Literally hanging on by the bark of the roots

Root architecture is often not appreciated, by sheer virtue of the fact roots cannot be seen readily. Ground-penetrating radar can obviously allow us to interpret, with the assistance of a tomogram, what the root structure is expected to look like, though it is only when trees are up-rooted, or have some of their roots exposed, can we really begin to appreciate the durability, versalitity, and character of them. Below are some great examples of exposed root plates, courtesy of bank erosion (be it by a river, suspected quarrying, animal disturbance, or other means), from Ickworth Park, Suffolk, UK.

tree roots exposed 1
A huge oak tree perched precariously upon a mound, complete with young lambs. Note its exposed roots. Looks quite severe, no…?
tree roots exposed 2
Well here’s the other side! How is this thing still standing, and with a very full crown? Madness! A testament to the tree, no doubt. Sheer love sheltering underneath the root plate, though if any more soil is lost this tree may topple.
tree roots exposed 3
Looking up slightly gives us the view a lamb may have of the tree. On the other hand, it gives us the view of a person crouching down to take a photo…!
tree roots exposed 4
This image gives us a sense of scale of the level of root plate exposure. That gentleman in the blue jacket was at least 6ft tall.
tree roots exposed 5
A look on the up-hill side.
tree roots exposed 6
And another. Note the nettle growing from within one of the roots. A mini niche ecosystem, of sorts.
tree roots exposed 7
A lofty beech tree sits next to a river…
tree roots exposed 8
…and could probably get arrested for indecent exposure.
tree roots exposed 9
This veteran oak sits on the outside edge of a meander. In time, the bank beneath has eroded, and if this erosion isn’t halted the oak will fall.
tree roots exposed 10
From this slightly ‘aback’ angle, we can appreciate just how serious the situation is.
tree roots exposed 11
Holding on for dear life!
tree roots exposed 12
The crown is certainly not in great condition. A lack of vigour may worsen its prospects for clutching on to life.
Literally hanging on by the bark of the roots

Lambs love trees, too

Only a quick post today as I’m absolutely shattered after walking around Ickworth Park for around eight hours with the Ancient Tree Forum. The park is currently being grazed with sheep, and lambing season is in full swing. The little ones have taken a liking to the base of large lapsed oak pollards, to shelter from the generally dreary weather of early April (as the old ‘April showers’ saying goes). Be it a basal cavity, or just a gap between two buttress roots, nothing is off bounds for these little lambs. Of course, this is actually bad for the tree, as the high amount of lamb excrement nitrifies the soil quite significantly, and there is also some compaction damage and bark damage as a result of the presence of the lambs (and sheep), too. Of course, for the lambs, the relationship is wholly beneficial.

trees and lambs 1
Two lambs drawn to the base of a very large oak tree.
trees and lambs 2
A closer look shows how the lamb on the right is sitting within a gap between two buttresses.
trees and lambs 3
These lambs are drawn to the fallen deadwood. The high abundance of nettles around the tree suggests a lot of historic grazing activity.
trees and lambs 4
A lamb sits within a large cavity at the base of a veteran oak. Again, note the nettles.
trees and lambs 5
Slightly closer, in this image.
trees and lambs 6
“Oi mate, sod off from my tree, okay!?”
Lambs love trees, too

Getting to a woodland – who and when?

In an age where life is rather tumultuous and detached from nature, many individuals may opt to visit woodlands and forests as a means of escaping from the frenetic day-to-day living of an urban environment. In fact, alongside the rise in urban populations there has been a rise in the number of individuals using woodlands and other green areas, and such a growing desire to visit has led to these sites becoming highly-used and thus pressured. Sites nearby to urban fringes, and those that are readily accessible via transport means (cars, trains, etc) may be most readily used, given that there is only so much time in a day and many individuals will only visit such sites for a few hours (oft during fair weather) before returning home.

To probe further into exactly what drives woodland visitation by individuals, we can look at a new article published in the journal Urban Forestry & Urban Greening. This specific study looks at woodland sites in Wallonia, Belgium, and delves into who visits woodland sites (demographics), and how they get there (transport). The region of Wallonia is covered by woodlands (around 30% of total land cover), and these woodlands are found quite far from any major cities (75% of woodlands are located in rural locations). A total of 40 sites were used for the study, and 14% of these were in peri-urban or urban areas. These (peri-)urban sites accounted for nearly 50% of all surveyed participants. Results from the questionnaire are shown below.

woodland visitation people
Who visits Belgian woodlands, and how do they get there?

From the above data, we can spot some very interesting things. For instance, many more people visit woodlands during the weekend, suggesting that the visits are undertaken outside of typical working times. Given the average age for visitors to the woodland sites is far below the retirement age, we can safely assume that this is the reason for the preference of weekend visits. This assumption is further supported by the fact that 68% of all woodland visitors are currently employed. We can also observe how the fewest number of visitors visit the woodlands during autumn, which is actually somewhat curious as autumn colours are absolutely stunning. It is arguably not at all strange, however, that the spring months attract the greatest number of visitors, though winter-time visits for walkers is actually higher (by a few tenths of a percent).

In terms of how people get there and who with, we can see that there’s generally a mix of sole individuals, families, couples, and groups who frequent the sites, though this is not really where it’s particularly interesting. Instead, it’s when we look at how they get there that it gets quite cool. For example, we can observe how sole individuals will access the woodland by foot or via bicycle, which is likely because of the ease of access via such means. Driving a car is probably not so enticing, as there’s less of a push for time and fewer people to suit on the trip, so it’s not as if once a child gets bored and starts throwing a tantrum that the entire family has to go home – there’s more freedom involved with accessing the woodland. We can also spot how a good third of families will cycle to a woodland, indicating that there is a desire for a family outing to coincide with a sort of ‘keeping fit’ mentality. Granted, cycling may generally be more feasible for families with slightly older children, as toddlers, whilst able to cycle, cannot cycle too far, and nor is it probably practical to have a young child cycle to a woodland unless it’s very close by (which isn’t usually the case, in this region of Belgium).

