(Arboricultural-styled) 'Fact of the Day'

[Kveldssanger]

05/04/16. Fact #186.

 

I’m currently reading a really quite brilliant book entitled Urban Tree Management – for the Sustainable Development of Green Cities, and there is a section where the authors discussed what the main types of land classifications are in the urban environment. A less science-heavy post than normal, hopefully this is both a nice change of pace and thought-stimulating.

 

Firstly, we can readily observe how urban areas with have what is considered a ‘road traffic area‘. As the name suggests, these areas are situated adjacent to highways, be such highways main routes or residential ones, and the role of trees in this setting may be to: (1) provide amenity value; (2) reduce the speed of traffic; (3) cast shade and cool the highway and surrounding pavements; (4) act as a visual screen; (5) filter out particulate matter; (6) ‘catch’ rainwater in the foliage crown and branching structure so to reduce the stress upon stormwater management systems, and; (7) act as a connective structure between parks and urban woodlands for the benefit of biodiversity. However, these numerous reasons for why trees exist in the urban landscape are pressured by the numerous stressors, such as air pollution, de-icing salts, compaction, impervious surfaces (drought), low humidity, high levels of light reflectance, and high temperatures. In this sense, the roadside tree has a very tough time, and survival rates may be low in the long-term (or short-term), and only those that are truly hardy may be able to reach a good age and retain their dignity in the process.

 

Alongside the road traffic area, we have the ‘densely built-up area‘. As noted by the authors, and also rather self-explanatory in nature, the two go hand-in-hand; particularly in the inner-city area. Therefore, many of the same stressors apply, and many of the reasons for planting also apply, though because such areas are going to be (at least, in part) residential, considerations must be given to ensuring that the constituent trees ideally don’t trigger allergic reactions in residents (Platanus spp.), emit awful smells (I’m looking at you, Ailanthus altissima), or block light to properties to a marked degree (densely-crowned species, including Quercus spp.). Perhaps, the densely built-up area is even more tricky (in both landscaping and tree survival) than the road traffic area, in terms of landscaping.

 

Within these densely built-up areas, we may sometimes come across a green roof and / or container plants, to add greenery to the landscape. Trees and plants can, and do, exist upon the rooves of properties in inner-city areas (such as in London, where there was the Derry & Tom’s roof garden, and the Selfridge’s roof garden in Oxford Street), though these areas are very limited in terms of root space and will likely be prone to higher wind speeds (because of the higher elevation and exposed setting). This means that drought is a very real risk, and as such tree species selection may be limited to smaller species, and those which are tolerant of dry conditions.

 

We then have the ‘sparsely built-up area‘, which may include residential areas (detatched and semi-detatched houses), small parks and green spaces, and ‘green corridors’ that may stretch across these areas. The stressors associated with this area type will be lesser than those in the densely-built up areas, though will still nonetheless exist. Trees planted in private gardens may have more scope to grow and fulfil longer lives, compared to roadside trees that may have more limited rooting space and have to compete with underground services (and the associated trenching that goes alongside repair works). Garden trees will also exist to serve a variety of purposes (depending upon personal preferences), ranging from amenity value and fruit crops, through to ecological value and adding privacy to a dwelling (conifer hedges). Old trees from prior land use (normally agriculture) may also remain, and at times form tree avenues.

 

Retained oaks from a very old highway, now sitting within a sparsely built-up area.

 

Outside of the built-up areas used for residential means, the urban landscape may also be host to industrial parks and commercial parks.Often, these sites are very large, and are covered with large warehouses / units, which all have their own (or shared) car parking areas. For the industrial park, trees that are planted may very well be both quick-growers and reach a large mature size, as a means of softening the visual impact of huge warehouses and the many coming-and-going articulated lorries. On the other hand, whilst commercial areas may utilise trees for similar purposes (amenity and softening), the tree species used are likely to be far more ornamental (perhaps fastigiated cultivars will be used, or sterile varieties of fruit trees). It is therefore unlikely that large trees will be planted, unless there is a central area that can support a large mature tree, or silva-celled systems have been installed complete with underground irrigation systems connected to stormwater reservoirs. For commercial parks that have indoor areas, trees can still be used, though they may be potted, be fake trees (made of plastic), or be small in nature (asides from during the Christmas period, where large conifers may be brought in temporarily).

