 40m away, it highlights how the above-ground isolation of individuals in a stand masks the intricately-connected nature of the individuals beneath. Thus, we must really see a forest as a network, in place of individual trees.

 

The fact that older individuals were found to have many more connections, on average, than younger ones, also highlights the criticality of retaining older specimens in a stand – if only for the benefit of safeguarding ectomycorrhizal networks that aid with younger specimens obtaining required resources for their growth. However, we must also recognise that the mycelial networks of the two Rhizopogon species studied benefit hugely from the older trees, and retaining them is also of benefit to their existence. Targeted felling of large individuals, therefore, could wreak havoc (and rather quickly) upon the entire system, as the stand’s resilience is built upon these (and related) ectomycorrhizal networks that have established and persisted for a long time.

 

Even if, as the authors suggest, a connection does not provide the young tree with resources, it will still benefit from connecting with a well-established ecotmycorrhizal genet that is itself healthy and fully-functioning as a result of obtaining its carbon from upper-canopy, mature Douglas fir. It does not pay to be isolated from the crowd.

 

Source: Beiler, K., Durall, D., Simard, S., Maxwell, S., & Kretzer, A. (2010) Architecture of the wood‐wide web: Rhizopogon spp. genets link multiple Douglas‐fir cohorts. New Phytologist. 185 (2). p543-553.

[Kveldssanger]

25/02/16. Fact #160.

 

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).

 

References

 

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.

 

Boddy, L. & Rayner, A.. (1983) Origins of decay in living deciduous trees: the role of moisture content and a re-appraisal of the expanded concept of tree decay. New Phytologist. 94 (4). p623-641.

 

Fox, R. (ed.) (2000) Armillaria Root Rot: Biology and Control of Honey Fungus. UK: Intercept.

 

Garrett, S. (1970) Pathogenic Root-Infecting Fungi. USA: Cambridge University Press.

 

Lonsdale, D. (1999) Principles of Tree Hazard Assessment and Management (Research for Amenity Trees 7). London: HMSO.

 

Rayner, A. (1993) New avenues for understanding processes of tree decay. Arboricultural Journal. 17 (2). p171-189.

 

Rayner, A. & Boddy, L. (1988) Fungal Decomposition of Wood: It’s Ecology and Biology. UK: John Wiley & Sons.

 

Schwarze, F., Engels, J., & Mattheck, C. (2000) Fungal Strategies of Wood Decay in Trees. UK: Springer.

 

Shigo, A. (1986) A New Tree Biology. USA: Shigo and Trees Associates.

 

Wargo, P. & Shaw, C. (1985) Armillaria root rot: the puzzle is being solved. Plant Disease. 69 (10). 826-832.

[Kveldssanger]

26/02/16. Fact #161.

 

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.