There is a general trend of people going to woodlands to relax, as well. Unsurprisngly, this markedly changes for those who cycle, where it shows that over 95% of those who get to the woodland via bicycle will go there for a sporting purpose. This adds weighting to the idea that families who cycle there have older children (or even a family comprised of only older generations). Such a disparity in results for those who cycle is however surprising, as it suggests only 2% of those who cycle to a woodland do so in order to relax. Perhaps there is no such thing as a leisurely bike ride, therefore?

The male:female ratio of woodland visitors is also quite distinctly different. According to this study, a good two-thirds of visitors are male. This ratio stayed true for the means of access to the site being on foot or via car, though rose sharply to 88% in favour of males if access to the woodland site was via bicycle. Maybe this has something to do with the high use of woodland sites for sporting, when the visitor arrived via bicycle. On a broader scale however, the reason for fewer females visiting is curious. Perhaps it has something to do with females feeling more wary of being alone in woodlands, or instead preferring to visit other places during free time.

woodland biking sport
Maybe cyclists arrive at woodlands in order to race or ride with or against other cyclists. Source: National Trust.

With regards to the distance travelled to get to the woodland, the data provided is not really eyebrow-raising. An average walk of 4km to get to a woodland site is somewhat fair, and it cannot be thought that anything other than a car journey would bring in the highest distance travelled to reach the desired woodland site. Walkers spending the least time (111 minutes) at a woodland is therefore not surprising, as they’ll spend probably the most time either side of their visit getting there and back. Walking will also expend the most effort in travelling (though not necessarily the most calories burned, which likely is held by those who cycle). Despite this, an average stay of 137 minutes (a little over two hours) is quite reasonable, and suggests that visitors seek a more instant impact in place of spending a whole day on site. Of course, the size of the woodland would have an impact, in this regard. If you’ve walked an entire woodland in around two hours, you may as well go home, after all. The purpose of the trip also defines the length of stay, and as it as generally the case that people were there to relax (excluding cyclists – though sporting is a form of relaxation or recreation) then dragging-out a stay may end up being stressful, as remaining somewhere only for the sake of remaining there is counter-intuitive.

If you want a more detailed breakdown of this paper, then I advise you locate a copy. I downloaded the entire journal whilst it was temporarily set as Open Access, though it appears to have once again been ‘closed’ (as of yesterday). It’s a good read, and not too heavy on the science-y stuff.

Source: Li, S., Colson, V., Lejeune, P., & Vanwambeke, S. (2016) On the distance travelled for woodland leisure via different transport modes in Wallonia, south Belgium. Urban Forestry & Urban Greening. 15 (1). p123-132.



Getting to a woodland – who and when?

Inonotus hispidus on ash – a very fine example

When family members call you up to say they’ve seen a “weird-looking thing on a tree” that’d probably a fungus of some sort, perhaps it’s too evident that you like searching for wood-decay fungi too much. But then again, why shouldn’t we love them? We need to be able to recognise them, understand what they do, why they do what they do, and what we need to do (if anything) in response to a positive identification. Trees support so many other organisms that, when learning about trees, we must learn about everything else as well.

So anyway, many thanks to the family member that gave me the heads-up on this ash tree last summer, as it’s probably the best example of Inonotus hispidus I have come across on my travels.

Inonotus hispidus Fraxinus excelsior 1
Residing within a mature hedgerow (pre-Elizabethan times), this storm-damaged ash is host to many sporophores of the fungus Inonotus hispidus.
Inonotus hispidus Fraxinus excelsior 2
Here, we see the largest sporophore emanating from behind a bark-covered region of the tree.
Inonotus hispidus Fraxinus excelsior 3
Above, we can see a smaller sporophore.
Inonotus hispidus Fraxinus excelsior 4
More smaller ones can be seen further round still.
Inonotus hispidus Fraxinus excelsior 5
This angle shows most of the sporophores present upon the ash tree, and gives a sense of the vastness of the mycelial mass within the host.
Inonotus hispidus Fraxinus excelsior 6
A side-on shot of the largest sporophore shown in the first image.
Inonotus hispidus Fraxinus excelsior 7
Another one, to round up. Note the very orange colouration of the sporophores, too.
Inonotus hispidus on ash – a very fine example

UK case law relating to dangerous trees

Stemming originally (after the ruling of Rylands v Fletcher [1868] relating to a landowner’s duty of care) from Giles v Walker [1890] and Noble v Harrison [1926], the latter having the judge conclude that “I see no ground for holding that the owner is to become an insurer of nature, or that default is to be imputed to him until it appears, or would appear upon proper inspection, that nature can no longer be relied upon…“, and slightly more recent court cases, beginning with Brown v Harrison [1947] and Lambourn v London Brick Co Ltd [1950], both of which concluded in line with Noble v Harrison [1926], provide background guidance for modern-day court cases. Such modern-day cases also take the lead from the cases of Goldman v Hargrave [1967] and Leakey v National Trust [1980], of which the latter drew influence from the former.