 

Large poplars and leyland cypress in an industrial area, shielding the large factory units.

Back in the outdoors, public parks, ornamental gardens, and cemeteries, will often contain large tree populations. Here, the planting of trees is usually for amenity purposes, though the lack of competition for aerial and rooting space with built structures allows for very large trees to be planted – even avenues. Cemeteries may be host to lines of sequoias, for example, whilst parks and gardens may display unique cultivars of popular tree species, or even contain exotic tree species. Closed cemeteries may, by their very nature, be home to some of the largest trees, mature in age, and some of these may even be very old (ancient) yew trees (Taxus baccata). Public gardens are likely to have many more smaller trees, though the Chelsea Physic Garden in London has some fabulous large mature trees – cork oak (Quercus suber), ginkgo (Ginkgo biloba), magnolia (Magnolia grandiflora), swamp cypress (Taxodium distichum), and sweet gum (Liquidambar styraciflua), to name but a few.

 

A huge giant sequoia in an urban park, planted for ita amenity value and ‘presence’ (status).

 

Cutting through towns and cities, we may also come across ports, channels, and waterways. Here, riparian trees that are ideally native (willows, alders), but also non-native species that aren’t invasive (wingnuts), may most often flourish, though because these areas are likely to suffer less markedly from pollution, there is scope for the selection of a wide range of tree species. Despite this, water-borne pollutants may disrupt rooting environments, particularly nearby to ports. Therefore, the scope for tree species selection varies depending upon the context of the site. Where watercourses run through a large park area, perhaps the largest trees may be able to flourish.

 

Remnants of old woodlands and forests may also grace urban locations, and these are classed as urban forests. These locations may also be man-made however, having been planted on old areas of plotland, parkland, or otherwise. Tree species here may often be exclusively native, though garden escapees may also frequent such areas. Depending upon the popularity and accessibility of the urban forest, soil compaction and understorey disturbance (dogs) may be a particular problem, as may fly-tipping and other pollutants emitted from nearby areas (such as roads). Sporting activities may also take place in urban forests, with off-road biking being a notable activity. Alongside human pressures, the crucial role the urban forests play in retaining relicts of biodiversity (forest islands) must be recognised, and from these ecological hubs trees in the wider landscape may provide connectivity between other urban forest islands. At times, these urban forests may be found within parklands, or adjacent to them.

 

Brownfield (derelict) sites that have been left undisturbed can also be an important area where trees are supported. Often, the ground is polluted, compacted, and largely unsuitable, so species found here will likely have succeeded naturally and consist of early successional species: alder (Alnus spp.), ash (Fraxinus spp.), birch (Betula spp.), elder (Sambucus nigra), hawthorn (Crataegus spp.), poplar (Populus spp.), and willow (Salix spp.). These regenerating sites may also be important ecologically, and scrubbier areas may support reptiles. Rubble left over from past land use can also be suitable for insects, who utilise these niche habitats for shade and shelter.

 

Lastly, we have allotment gardens. In these areas, which are usually small plots, large trees are almost certainly not going to exist. Instead, fruit trees may be commonplace, and at times old mature apples (Malus spp.), pears (Pyrus spp.), and plums (Prunus domestica) can be found.

 

There likely exist many more niche areas within the urban landscape, though hopefully this post has given us all some food for thought. Buy the book (linked below), for many more thought-provoking reads.

 

Source: Gillner, S., Hoffman, M., Tharang, A., & Vogt, J. (2016) Criteria for species selection: Development of a database for urban trees. In Roloff, A (ed.) Urban Tree Management – for the Sustainable Development of Green Cities. Singapore: Wiley Blackwell.

[Kveldssanger]

07/04/16. Fact #187.

