(Arboricultural-styled) 'Fact of the Day'

[Kveldssanger]

05/03/15. Fact #167.

 

From the popularity that was associated with my post on wind-related tree deaths in the USA, I thought I’d build slightly on how wind can create hazards by going over a really interesting article in the book Tree Structure and Mechanics Conference Proceedings: How Trees Stand Up and Fall Down. The article looks at how, over the period of 1992 to 1999 in Rochester, New York, wind gust speed impacted upon the rate of street tree branch failures. Because data relating to such failures was held electronically, and old climatic data is ‘easily’ obtainable, the methodology was very simple and solely involved relating branch failure date to the wind speed in that area at the same time. The study is also not lengthy and was in fact created on the back of this study, but the data contained within is absolutely brilliant and so it’ll certainly be of interest to many reading this.

 

As a means of setting the scene so to speak, the authors first establish two things: (1) when branches failed, and (2) when wind gusts were deemed significant (in this case, over 40mph for over 5 seconds).

 

In relation to the first point (see the below graph), it was found that branch failures were most frequent during the months when foliage was present upon the trees (May to October / November) – specifically, July, August and September (even including the 243 failures in strong winds in September 1998) had a very high number of branch failures in comparison to other months. In a way, this is to be expected, as full leaf crowns will drastically increase wind resistance of a crown. Branches will therefore be loaded not only by greater weight from the leaves they are supporting, but the leaves they are supporting will have a large surface area that can readily catch in the wind (which will further increase the loading upon the branch, pushing it closer – or beyond – it’s safety factor). Furthermore, if it’s raining, then the water caught by the leaves will yet further increase the weight the branch will bear.

 

The number of branch failures, per month, in Rochester. Note that 243 failures occurred during September 1998, as a result of strong winds.

 

However, when looking at when the strongest wind gusts were, we can observe something peculiar – the frequency of such gusts is almost an inverse of the rate of branch failures across the calendar months (see below). This suggests that it is not the frequency of strong wind gusts that correlates with branch failure, but simply the occurrence of such a wind gust.

 

The number of wind gusts of 5 seconds or greater, and how strong such gusts were (see legend key).

 

From here, the authors then look to segment the total number of branch failures into categories relating to wind speed. As somewhat expected, there is an exponential increase that begins at around 40mph, and drastically starts to rise at 60mph. The lack of 70-79mph data is because no such gusts occurred during the survey period. The data is shown below.

 

The number of branches failures in Rochester’s street tree population, and at what wind speed the branches were overloaded at.

 

Pulling apart this data further, the authors sought to compare at what wind speeds branches failed at, during both the period in which foliage was present, and when foliage was not present. Results for the branch failures when foliage was present (shown below) are interesting, in the sense that it appears to be that wind gusts alone are not the cause of failure (as noted by the high spike of branch failures at 43mph). Of course, there is marked correlation between wind gust speed and branch failure rate. The authors in fact suggest that the effect of the observed wind gusts on branches may be exacerbated by simultaneous (or prior) precipitation, drought (where wood rays become very dry), and previous stronger and more sustained gusts. Therefore, calmer winds in ‘right’ conditions may at times more readily cause branch failure than when stronger wind gusts occur and such aforementioned influencing conditions are not evident.

 

Branch failure frequency during the time when foliage was still present upon the tree, in relation to the wind gusts that caused such failure.

 

Diverting attention to the leafless period, there seems to be little correlation between branch failure and wind gust speed (as shown below). Perhaps, other climatic faactors (snow, ice, etc), in conjunction with wind loading, will induce failure in such instances; and in times where snowfall or ice accumulation is significant, branches may certainly fail under such extreme loads.

 

Gusts that induced branch failure during the leafless period.

 

So what can we gain from this study? Hopefully, a lot. Admittedly, the sample size was limited (only eight years and in one city), though it goes to show how, at least for broadleaved trees, wind gusts during the summer will indeed be more likely to cause structural failure. Without question, we must not ignore other loading factors that may combine with wind, during the growing season (most probably precipitation). During the winter (leafless period), it seems wind gusts alone are far less likely to correlate to any degree with branch failures, and instead wind gusts may be significantly aided, through the presence of snow or ice, if branch failure does occur. Failure may even occur independently of wind gusts.