Before visiting more contemporary cases however, further exploration is warranted into older cases. As noted in the case of Rylands v Fletcher [1868], a landowner must accept that he or she has a duty of care to those using neighbouring parcels of land for their own purposes, by ensuring trees upon their land (that may affect adjacent properties) are as safe as is reasonably possible. This sentiment is echoed in Donoghue v Stevenson [1932], where the judge remarked that a tree owner has a duty of care to “avoid acts or omissions” that can be reasonably foreseen to, if allowed to persist, cause harm to a neighbour. Of course, only where a defect is outwardly evident to a reasonable and prudent landowner will a successful case be made, as was recognised in the case of Kent v Marquis of Bristol [1940]. In this instance, it was ruled that if the elm tree in question had been inspected, then it would have been outwardly recognisable that the cavity present upon the structure was a hazard, and therefore should have been dealt with prior to a subsequent incident occurring.

A willow (Salix alba) with a significant hazard beam. Because of the target area beneath, the hazard was removed, in this instance.

Adding to this, where a tree becomes defective as a result of vandalism or at the hands of a trespassing individual, as soon as the landowner becomes aware of the issue (or should be reasonably expected to already be aware of the issue) then the tree must be made safe. This was the case in Sedleigh-Denfield v O’Callaghan [1940], where the local authority trespassed onto a parcel of land and undertook works to ensure a ditch drained properly (by installing a culvert and grate), unbeknown to the landowner. However, such works proved to be ineffective because of the improper installation of a grate, which led to the culvert frequently being blocked. Because of this, the land frequently flooded, and eventually onto a neighbouring property where extensive damage was caused. The defendant on whose land the flooding emanated from claimed that he had no knowledge of the work being undertaken by the local authority, though the judge ruled that because the defendant had continued the allow the nuisance (which should have readily been identifiable) that he was to be liable for the damages. This has implications for the arboricultural industry, as it suggests that as soon as one is aware of a hazard relating to a dangerous tree, it must be appropriately actioned.

Similarly, where a tree has had pruning operations undertaken unto its structure, which have had the impact of making the tree less structurally safe (such as topping causing an abundance of sprouts, though even if pruning the tree in accordance with BS 3998), there is added impetus to follow-up with further inspections to ensure the tree is not posing an unreasonable danger to the target zone beneath. Such a stance was held in the case of Chapman v Barking and Dagenham LBC [1997], and prior to this the case of Caminer v Northern & London Investment Trust ruled that whilst the lopping of the elm tree in that instance would be appropriate to remedy the situation that manifested when the tree failed and caused injury to the claimant, it would make the tree more dangerous in the long term and must therefore be inspected more frequently. In addition to this ruling, the case also held magnitude because the judge ruled that, as the tree was outwardly not defective, there was no reasonable justification in expecting the landowner to have instigated further investigation into the tree’s state. However, if the tree had warranted further investigation to such a layperson, they would have been expected to employ an expert to assess the tree more vigorously.

Reverting attention back towards more recent cases, Leakey v National Trust [1980] was concerned primarily over a landslip and who was liable, if anyone, for the damage caused. The court did however utilise the case to expand upon the issue of natural phenomena, which ultimately included relevant conclusions for trees. The court concluded that there is a general duty with regards to ensuring hazards, naturally-borne or man-made, do not stem from the land of the owner and affect neighbouring land owners or guests on the site. The duty is “to do what which is reasonable in all circumstances, and no more than what, if anything, is reasonable, to prevent or minimise the known risk of damage or injury to one’s neighbour or his property.” However, somewhat critically, it did determine that naturally-borne hazards that could have been be prevented can only having the ruling applied to them where “the defendant has, or ought to have had, knowledge of the existence of the defect and the danger thereby created.” As a result, one can expect that only hazards relating to trees that are relative to the expected knowledge of the owner of the land, or those employed to look after such land, to be actionable. Such a standard was in fact elucidated to in Khan & Khan v London Borough of Harrow & Helen Sheila Kane [2013], where the court ruled that the issue of reasonable foreseeability is not a subjective test, but is an objective test as to what ought to have been known to a reasonable person in the position of the defendant: “In this case, the relevant person is a reasonably prudent landowner.” However, a defendant’s subjective knowledge can impose a higher standard, the court found. Therefore, local authorities and organisations who are responsible for trees may indeed be held to such a higher standard. Such a precedent has since been mirrored by many other more recent cases, including:

  • Corker v Wilson [2006], where as the defendant had carried out informal observations of the oak tree, which had seen one of its limbs fail and strike a passing motorist, injuring him and damaging the vehicle, on an ongoing basis, and all the evidence was that the tree was in good health. The judge ruled that there was nothing about the tree which should have alerted the defendant, or led him to obtain a more detailed inspection by an arboriculturalist.
  • Atkins v Scott [2008], where the judge ruled that it was “neither probable nor reasonably possible for a competent inspector to have observed the crack in the branch that failed“, after the branch of an oak tree failed and struck and injured a passing motorist during windy weather.
  • Selwyn-Smith v Gompels [2009], where an ordinary landowner’s tree fell onto a neighbouring garage. The claim was rejected as the judge deemed that the defendant had no requirement to engage with an arboricultural expert “unless and until reasonable inspection by the standards of that [the defendant’s] knowledge discloses or should disclose that the tree might be unsafe”.
  • Ulsterbus Ltd v Sufferin [2010], in a case where a double decker bus carrying school children collided with a branch within the crown of a tree and caused both damage to the bus and injury to some passengers, after it veered slightly to the left to allow for a large van to pass heading the other way, saw the court rule that “the defendant in this case was not aware of the risk” and “should not have become aware with reasonable care of any danger posed by the branch with which the bus collided.” The court drew influence from British Road Services v Slater [1964], where the court ruled that the case should fail based on the fact that the claimant had pulled into his nearside to enable an oncoming vehicle to pass, thereby leaving the carriageway in part, in addition to the fact that both the claimant and the defendant had not, until this point, considered the tree a hazard.
  • Micklewright v Surrey County Council [2011], where it was found that extensive internal decay was a major factor in the failure of a large branch of a mature oak tree. The judge found that nobody had seen any external signs of decay and ruled that, even if the local authority had had in place a proper system of inspection, the extent of the decay, and the danger it posed, would not have been revealed.
  • Bowen (A Child) & Ors v The National Trust [2011], where the court ruled that a “risk assessment in any context is by its very nature liable to be proved wrong by events, especially when as here the process of judging the integrity of a tree is an art not a science, as all agree. [The court] accept[s] these inspectors used all the care to be expected of reasonably competent persons doing their job, and the defendant had given them adequate training and instruction in how to approach their task. To require more would serve the desirable end of compensating these claimants for their grievous loss and injuries. But it would also be requiring the defendant to do more than was reasonable to see that the children enjoying the use of this wood were reasonably safe to do so. I regretfully conclude that I cannot find that the defendant was negligent or in breach of its duty in respect of this tragedy.
  • Stagecoach South Western Trains Ltd v Hind & Anor [2014], where it was determined that “there was nothing that should have alerted her [the defendant], or put her on notice, that the tree was anything other than healthy, or required a closer inspection by an arboriculturalist.” This case related to a 150-year old ash tree, whose stem had fallen onto the railway line from the garden of the property, which then resulted in a collision with an oncoming train. The court in this case drew influence from both Corker v Wilson [2006] and Selwyn-Smith v Gompels [2009], concluding that a system of informal observations by the landowner was adequate, and that an inspection by an expert arboriculturalist was only necessary if there was something revealed by the informal inspection which suggested that a more detailed inspection was required. The arboriculturalist was also found to not be liable for any wrongdoing, having worked on the tree more than one time prior to the incident. Again, there were no outward signs to suggest to him that there was evident risk associated with the ash’s presence.
This veteran beech (Fagus sylvatica) lapsed pollard is riddled with Ganoderma, and thus it has received a slight crown reduction to reduce the level of risk associated with the fungal decay and the sheer weight of the many stems.

However, in specific instances, it has been deemed that the land owners have not done enough to prevent hazards from materialising. Such cases include:

  • Quinn v Scott [1965], where it was decided upon that because the decay of the tree was so visibly apparent, it should have been felled in response to the clear hazard presented. The judge stated: “The duty of the Trust is to take such care as a reasonable landowner – and that means a prudent landowner – would take to prevent unnecessary danger to users of the highway adjoining the Trust’s land. There is not to be imputed in the ordinary landowner the knowledge possessed by the skilled expert in forestry… But, in my opinion, there may be circumstances in which it is incumbent on a landowner to call in somebody skilled in forestry to advise him, and I have no doubt but that a landowner on whose land this belt of trees stood, adjoining a busy highway, was under a duty to provide himself with skilled advice about the safety of the trees”.
  • Chapman v Barking & Dagenham LBC [1997], where it was concluded that “the defendant council did not at any relevant time appreciate the distinction between making lists of trees and routine tree maintenance, and systematic expert inspection as often as would reasonably be required”, thereby finding them liable of negligence and not remedying a hazard that could have readily been identified.
  • Poll v Bartholomew [2006]. This High Court case covered the standard of the duty of care and decided that, in this set of circumstances, a drive-by check was not a sufficient level of inspection and the claimant succeeded. The claimant had collided with a fallen tree that had extensive decay, though this was not picked up on by inspectors given the poor extent of inspection. This case is particularly important as it suggests different levels of inspection and competence are to be applied, depending upon the knowledge extent of the owner.

However, the case of Stovin v Wise [1996] raises an interesting point, in that “The mere existence of statutory power to remedy a defect cannot of itself create a duty of care to do so. A highway authority need not have a duty of care to highway users because of its duty to maintain the highway“. For example, under Section 154 of The Highways Act 1981, a local authority is not necessarily to be held liable if they do not exercise the power under Section 154 to remove a threat to the highway and its users.

UK case law relating to dangerous trees

The human health impacts of tree mortality – a case study

The presence of trees in the urban environment is certainly beneficial for the health and well-being of local residents, who will, generally-speaking, benefit more than they suffer as a result of the trees’ presence. Somewhat anecdotally, it has also been shown that it is the presence of healthy trees that has such beneficial impacts. In this sense, if tree populations suffer at the hands of a biotic or abiotic stressor, visibly decline in health, and potentially die in time (even on a massive scale), then the impact will also be ‘felt’ by the local residents.

As a means of adding weighting to this statement, we can look at a US-based study that assessed whether the presence of the tree pest emerald ash borer (Agrilus planipennis), as it ravaged urban ash (Fraxinus spp.) populations, brought about adverse health responses in humans as well. The beetle has spread quite rapidly in the US since its arrival in 2002, and now occupies an area of land quite massive in scale (as shown by the map below). Specifically, the study looked at whether the presence of emerald ash borer had associations with the rate of mortality caused by cardiovascular and lower-respiratory-tract issues. The authors of this study selected these causes of death, because they are the first and third highest causes of death, respectively. Furthermore, there is reason to suggest that these two causes of death can be influenced by the presence (or lack) of trees.

emerald ash borer map USA
A map of where the emerald ash borer has been found in the USA, on a county level.