 

The services that trees provide in the urban setting are largely well-recognised, and it can therefore not be debated as to whether tree presence in the urban environment is optional – it’s mandatory. However, because the urban environment is very harsh and vastly different from a natural setting in which trees have adapted (and evolved) to grow, species selection may be limited. On a similar level, species selection may be limited by available space, concerns regarding subsidence, root damage to built structures, and so on. Therefore, a species palette may be quite limited when it comes to selecting the trees to plant, and it may even consist of cultivars that are more ‘fit’ for urban use. Furthermore, it is unlikely that the palette will consist wholly of native tree species, perhaps because they cannot cope with the urban environment, or because they lack the necessary morphological traits required (such as a compact, upright crown). The desire for amenity may also see exotic trees used, as they are inherently likely to be more intriguing.

 

The limited available options for urban tree species will lead to a limited amount of diversity within an urban landscape, with regards to tree species present (generally-speaking). In particular instances, and notably where avenues are planted, dozens (or hundreds) of trees may exist of the same species, and if they are cultivars or clonally propagated then genetic diversity may also be very limited, if not identical throughout the avenue. Such a lack of diversity, on all levels (from species to intra-species genetics), is always going to be a problem, as the tree population is left at a higher propensity of suffering significantly from a pest or disease outbreak. On the species level, if 50% of an area’s trees are of one particular species, and a pest or disease attacks that species, all are potentially at risk. This risk increases further if the trees wholly (clonal), or only partially (seed sourced from same mother), share genetic properties. Essentially, a street graced with healthy trees one year may be void of trees the next (and we can observe how the emerald ash borer did this to avenues of ash in the US). For this reason, current literature advises that a great diversity of trees are planted in urban areas, to reduce this risk.

 

With all of this context in mind, we can now look at a study, undertaken across ten major cities over Scandinavia, that looked into the diversity (unfortunately, not on a genetic level) of constituent tree populations (see below map). In total, a little over 190,000 urban trees were featured in the study (not all cities had all their trees recorded, with some cities only possessing accurate inventories for 20% of their tree population). This figure could be split into street trees (123,000) and park trees (67,000).

 

A map of the ten cities.

 

After analysing all of the tree data, it was found that Tilia spp. (linden or lime) was most frequently used across the ten cities, accounting to a total of nearly 24% of all trees (though, in Helsinki, it accounted for nearly 45%). When grouped with Acer spp., Betula spp., and Sorbus spp., this amounted to nearly 60% of all trees, on average. Evidently, there is a clear reliance upon these four tree genera, which can thus be regarded as ‘staple trees’. In this sense, if any pest or pathogen were to use these genera as hosts, there is great potential for huge levels of damage.

 

Focussing more intricately on these four genera, it was found that Tilia x europaea (European lime) amounted to 16% of all trees (and was the most common species in five of the ten cities), followed by Acer platanoides and Betula pendula, which both constituted for 9% of the total tree population. In two other cities, Tilia spp. were the dominant species, so in seven of the ten cities limes were most abundant.

 

With regards to differences between street tree populations and those in parks, the general trend was that parks had a higher diversity of species – though not all cities had records of their park trees; either at all, or had only completed partial records. Arhus had by far the greatest number of park species, with an apparent total of 599 different species (because of a botanical garden). This towered over any other city, as can be seen below. For street tree diversity, cities had an average of around 50-60 different constituent species, though Malmo had the most at 113.

 

Tree species diversity across the ten cities. Values of zero indicate no records were available.

 

Of the actual diversity between species within streets and parks, Malmo ranked highest with nearly 45% of its park trees consisting of tree species not amounting in numbers to over 2% of the total tree population. This figure dropped to 36% in street scenes, suggesting Malmo’s streets are less diverse but, if particular pests and diseases were to strike, the city may potentially only lose (or have at high risk of mortality) a small segment of its overall tree population. However, only seven different tree species acconted for half of the city’s tree population (higher than all other cities), which could still leave the tree population at marked risk if the ‘wrong’ aggressor were to strike. For the other cities, that risk is even greater.