 

Source: Luley, C., Pleninger, A., & Sisinni, S. (2002) The effect of wind gusts on branch failures in the city of Rochester, New York, U.S. In Smiley, E. & Coder, K. (eds.). Tree Structure and Mechanics Conference Proceedings: How Trees Stand Up and Fall Down. USA: International Society of Arboriculture.

[Kveldssanger]

07/03/16. Fact #168.

 

Unless the substrate on which a seed resides is preferable, the germination and successful recruitment of the seedling will likely fail. Compiled with the fact that different species’ seeds require different substrates and conditions to germinate and establish, recognising exactly what constitutes preferable is important. For smaller-sized seeds (such as birch), which will lack nutrients that are internally available with larger seeds (such as acorns), it is typically accepted that a more nutrient-rich and external substrate is necessary. Whilst this nutrient-rich substrate may very well be the soil itself, it may even more often not be the soil but decaying deadwood that, courtesy of decomposers (bacteria, fungi, insects), has had its locked-away nutrient mineralised (made freely available for uptake by other organisms). In this sense, for smaller-sized seeds, a change in seedbed (substrate) and microsite conditions may very likely signal a change in the rate at which such seeds germinate.

 

The focus of this post is a study that invesitgated how different seedbed conditions impacted upon the rate of seedling recruitment, performance, and morphology of Betula alleghaniensis (yellow birch). The study was undertaken in 2010 within Quebec, Canada, which is part of the species’ native range, and the seedbeds were all located in forest stands dominated by Acer saccharum (sugar maple). As a general rule of thumb, yellow birch prefers nutrient-rich substrates (notably decaying deadwood, though also soil) that have good light availability and are largely leaf litter-free; the same can most probably be said for many other species of the genus Betula, too. Therefore, the main aim of the study was to ascertain whether there was a difference in suitability between these preferable substrates available in such an area of the species’ native range.

 

Within the overall study location, four areas were selected. All had ‘recently’ been harvested with a selective cutting method, with two areas having been cut in 2004 (new cut) and another two having been cut in 1994 and 1995 (old cut). Sugar maple was the most dominant tree species on all sites, with yellow birch also featuring prominently. Other tree species were present, including (but not exclusively) Acer rubrum (red maple), American beech (Fagus grandiflora), and Thuja occidentalis (eastern white cedar). In each area, yellow birch presence (a total of 1,015 individuals) within the understorey was measured, and all that ranged from 15-330cm in height were recorded. Where each yellow birch was recorded, the seedbed (mineral soils, mosses, deadwood, moss-covered deadwood) and microsite types (pits, stumps, fallen trunks, etc) were also noted, as was the level of canopy ‘openness’. Where a seedling was found upon deadwood, the level of decay (from 1-5) was also recorded, as was from what species the deadwood came. To ascertain performance and morphology, a total of 274 of the yellow birch were also removed from the site, and assessed for their growth increment, their number of branches, and their age. These individuals were also dissected into their roots, stem, branches, and leaves, and each part was weighed after being dried to measure dry-weight biomass. Roots were also measured and segmented into categories associated with their diameter.

 

A view of the understorey of a stand of predominantly sugar maple and yellow birch in Quebec, Canada. Source: Wikimedia Commons.

 

Explanations relating to the methodology now over, we can begin to assess the results.

 

Firstly, it was found that seedbed and microsite types were different between the old and newly cut stands. From this, we can perhaps recognise that harvesting of a site can have an effect upon recruitment of yellow birch seedlings. Looking more into this this difference, the authors found that moss-covered deadwood was the principal substrate upon which yellow birch seedling would grow (46% and 29% of all seedlings in old and newly-cut stands, respectively), though mineral soils would support more seedlings in newly-cut stands (at 41% of all seedlings found, compared to 34% in older cuts) – probably because of less deadwood being available. Curiously, deadwood lacking moss was less often used as a substrate, with only around 20% of seedlings in each stand type growing upon it (below, the upper graph demonstrates these differences). In terms of what microsite was preferred, skid trails were most frequently found to accomodate seedlings in newly-cut stands, and in older stands skid trails were second-best to stumps (below, the lower graph demonstrates microsite differences). However, across both site types, skid trails supported almost equal amounts of seedlings (of which most grew directly within the soil), as did trunks. Perhaps, the suitability of skid trails is clear, as they may very well generally have higher light levels reaching the forest undercanopy.