In all of the US states that had at least one case of the emerald ash borer, the authors analysed information, from 1990-2007, outlining the cause of death for citizens. With this data, the authors looked at whether the presence of the beetle, and how long it had existed in a state, had any influence upon the mortality rates associated with the two causes of death mentioned. These comparisons were then related to the estimated abundance of ash trees in the states, and demographic data obtained via census records.

Following on from data analysis, the authors identified that the presence of the beetle had a more significant impact upon respiratory-related mortality rates in wealthier counties (where there was a greater access to ash trees, compared to poorer counties) and, on average, there were 6.8 more deaths per 100,000 individuals in states host to the pest than prior to the state’s date of infestation. However, the impact of its presence increased over time, therefore meaning that areas that have been infested for longer will experience a higher average mortality rate than those areas more recently infected (as can be seen in the below table). At the time of the study, the authors therefore estimated that 6,113 respiratory-related human deaths had been caused by the presence of the emerald ash borer between 2002-2007, because of its impact upon its host ash trees (which almost always will die, or are cut down upon detection of them being infected). Therefore, we can perhaps observe that dying trees (and an increasing lack of trees) means dying humans. However, the fact that there is an anticipated 2-5 year lag associated with beetle presence and human mortality rates, the real effects of the ash borer upon human health may not yet be fully appreciated. Curiously, the authors also suggest that the media coverage of the ash borer’s presence may induce stress in some individuals, and such stress may potentially exacerbate (or create) health issues. Perhaps this highlights the emotional relationship people have with trees, and at times there may even be a sort of cross-kingdom empathy (and associated grief).

emerald ash borer human impact
The impact upon respiratory-related human mortality rates caused by the emerald ash borer.

The observed impacts upon cardiovascular-related mortality rates per 100,000 individuals was even higher. In counties host to the emerald ash borer, an additional 16.7 deaths per 100,000 can be attributed to the pest’s presence. Therefore, 15,080 deaths can be directly linked to the effects of the borer, meaning that a total of 21,193 individuals have suffered mortality, between 2002-2007. Much like with respiratory-related deaths, the duration of time for which the beetle has been present has an impact upon the rate of mortality (see the below table). It was also found that individuals in counties with moderate levels of average income were most markedly affected.

emerald ash borer cardiovascular death
The impact upon cardiovascular-related human mortality rates caused by the emerald ash borer.

Not provided in the journal, I decided to plot both sets of figures relating to the mortality rate increase observed with the presence of the borer, and assess the two lines (as shown below). What we can crudely see is that, as the years progress, the dispiraity increases between the two data sets, in favour of cardiovascular-related deaths. However, respiratory-related deaths consistently remain, across the six year period, 39-41% lower than cardiovascular-related deaths. Therefore, enriching cardiovascular health may likely be a more significant focal point with regards to any mitigation measures that may take place, though we must obviously be aware that there is likely going to be a levelling-off threshold, by where no more ash trees exist and therefore the mortality rate cannot suffer any further (accompanied by lag times). During this time, re-planting may of course occur, and offset any adverse impacts associated with ash mortality. Of course, these new trees will take time to mature, and therefore it may be many decades before health impacts begin to markedly reverse.

emerald ash borer comparison
Comparing the two data sets (blue line: cardiovascular mortality rates; orange line: respiratory mortality rates).

There is no question that such data is indeed very interesting, and the results don’t necessarily remain limited to the emerald ash borer. Across the world, we can observe trees dying or being removed because of pest or disease outbreaks, so one can suspect that similar impacts may be associated with, for example, ash removal because of ash dieback, elm removal because of Dutch elm disease, or tree removal caused by Xylella fastidiosa. Granted, the actual impacts may differ (either be more severe, or less severe), though there will certainly be impacts. The findings that more affluent individuals were more adversely impacted by the emerald ash borer is very interesting, though perhaps not surprising, as environmental inequality certainly exists within urban districts (with more affluent areas having a more abundant tree presence). In this sense, less affluent neighbourhoods may already be suffering as a result of other stressors, and the lack of ash trees in the near locality means that they simply aren’t impacted by their death, because they are not directly experiencing such significant tree mortality. Other socio-demographic issues may also be implicated in this equation, such as the highest level of education an individual has. Similarly, the reduction is ecosystem sevices (pollution removal, encouragement to exercise outdoors, and so on) associated with the death of ash trees may have an impact upon respiratory and cardiovascular health, and this impact will be most pronounced where the most trees have been lost, which is (in this case) in the more affluent areas.

agrilus planipennis
ATTENTION: Have you seen this pest? It is wanted for the death of over some 100,000,000 ash trees, in the USA. Source: Aetree.

Source: Donovan, G., Butry, D., Michael, Y., Prestemon, J., Liebhold, A., Gatziolis, D., & Mao, M. (2013) The relationship between trees and human health: evidence from the spread of the emerald ash borer. American Journal of Preventive Medicine. 44 (2). p139-145.

If you found this interesting then please considering sharing it, reading the source article, and leaving a comment below (or over on Arbtalk). Thanks.