 

Across eight of the cities (excluding Oslo and Tampere’s street tree populations), non-native tree species were more common than native tree species. However, in terms of actual tree abundance, native trees outranked non-native trees in all cities besides Arhus’s parks (not surprising, given there were 599 differnet species found in a botanical garden!). From this, we can assert that there is a greater tendency to select non-native tree species, though in lower numbers. Perhaps it is inevitable that the diversity of non-native species will trump native species as well, given the number of non-native species available for selection is vastly higher (for example, the UK only has 33 native tree species, though hundreds of viable non-native species exist for urban planting). Evidently, there is still a focus on ensuring native trees comprise the majority of actual tree numbers, which is beneficial for biodiversity. However, if an exotic pest or disease emerges, these naive trees may be struck hard (ash dieback, for example), and large losses may be incurred.

 

The total number of tree species in park locations across those cities that possessed such data (complete or incomplete).

 

The abundance of native and non-native tree species in the same cities.

 

The authors also found that some tree species were isolated to particular areas (or even a single area). For example, beech (Fagus sylvatica) amounted to 7% of all park trees in Malmo, though these were isolated to one single park. Unfortunately, the exact distribution of tree species wasn’t ascertained in the study, so beyond this example there is no data. However, it does highlight that certain tree species may only be very locally abundant. This may mean that the loss of the tree species isn’t damaging to the city as a whole, as tree cover is still maintained in all other areas, though it does mean that local tree cover is almost wiped-out, and a hugely concentrated inoculum base exists for a pest or pathogen.

 

In light of all the data provided for the study, one can certainly observe that whilst there is diversity in many cities, there is a tendency for particular genera and species to be selected in great abundance (such as lime; notably Tilia x europaea). Whilst this may be because of the practicality of using such species (survivability, growth form, amenity value, tolerance of poor conditions, etc), if a pest or disease were to use these species as hosts then there is a serious issue at hand. For this reason, it can be suggested that investigations into identifying additional species (and genera) should be undertaken, in an attempt to enrich diversity and lessen the reliance upon a select few genera and species. Of course, such an enriched diversity will come at the utilisation of non-native tree species, which runs the risk of invasive populations forming and damaging surrounding rural and woodland / forest regions. Biodiversity may also suffer, as there are few obligate associations between these exotic trees and native bodiversity.

Source: Sjöman, H., Östberg, J., & Bühler, O. (2012) Diversity and distribution of the urban tree population in ten major Nordic cities. Urban Forestry & Urban Greening. 11 (1). p31-39.

[sean]

Great stuff again mate. Think I will be getting said book.

[Kveldssanger]

It's really very good, so you won't be disappointed! Covers a huge range of topics, as well.

[Kveldssanger]

08/04/16. Fact #188.

 

Courtesy of forestry practices, and also the general clearance of ageing trees either to facilitate development or to remove a high level of supposed risk from their retained presence in urban and woodland environments, there is a growing lack of a future veteran / ancient tree population. This is concerning, as such trees have huge ecological value, and typically a value that is far greater to biodiversity than younger trees (notably for saproxylic insects and cavity-nesting birds). At this moment in time, the majority of existing veteran and ancient trees are likely native species, by sheer virtue of the fact that introduced species likely do not predate the 1600s-1700s (and those that are of marked age are generally few and far between, having been planted only in high-profile areas). However, given many environments now feature tree species that have been introduced artificially, future veteran and ancient tree populations may have to exist beyond just those species which are native to an environment. Similarly, if veteranisation techniques are to be practiced in order to artificially bridge the gap between two generations of ‘true’ veteran trees, such practices may have to also be undertaken on introduced tree species. This will in fact be particularly important, as many native tree species suffer from high mortality rates associated with exotic pests and diseases (Dutch elm disease, ash dieback, acute oak decline, and so on, in the realm that is the UK), which may increase our reliance upon exotic tree species.

 

In order to assess whether an introduced tree species can support native biodiversity, the authors of this study sought to ascertain whether veteranisation techniques (the creation of basal water-filled cavities and trunk cavities for nesting birds) undertaken to the London plane (Platanus x acerifolia) would yield positive results for select bird species, and for how long. This species was chosen as, in the study site of northern Italy (Bosco Fontana), the plane is considered a pest that is damaging the native ancient and semi-natural Quercus robur stands (alongside an invasive tree species: Quecus rubra). Ecologically, the implications of such an un-natural stand formation and succession could be considered highly significant, as there are almost certainly going to be a huge number of obligate associates to the native oak species, which in turn attract predators and parasitoids, and so on and so forth. Conversely, no species will have co-evolved with the London plane or red oak, and thus associations may very well be few and far between.