 

How the seedbed type (DW – deadwood; MDW – moss-covered deadwood; MS – mineral soil) impacted upon seedling recruitment of yellow birch in newly-cut stands, and those cut some time ago in 1994 and 1995.

 

How different microsite types supported different amounts of yellow birch (PM – pits and mounds; S – stump; ST – skid trail; T – trunk; WD – woody debris).

 

In terms of the age distribution of seedlings across both stands, seedlings in the stands cut in 1994 and 1995 (15-16 years prior to the study) were, on average, 12.6 years old, and for newly-cut stands (cut six years prior to the study) they were 9 years old on average. This in itself is interesting, as it suggests some of the seedlings in the newly-cut stands pre-existed the change in site conditions when harvesting took place (though, of course, where seedlings were found in skid trails they surely could not have pre-existed harvesting). As for the average height of seedlings and how open the canopy was above their location, no marked differences were found between both stand types, though mineral soil did support seedlings of greater height (on average) when compared to deadwood and moss-covered deadwood.

 

Focussing on those individuals found on deadwood, over 60% were found growing on conifer logs. Because the dominant species in the forest stands were not conifers, this suggests that coniferous species occupying a smaller total population of no more than 20% of the total amount of trees (including eastern white cedar) more markedly supported the regeneration of a species dominant in the stand (yellow birch). This alone is important for forest managers and conservationists. The inverse was actually found with sugar maple, where its deadwood only supported around 10% of all seedlings – this is in spite of it occupying 50% of total the total number of individuals in the stand. Away from the species of deadwood, more heavily-decayed deadwood was found to support more yellow birch seedlings (see below graph).

 

How different decay classes of deadwood (1 being the lowest) supported different numbers of yellow birch seedlings.

 

Looking at morphological traits of the seedlings, it was found that those seedlings growing within the soil had the greatest growth increments. However, those growing upon deadwood (both covered and not covered in moss) had a higher fine mass root ratio, meaning that their root systems were more developed than those growing in mineral soil of an identical height. Not only this, but leaf area ratio (total leaf area compared to overall seedling dry mass) was highest in seedlings growing on moss-covered deadwood. This suggests that, whilst height growth is reduced, the seedlings have better root systems and more leaf area (aiding in resource uptake and photosythesis, respectively), and may therefore be in a better ‘condition’. After all, height is not the only determinant in the quality of a seedling.

 

From these results, what can we conclude? Principally, the benefits of heavily-decayed deadwood cannot go ignored. Evidently, deadwood, and particularly moss-covered deadwood (which will typically be more decayed, anyway – higher moisture levels), is critical for the recruitment of yellow birch, and even in spite of it occupying only small to moderate levels of the forest floor. This is likely because decayed deadwood is more readily penetrable by roots, has more nutrients available for uptake, reduced competitin for resources with other plants, and retains higher levels of moisture which seedlings require. In this sense, much akin to how the less plentiful coniferous deadwood is more important for yellow birch recruitment than broadleaved deciduous deadwood (probably because coniferous deadwood decays more slowly, thereby providing a viable substrate for a longer period of time, which enables a seedling to develop a better rooting system and anchor into the mineral soils beneath), deadwood is more important than the more spatially abundant mineral soils. This clearly demonstrates how more ‘niche’ substrates need to be conserved in forest ecosystems, in an attempot to retain ecological integrity. Just because something is found in the greatest abundance does not mean it is the most important thing for the ecosystem – perhaps, even the opposite!

 

Here, a yellow birch seedling that started its life growing on the now heavily-decayed stump, has developed ‘stilts’ from where the roots anchored into the mineral soil beneath and the stump then rotted away. Source: Dr John’s Blog.

 

In terms of whether harvesting impacts upon seedling recruitment – yes, and no. Of course, the presence of seedlings growing upon stumps was notable (particularly in older stands, where stumps were of course likely to be more significantly decayed), and such stumps would not exist to any marked degree if harvesting operations did not occur. Granted, one must recognise that stumps may have been more routinely used as a substrate because trunks were likely less available, and therefore seedlings may only be using what is available to them. That aside, the authors comment that yellow birch may be less reliant upon harvesting to free-up the canopy and allow more light to penetrate through, and instead grow successfully and retain a constant seedling cohort if suitable substrates and microsites persist (largely, deadwood that is well-decayed – canopy openness seems not to be highly significant). This is because many seedlings in the newly-harvested stands pre-existed the harvesting date (asides from those growing in skid trails, directly within the soil), suggesting it wasn’t the harvest operations that most actively facilitated seedling recruitment levels.