The human health impacts of tree mortality – a case study

Book review: Urban Tree Management (Roloff, 2016)

There is a growing source of literature available, almost at one’s very fingertips, detailing the ever-expanding knowledge we have of urban trees. Usually, these books will focus on a specific aspect of (by-and-large) urban trees, such as David Lonsdale’s Principles of Tree Hazard Assessment and Management, Ed Gilman’s An Illustrated Guide to Pruning, James Urban’s Up by Roots, John Dover’s Green Infrastructure, and Watson & Himelick’s The Practical Science of Planting Trees. However, books that draw in information (at times, courtesy of numerous contributors) on many aspects of urban tree management do also exist (and there are certainly a few), and I would probably champion this book as one of the very best out there at this moment in time. The chapters on urban trees and drought stresses (chapter 5), urban tree genetics (chapter 16), the governance of urban trees (chapter 17), and how to construct an effective urban tree education program (chapter 20), were perhaps the most interesting for me, though I cold not say that any chapter in this book was out-of-place or not of use. I certainly see future referrals to this book, when it comes to considering discussion topics and writing essays, being very common. Similarly, I am certain that it’ll become a very well-used book for students who are studying urban tree management / urban forestry. Its scope really is that vast.

In terms of knowledge gained from the book, I’d probably say there’s so much I have learned, considered, and entertained, that I don’t know where to begin. I found myself re-reading a few sub-chapters, just to re-affirm and re-absorb what I had just read (not wanting to miss out on any vital slice of the knowledge pie). At times, what was being presented in the book was what I already knew, and I don’t doubt that others who immerse themselves in literature and seek to learn every day will experience the same feeling when they read the book, though this exact sensation was largely limited to isolated sections. Often, I may have been reading something I already had recognised, considered, and partly understood, though not in the angle it was being presented; or, I had not even not considered it at all. The wide range of authors of the chapters (there must be at least 15) probably helped in this regard, as it didn’t limit the book to the knowledge of just one or two individuals. Perhaps this also kept the book feeling ‘fresh’.

Urban Tree Management book
The book in all its glory.

This book balances the use of text and the inclusion of tables and figures very well, and in many instances one can find that the text is actually very much simplified by an included table alongside. The images used are large enough to be able to see what is going on within them, and the book being printed in colour certainly aided with that. The book was also very well proof-read, though I did notice a short segment of German within one of the chapters. Obviously, the author of that chapter had written some notes in his first language, and not removed them prior to the book being printed. Beyond that however, there really is hardly anything that cannot be understood (and in many instances sentences are composed in a way that even a layperson could understand), or appears out-of-place (gramatically). The sub-chapters are also nice and digestible, with no feeling of being over-burdened by information or ‘walls of text’.

At a bit of a tangent, I am pleased that the Germans actually wrote a book in English. Browsing through the references to this book, I grew quite envious of the plethora of books listed that were all written in the German language (of which I cannot speak). Evidently, there is a significant amount of material written by German academics and industry professionals that has never reached a wider audience, and like The CODIT Principle, which was translated from German and published by the ISA last year, I’d be keen to see other publications translated, too. They know their stuff, that’s for certain! In the meantime, the fact that this book drew material from these German publications is good, as it gives these books an indirect form of accessibility to a far wider audience.

You can buy the book here, if you so desire. Considering the information within, it’s a small price to pay.

Book review: Urban Tree Management (Roloff, 2016)

Upcoming book to consider purchasing on urban forests

A book entitled Urban Forests: Ecosystem Services & Management is set to be released on 21st July 2016. Pressed by CRC Press, I have pre-ordered a copy (price is £80 + P&P), and I suspect some of you who read this blog may also be interested in acquiring a copy once it is pressed. The book can be pre-ordered here, from the publisher. Of course, other book stores will sell it too, I would very much imagine.

Upcoming book to consider purchasing on urban forests

The role of urban trees – is it all too anthropocentric?

This post is essentially a thought-piece, so in place of a referenced post it’s more of a ‘brain dump’ (of which I cannot promise absolute coherency and flow). I suppose the purpose of this post is therefore to stimulate some form of discussion, at least internally (though ideally also externally) – this discussion will, ideally, be laden with academic references and personal experiences.

Over the last year in particular, as I have begun to really begin researching the role urban trees have, and what they are perceived to provide in the planning stages of developments, I have become increasingly concerned with the seemingly over-riding desire to use trees for the benfits of humans. In this sense, their role is one of a sort of dynamic ‘furniture’, be it a street that is furnished wth trees, a garden, an urban park, or even an urban woodland or forest. Attention is paid to what trees give humans, and this may range from the already-mentioned aesthetic aspects, to the social ecology of humans (promoting integration between individuals, and outdoor recreation), and to the economic benefits (reduced recovery times in hospitals, a better sense of self-esteem and associated lower levels of stress, the encouragement of consumer spending, raising the value of a property, and so on). The list, with regards to the anthropocentric provisionings of trees, is quite literally vast. However, at least anecdotally, there seems to be little desire to utilise trees in the urban environment for the benefit of biodiversity and, even if there is a desire, it seems to rank beneath the value trees directly provide to humans. We can even see this with (peri-)urban woodlands, which are utilised generally for recreational purposes, and these (usually intensive) recreational activities are very much damaging to lower tier plants and soil flora and fauna. In turn, these adverse impacts cause the tree populations to suffer (be it from seedling recruitment, reduced nutrient uptake, water uptake, and so on).

Whilst I fully understand why trees are viewed in such an anthropocentric manner, we must not forget that our urban and peri-urban environments reside within a mosaic of habitats at the landscape-scale, and biodiversity does not stop at the doorstep of our ever-expanding towns and cities. By-and-large, native faunal biodiversity has evolved alongside the presence of native floral biodiversity, and this relationship over many millennia has produced thousands of obligate associations, and has formed the basis of functioning terrestrial ecosystems. In our urban areas, this relationship is sometimes aggressively severed by the abundant presence of exotic tree species that lack any obligate associations with native fauna (and have the potential to be invasive), a lack of tree species diversity, and fragmentation that creates many ‘urban tree islands’ (oft found in tree-less streets and estates), which are somewhat akin to their ‘forest island’ counterparts. At the same time, ivy stems are removed from trees, and usually under the guise of the ivy reducing amenity value in an urban system that thrives off of order and uniformity, or the concern that the ivy will kill the tree (which is usually not going to be the case). Even where the means of servance is risk-based, is there justification in severing the stem at the base? Probably not.