 

Therefore, the benefits of (well-decayed) deadwood are yet further accentuated. As if there wasn’t already enough justification to retain deadwood, we can now add this study to the ever-expanding arsenal that is amassing at the gates of the forest manager. Without such deadwood accumulation (and notably of coniferous species), the recruitment and performance of yellow birch seedlings may begin to suffer. In time, the entire composition of the forest may begin to alter, if such a suitable substrate becomes lacking. Without doubt, one must allow for a stand to succeed at all times, though there is also a need to conserve and manage with responsibility. For yellow birch, reducing the presence of coniferous species within the stand and not allowing deadwood to accumulate and decay in situ would be an ecological travesty.

 

Source: Lambert, J., Ameztegui, A., Delagrange, S., & Messier, C. (2015) Birch and conifer deadwood favour early establishment and shade tolerance in yellow birch juveniles growing in sugar maple dominated stands. Canadian Journal of Forest Research. 46 (1). p114-121.

[Kveldssanger]

Really good study, that! Right up my street, with its implications for conservation. If you read any of this thread, read the one above!

[Kveldssanger]

08/03/16. Fact #169.

 

Trees located within the urban environment are, by default, more of a risk to public and property. This is because beneath many such trees are permanent (or more frequently temporary) target zones, which would not necessarily be found to such a degree in a rural setting. Having said that however, many urban trees have a very low level of risk, and are thus deemed to hold an acceptable level of risk. Granted, some urban trees present more of a risk, and the manifestation of a greater level of risk can occur through a variety of different means. One significant contributing factor to the increaed risk of an urban tree, and which is the focus of this post, is fungal decay. Where trees have been wounded, are stressed, or may even be largely in good condition, fungi can enter into and establish within the structure of a tree, break down the lignin and / or (hemi-)cellulose within the wood, and thereby reduce the tree’s structural integrity. In turn, the associated hazards increase. Therefore, understanding more about fungal decay is important, and the authors of the study discussed below pursue a greater undestanding of fungi in our urban trees.

 

By using some of the trees in Helsinki, Finland, the authors sought to gain more of an understanding about the abundance and patterns of decay of certain common wood-decay fungi found in the city. In total, 194 trees (76 of the genus Tilia, 60 of the genus Acer, and 58 of the genus Betula) in both parks and streets were assessed within the study, of which all were showing signs of health decline, or were outwardly hazardous. During the inspections to determine that such trees were indeed hazardous or in decline, information was gathered in relation to the external symptoms (fungal brackets, cracks, cavities, etc) and their exact locations (including height upon the structure) upon the trees. Subsequently, all were felled over a period of time between 2001-2004, and from there the research began into assessing what fungal species caused the decay (at times, DNA sequencing was needed, and notably for differentiating Ganoderma species from one another), and how extensive horizontal decay was within the tree’s structure (by taking cross-sections around areas of dysfunction).

 

Following on from the investigations post felling, the authors identified 13 fungal species and genera within the host Acer spp., Betula spp., and Tilia spp. Pholiota sp. was most frequently found to have colonised all three tree genera, and often other fungal species were found alongside. Other fungal species, such as Piptoporus betulinus, were only found upon trees of the Betula genus, though of course the host preferences (not all fungi are generalists) of different fungal species will determine, from the limited range of tree species assessed, what fungi could possibly be present on each of the three genera, and how often (Rigidoporus populinus, for example, was predominantly – but not exclusively – found upon Acer spp.).

 

Interestingly, Piptoporus betulinus was also never found alone within its host, which therefore suggests other fungi will also have contributed to decay within the Betula spp. featuring in this study (this may align with its strategy of specialised opportunism, where the fungal spores wait for the host Betula sp. to become stressed before attacking). In fact, the only three fungal species isolated from the trees of this study that occurred alone around 50% of the time were Ganoderma applanatum (syn: G. lipsiense), Hypholoma sp., and Phellinus igniarius. This perhaps suggests they they are either far more competitive and defend their ‘patch’ aggressively, they enter into trees and take advantage of substrate conditions other fungi cannot similarly exploit, or succeed into substrates other fungi had already decayed (and then exited). Below, the table (pardon the small size) outlines all fungal species isolated, and from which hosts.