Granted, with the fact that many invasive pests and pathogens are wiping-out native tree and plant species with relative ease, the ideal reliance upon native tree species is perhaps going to become an ever-growing problem. For example, ash dieback (an oriental plant pathogen) is expected to cause massive mortality of the European ash (Fraxinus excelsior), of which there is currently no means of remedy. It is suspected that oriental ash species will possess resistance to the pathogen, as they have evolved alongside the pathogen and thus have developed means of effectively limiting its impact on the population level (the same applies to ash trees and the insect pest emerald ash borer), though the introduction of such exotic ash species will not reverse, by any stretch, the intricate associations European ash have sustained with, for instance, obligate lichen species (of which many are endangered and will go extinct if the European ash die). One means of retaining the presence of native urban tree species is to create cultivars that are resistant, at least partially, and in the short-term this is ideal. However, once the pathogen has been exposed to the specific cultivar(s) over a period of time, it will itself adapt to the change in host type and likely once again ravage the (genetically undiverse, as many cultivars share some, or even all, genetic qualities) tree population(s). Even in recognising this, there still appears to be little consideration given to selecting resistant exotic tree species that can provide marked ecological benefits for urban biodiversity, with more focus instead being simply on sustaining a form of urban forest that gives benefits to humans (who generally don’t discriminate between tree species, but simply demand tree presence).

emerald ash borer urban
An avenue of ash being killed by the insect pest Agrilus planipennis (emerald ash borer). Once these ash are removed, the entire area will be significantly lacking in large trees, which will have significantly adverse impacts upon biodiversity. Source: ACTrees.

Beyond tree breeding for resistance, other cultivars and clones exist that have ‘urban form’, and these cultivars may also be at huge risk if exposed to an associated pest or pathogen. As urban environments suffer from in-fill and denser developments, restricted space leads to specific cultivars being selected that have fastigiate form, can tolerate root compaction, and so on. Where these cultivars exist as monocultured avenues, there may be an even more marked problem. Of course, standard tree avenues of a signle species are also very susceptible (notably if they are very similar, on a genetic level), and could quickly cease to be. This would then mean any biodiversity associated with the avenues is lost, too – notably, as dead trees are usually removed entirely from an urban site, even in spite of deadwood being of huge ecological importance.

Excessive risk management in urban settings, primarily out of the fear of litigation, is another huge issue that does not reconcile with the need for urban forests to provide for biodiversity. Older trees are usually going to present the greatest risk, due to their stag-headedness and other retrenchment processes that see them become what is considered ‘strucurally unsafe’, though it is these trees that give the most benefits to biodiversity – namely, saproxylic insects and fungi, though also other species that have had dozens (or hundreds, or even thousands) of generations exposed to the specific old trees in situ. Such risk to humans generally sees the trees managed, perhaps via crown reduction (or even felling), and in the case of reduction (and felling) work deadwood (which is ecologically crucial) may likely be removed if a target area exists beneath. At times, this deadwood won’t even remain at the base of the tree (if only reduction work is set), even in large parks (where retention is particular feasible), and this essentially creates an ecosystem (from single tree scale up to city-wide scale) with a huge void of deadwood-associated species. Outside of parks, the grabbing of large pieces of deadwood by members of the public that are destined solely for the log burner is a similar ecological travesty (this is an education issue).

A line of five Salix alba within a well-used urban park that were monolithed, as a result of extensive fungal decay caused by Ganoderma australe. These willows could easily have been felled, though the retention of the very large decaying stems is of marked benefit to saproxylic species.

Leaving trees themselves temporarily, the observation of most (if not all) urban cities having a fascination with intensively-cut amenity grass is another ecological issue. Large tracts of grass are usually cut for no reason other than to enable for the scene to look formal, and whilst this may be understandable in areas in parks where sports recreation is undertaken (which means management isn’t solely for formality), huge roadside verges are also cut in the same manner. Such verges, or even areas of parks not used for sports recreation, really need not be cut, and instead could be left unmowed for most of the growing season and allowed to ‘wild’ for the benefit of biodiversity (arthropods and small mammals, namely). Scope exists to keep grass long beneath many trees, in fact. Such a practice would in part resolve the issue of ground compaction around trees, and if the tree harbours many insect species, the wildflowers and ‘weeds’ may provide a source of nectar for such insects, and providing the conditions for a small micro-ecosystem to manifest on the single-tree level.

During urban development processes, we also observe areas of woodland, pasture, parkland, or otherwise, removed for housing and other forms of development. At times, relicts of former land use (namely trees) are retained, though not always in good conditon and certainly in isolation from their once more diverse ecological setting. The general bias in governmental (worldwide) plans for economic development sees attention to the ecological aspects associated with such developments lacking, and the lack of desire, scope, and ability to ensure tree planting, protection, and retention measures are properly developed, understood, practiced, and enforced, means that many new urban developments lack mature trees and are graced by only an insufficient number of new trees that are usually improperly cared for, and are often unable to provide, even long-term (if they survive), for local biodiversity. Small gardens so highly common in newer developments, compiled with a lack of roadside verges (maximising space for housing and off-road parking) and an abundance of aerial and subterranean services, means that scope for planting large (or even small) trees may not even exist. What hope is there ever going to be for these settings, on an ecological level? Rarely is there scope for demolishing these areas and entirely re-building, given the costs involved. The fragmented nature of the development process also doesn’t serve to aid with the provisioning for biodiversity, with horticultural and arboricultural measures usually only being considered during the latter stages of the process. Even when they are considered, it is questionable as to what weight they have, unless the site is particularly contentious or the potential customers are very affluent and demanding of high-quality landscaping and an associated mature tree presence.

housing development trees
A quite newly-built housing development that is largely void of space that can accommodate trees. We can see a few large trees in the background, though a lack of a roadside verge (on the property side), and a front garden of any decent size, severely limits the potential for any sort of tree planting. Source: Architecture Photos.