 

For a better view of this table, please visit the journal article itself (linked at the bottom). Armillaria spp. does not include A. mellea nad A. ostoyae, but instead mainly A. cepistipes and A. borealis (both weak pathogens, which are usually saprophytes).

 

Additionally, only certain fungal species could actually be readily identified by their sporophores (including Rigidoporus populinus and Ganoderma applanatum), because not all fungi had produced sporophores at the time of inspection and felling (and if they did, they may have been few and far between – Hypholomoa sp. never produced fruiting bodies, and no symptoms were externally evident to suggest it was there). However, the authors do note that sporophores were found on the trees where decay was most extensive in the radial direction, and this may indeed make sense when one considers that fungi will not produce a sporophore (an exit strategy) unless there is a need to do so (such as running out, locally, of substrate – indicating wider radial spread in the direction of the sporophore). Not only this, but even where fungal sporophores were not evident, crown dieback could be observed as a result of decay by Piptopirus betulinus and Inonotus obliquus, for example. The table beneath further outlines on how some fungi, whilst present in the tree (either outwardly or via laboratory analysis), did not cause crown symptoms to show. For many fungal species, the crown symptom rate is actually very low.

 

A break-down of how different species of fungi could (or could not) be observed within a tree.

 

In relation to the extent of decay, most trees were either hollow (57%) or were significantly decayed (35%), and particular fungal species were only found in hollowed trees (including Armillaria spp., Phellinus igniarius, and Pholiota spp.). Conversely, species such as Ustulina deusta (syn. K. deusta) and Ganoderma applanatum were found only where the tree was in advanced stages of decay, and variation existed between how frequent their presence was (the latter was more routinely found as a primary decay agent, whereas the former was not found in discoloured wood but only where wood was already very decayed). Of the 8% of trees where wood was only discoloured, Piptoporus betulinus and Ganoderma applanatum were two examples of fungal species that could be found. From this data, one can recognise how different fungi will occupy different stages of the decay process, and therefore a succession of sorts may perhaps occur amongst individual species.

 

Building upon the above, the authors also recognised how different fungal species would create different radial decay patterns (see the below table). Species including Cerrena unicolor and Ganoderma applanatum were observed to more readily create decay cross-sections of greater radial spread, for instance (and also invade the vascular cambium). However, most species were found to extend out to across more than half of the cross-sectional area of a tree’s main stem, which means the critical t/R value of 0.3 (Mattheck’s hollow tree failure theory) is quite significantly encroached upon, and for 5-6 of the fungal species here, surpassed. In terms of the location of decay, it was found that Ganoderma applanatum was usually found in the lowest 1m of the main stem, which means entire tree failure is very possible when decay is very extensive. Other fungal species were found in different locations, including branch forks (Rigidoporus populinus). In such an instance, decay may only induce failure further up the tree’s structure.

 

Comparing different fungal species observed and how far out (radially) they were observed to go, at a cross-section.

 

Looking further at the table above, a few additional observations can be made. Firstly, as even stated by the authors, Pholiota spp. may be pathogenic within the rooting system of a tree, and because this study only assessed the above-ground structure, exact extents of decay within host trees may not have been fully understood. Armillaria spp. is also a root pathogen, and the same comments apply to that genus as well (thought A. mellea and A. ostoyae were not isolated in this study, so Armillaria spp. may be under-represented for severity). However, in general, the table does demonstrate that fungi may very well facilitate failure in the stem or branching area, and particularly for those that extend over the 70% threshold – Ganoderma applanatum may very well cause basal failure, whilst Rigidoporus populinus may cause failure in the lower crown (at a branch junction).

 

In recognising all of this, we can begin to appreciate how fungal strategies (some are notably pathogenic, whilst others are not so) have significant impacts upon how we manage urban trees, and therefore we must understand specific species’ strategies and cater management to meet the ‘needs’ of the tree. There is little use in not discriminating between different fungi, as some (Cerrena unocolor, Ganoderma applanatum, and Inonotus obliquus) certainly have the potential to be more hazardous than others (Pleurotus spp.). Additionally, as some fungal species (Hypholoma spp.) will more typically (or exclusively) succeed into trees already extensively decayed or hollowed (see the below table), recognising their presence may be important for understanding what condition the host may be in. Granted, this is likely common knowledge, though it is still important to recognise this over and over again, as it really is a main crux of tree management for safety reasons in urban areas.