Not wanting to continue and make this post into something akin to War and Peace, I’ll round up here. There is no way I can cover everything I want to without making the post much longer, and that would go against the point of a blog post. I suppose the real crux behind all of my points here is that there is a general desire for amenity and human practicality when it comes to creating and managing tree populations, in place of the desire being to principally provide on an ecological level. In my eyes, this is counter-productive, as whilst it satisfies the need for humans it neglects needs of the ecosystem, which we, as humans, can never escape. Our urban environments are ecosystems just as much as our forests are, and if the health of the ecosystem declines then our own health will decline alongside. Trees, across the world, form the basis of many terrestrial ecosystems, and therefore they must also form the basis of our urban ones, and it is important that this is recognised and held in the highest of regards by those in the realm of decision-making and influence (this extends to pretty much every individual). It is also important that the tree species chosen are chosen for ecological reasons, as if they are chosen primarily for other reasons then the area will fall foul to ecological decline (in comparison to an area developed with ecology in the highest mind).

The role of urban trees – is it all too anthropocentric?

Fungus spotlight: Phaeolus schweinitzii

Common hosts: Conifers such as Larix spp., Picea spp., Pinus spp., and Pseudotsuga menziesii (Dewey et al., 1984; Levy, 1982; Lonsdale, 1999; Schwarze, 2008; Watson & Green, 2011) are frequent hosts, though it may also infect broadleaved species such as Prunus spp. and Quercus spp. (Schmidt, 2006).

Phaeolus schweinitzii Pinus 1
A slightly-less-than-fresh set of sporophores upon the stem wound of a pine (Pinus sp.)

Colonisation strategy: Heart rot strategist. If damage is present on the stem, entry can be achieved via the wounds (Lonsdale, 1999; Watson & Green, 2011). Alternatively, entry can be through the root system (Schwarze et al., 2000; Watson, 2006), where it usually follows Armillaria spp. (Starr, 2013; Watson & Green, 2011). Very commonly found in coniferous forest located on formerly hardwood forest soils (Schmidt, 2006).

Rot type: Cubical brown rot (Lonsdale, 1999; Schwarze et al., 2000; Watson & Green, 2011) with discernible mycelial sheets (Rayner & Boddy, 1988).

Phaeolus schweinitzii Pinus 2
A look at the underside of the sporophores shown above.

Significance: A loss of tensile strength triggers brittle fracture. As decay progresses, fracturing under wind loading becomes an increasing possibility at the base of the stem or in the root plate (Blakeslee & Oak, 1980; Lonsdale, 1999; Watson & Green, 2011). Decayed zones are viewed like a cavity, given how P. schweinitzii decay brings about the same properties as a hollow would (Lonsdale, 1999). A driver of standing timber damage in coniferous forests (Levy, 1982).

Phaeolus schweinitzii Pseudotsuga 1
A very degraded bunch of sporophores at the base of a storm-damaged douglas fir.

Location of decay: Decay usually occurs around the base of the stem and within the root system (Dewey et al., 1984; Schwarze et al., 2000; Watson & Green, 2011), though decay columns can reach as high as 6-8m (Lonsdale, 1999).

Methods for prevention: Limit heartwood exposure where possible when pruning. If decay has established significantly, it may be worth felling depending on location. However, as its presence is rare on amenity trees (Lonsdale, 1999), such a responsive step is likely to be rare. The avoidance of planting susceptible species (Larix spp., Picea spp., Pinus spp., and Pseudotsuga menziesii) to the fungus is recommended in sites where risk of infection is likely (Dewey et al., 1984; Starr, 2013), if this is practicable.

Phaeolus schweinitzii Pseudotsuga 2
A cross-section of a ‘spent’ bracket, showing the tube layer.


Blakeslee, G. & Oak, S. (1980) Residual naval stores stumps as reservoirs of inoculum for infection of slash pines by Phaeolus schweinitzii. Plant Disease. 64 (2). p167.

Dewey, F., Barrett, D., Vose, I., & Lamb, C. (1984) Immunofluorescence microscopy for the detection and identification of propagules of Phaeolus schweinitzii in infested soil. Phytopathology. 74 (3). p291-296.

Levy, J. (1982) The place of basidiomycetes in the decay of wood in contact with the ground. In Frankland, J, Hedger, J, & Swift, M. (eds.) Decomposer basidiomycetes: their biology and ecology. UK: Cambridge University Press.

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

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

Schmidt, O. (2006) Wood and Tree Fungi: Biology, Damage, Protection, and Use. Germany: Springer.

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

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

Starr, C. (2013) Woodland Management: A Practical Guide. 2nd ed. India: The Crowood Press.

Watson, B. (2006) Trees – Their Use, Management, Cultivation, and Biology. India: The Crowood Press.

Watson, G. & Green, T (2011) Fungi on Trees: An Arborist’s Field Guide. UK: The Arboricultural Association.

Fungus spotlight: Phaeolus schweinitzii