 

It is necessary to note, however, that only three tree genera were studied in this situaton. Therefore, caution should be exercised in forming absolute conclusions, as many urban locations consist of tree species and genera over and above what was assessed in this case. Nonetheless, credit where credit is due – a fantastic study!

 

How different fungal species occupied trees at different levels of decay, indicating how identification of their presence may help detail the condition the tree is in.

 

Source: Terho, M. & Hallaksela, A. (2007) Decay characteristics of hazardous Tilia, Betula, and Acer trees felled by municipal urban tree managers in the Helsinki City Area. Forest Pathology. 37. p420-432.

[David Humphries]

An interesting read

 

 

 

.

[Kveldssanger]

Indeed! Hopefully of use.

 

Correction to above post:

...species such as Ustulina deusta (syn. K. deusta) and Ganoderma applanatum (the former, exclusively) were found where the tree was in advanced stages of decay...

[wmuller]

08/03/16. Fact #169.

 

Source: Terho, M. & Hallaksela, A. (2007) Decay characteristics of hazardous Tilia, Betula, and Acer trees felled by municipal urban tree managers in the Helsinki City Area. Forest Pathology. 37. p420-432.

 

This link is not working, do you other haven working link? Thank you.

[Kveldssanger]

Odd! The link works for me. Try accessing it from here.

 

https://scholar.google.co.uk/scholar?q=+Occurrence+and+decay+patterns+of+common+wood-decay+fungi+in+hazardous+trees+felled+in+the+Helsinki+City&btnG=&hl=en&as_sdt=0%2C5

[Kveldssanger]

10/03/16. Fact #170.

 

Following on from the mass mortality of elms across the UK’s landscape in recent decades (and also an earlier spate of the disease during the first half of the last century), there has been a huge desire to breed resistant stock, in an attempt to have elms grace the landscape once more. However, such a pursuit has been very slow in terms of substantial progress, and it is only in more recent years that more significant developments have begun to manifest in breeding resistance against Ophiostoma novo-ulmi. In fact, not all of these advancements have been in the UK – research across Europe and the US has really helped shape the way forward for the elm, given the disease was responsible for the loss of elms across both continents (Lamb, 1979; Venturas et al., 2014).

 

Because resistance to Ophiostoma novo-ulmi (DED) is considered to be largely polygenic (qualitative) – in the sense that resistance does not follow a major gene pattern, but instead by the effective production of phenolic compounds and signalling metabolites (compartmentalisation processes), xylem morphology (length, width), and so on – breeding resistant stock is automatically very difficult (Aoun et al., 2009; Ďurkovič et al., 2014; Martín et al., 2013a; Martín et al., 2013b). Therefore, for resistance to be identified, understanding what genetic markers to look for is necessary, and research is ongoing in this regard (Perdiguero et al., 2015). However, because the main manner in which polygenic resistance is cultivated is through vegetative propagation, there is huge risk of a future strain of Ophiostoma novo-ulmi, or even another pathogen, creating the same problem that arose over the last few decades with regards to elm mortality (Martín et al., 2013b). A small genetic pool across many individuals is not to be desired, and can spell disaster very quickly – as was the case for Ulmus minor, which had a history of vegetative propagation across the English landscape and suffered great losses as a result of DED.

 

With regards to identifying resistant individuals therefore, research has indeed been completed via the growing of cuttings from different mother trees for a variety of species (including Ulmus glabra, Ulmus laevis, Ulmus minor, and Ulmus pumila), and then inoculating the cuttings with Ophiostoma novo-ulmi (Solla et al., 2005). Results have shown that some cuttings of particular species will display complete resistance to the pathogen, though many cuttings of other species will not. The reason behind evident resistance is largely considered to be down to xylem width and length, with individuals that possess longer and broader vessels showing greater susceptibility to the pathogen (Pouzoulet et al., 2014). This may not exclusively be in the wood structure either, as leaf xylem structure also impacts upon resistance – ‘Dodoens’ is marked as a potentially important cultivar, in this regard (Ďurkovič et al., 2014).

 

An Ulmus ‘Dodoens’ at Royal Botanic Gardens, Kew, taken in 2012. Source: Davis Landscape Architecture.

 

To advance this displayed resilience of some cuttings, hybridisation projects have been undertaken that may even see resistant European elm individuals crossed with resistant Asian ones – this broadens the genetic resources an individual will have access to (Brunet et al., 2013; Santini et al., 2012; Solla et al., 2014). If such resultant individuals display resistance in the laboratory, then they will be propagated vegetatively and then be subjected to further testing in a more naturalised environment (Santini et al., 2010). Such a means of testing for resistance has produced some clones that may potentially be used in an ornamental setting or forestry setting, including ‘Ademuz’, ‘Majadahonda’, and ‘Morfeo’ (Martín et al., 2015). It is, at this point, important to recognise that the resistance of different clones may in fact vary across regions of the world, and that cultivars will also differ in their response to the pathogen – some will initially display low resistance but soon recover and exhibit few signs the following year, and some perhaps vice versa (Buiteveld et al., 2015). Furthermore, current research has only really focussed on trailing resistance in young specimens. Little evidence is available to show how these new cultivars will fare in the longer term (such as over many decades).

 

Concerningly, research by Hodgetts et al. (2015) in the UK found that freshly-imported ‘Morfeo’ clones were host to Candidatus phytoplasma ulmi, which is a pathogen that is controlled in the UK and therefore requires infected stock to be destroyed under a Plant Health Notice. Despite this, many older and pre-existing clones were not found to be host to the pathogen. Therefore, risk may also exist with regards to the movement of cultivars, which evidently may harbour exotic pathogens that may threaten the UK’s tree species. On a similar note, Ulmus americana ‘Princeton’ (a DED-resistant cultivar) has been discontinued by some nurseries (including Barcham) because of its high susceptibility to Candidatus phytoplasma ulmi.

 

Very recent research has also identified specific genetic markers in Ulmus minor that may suggest resistance (Perdiguero et al., 2015). Such identification, the authors allege, may aid significantly with the quest in finding disease-resistant cultivars, and such markers may also be transferable across species. At the same time, the genomics of the pathogen Ophiostoma novo-ulmi, now fully mapped, is paving the way for innovative research that seeks to identify specific markers that identify pathogenicity (Bernier et al., 2015). Research into the pathogen itself may therefore yield beneficial results in understanding how to breed for resistance, though only time will tell in this regard.

 

Even in spite of the cultivation of many individuals that display levels of resistance to DED, the fact that vegetative propagation is the main means of continuing to provide resistant elms means there is huge risk of elm populations lacking genetic diversity. Such populations are fragile, and can readily be wiped-out by a pathogen in a very quick period of time. Of course, looking to re-introduce elm to the landscape through means of cultivation is a noble pursuit, particularly when man was the main cause of the second DED outbreak in the UK, though it is perhaps naive to think that a similar thing could not happen again – and by planting clones, a similar mass-mortality event has a much higher likelihood of occurring. Hybridisation of elms is therefore potentially a way forward, in place of cloning. However, then we run the risk of a loss of true native progeny, and crossing species that would never otherwise have the ability to cross may be ethically obstructive for some.

 

Pessimism aside, understanding what drives resistance is important, and DED has triggered a great deal of research into finding resistant elms. The benefits of such research – very importantly – does not stop with elms, as research techniques can be replicated for other areas of disease research. Technological advancements and continued investigatory work into DED will therefore continue to yield good results, though potentially with greater magnitude. Judging by current research, finding truly resistant cultivars is indeed a possibility, and planting a large mix of them when they do arise may be the best way forward.

 

References

 

Aoun, M., Rioux, D., Simard, M., & Bernier, L. (2009) Fungal colonization and host defense reactions in Ulmus americana callus cultures inoculated with Ophiostoma novo-ulmi. Phytopathology. 99 (6). p642-650.

 

Bernier, L., Aoun, M., Bouvet, G., Comeau, A., Dufour, J., Naruzawa, E., Nigg, M., & Plourde, K. (2015) Genomics of the Dutch elm disease pathosystem: are we there yet?. iForest-Biogeosciences and Forestry. 8 (2). p149-157.

 

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[sean]

Interesting stuff. That's some research, good to see.

 

In response to the Fungi colonisation of urban trees