 

A map of the site, with the green dots being individual plane trees used in the study.

 

The 112 plane trees featured in the study were all of a DBH between 40-75cm, and spaced at least 20m apart from one another, in many segments of the study location (see above map). On each tree, between two and three basal pockets and one larger nesting cavity upon the trunk (1-5m up) were created. The basal pockets were designed to accumulate water and slowly rot (for the benefit of water-reliant fauna), and the largest slits were at the bottom of the trunk and decreased in size as they progressed upwards (unfortunately, no data is provided as to their benefits for biodiversity in the study). Upper nesting cavities were designed to house secondary cavity-nesting birds, and resemble naturally-occurring cavities frequent in veteran trees. Their sizes varied, in accordance with the intended bird species that would use the cavity as a nesting site, and these cavity types (and abundance) are listed in the table below.

 

Tree cavities and the intended nesting birds – an overview of the study cavity types.

 

For each tree, the artificial stem cavities noted in the above table were routinely monitored over three years for signs of habitation, and in a select few trunk cavities the internal cavity temperature was recorded for a period of six moths during 2001. Internal cavity temperatures were compared to nest boxes also installed on some trees. Two additional (partial) investigations were also conducted on the plane trees, in 2008 and 2010. The DBH of all trees was measured, to determine whether the veteranisation techniques had harmed the tree in any marked manner.

 

After a period of 8 years following on from the creation of the habitats, 77% of the plane trees remained alive, and the treatment type (trunk cavity sizes) and the size of the tree had an impact upon the mortality rate. Those that did die most routinely failed along the main stem, suggesting that the decay initiated by the habitat creation practices was a possible cause (though, not a significant attributing factor). Of the cavities within the main stem, the most constant temperatures were recorded for the large nesting cavity created for Strix aluco, suggesting that conditions within the host stem were optimal for habitation. Temperatures were also far more constant than those within nest boxes, which means that, wherever feasible, nest boxes should be avoided and instead nesting cavities constructed within the tree (even for exotic tree species).

 

Looking at bird species that colonised the cavities, it was found that five of the nine bird species targeted utilised the 87 (80%) of the artificial habitats (M. striata, P. caeruleus, P. major, S. aluca, S. europaea). For P. caeruleus and P. major, their presence significantly increased with time, and they were even found to use cavities created for other bird species. This trend (increased abundance as time progressed) was also observed for many other of the bird species found, as can be seen in the graphs below. However, by the eighth year, the cavities were largely unsuitable for nesting birds, perhaps because of the decay around the cavities.

 

The different graphs relate to the cavity sizes listed in the table earlier in this post.

 

Evidently, the use of the tree cavities by nesting birds means that exotic tree species can be relied upon to provide for some habitat, assuming they are continually created as the previous ‘batch’ become unsuitable (due to decay, insect infestation, and so on). Of course, some bird species didn’t use the cavities even though they were locally present, though given over half of the bird species did use the cavities it shows a good level of willingness by the bird species to associate themselves with the planes. From this, we can recognise that there is value in exotic tree species, and that veteranisation techniques should not necessarily be isolated solely to native tree species (if native tree species are even available, that is!). The authors do however note that this practice should be limited to broadleaved species, as the traumatic resin canals created by conifers flood the cavities with resin and this makes the cavities unsuitable for nesting birds. By a similar token, such practices should take place away from well-used locations, meaning that if the practices are undertaken in a woodland then they should be done away from pathways. This benefits both the need for risk management and nesting birds, which would generally be more suited to locations that are less disturbed.

 

Source: Zapponi, L., Minari, E., Longo, L., Toni, I., Mason, F., & Campanaro, A. (2014) The Habitat-Trees experiment: using exotic tree species as new microhabitats for the native fauna. iForest-Biogeosciences and Forestry. 8 (1). p464-470.

[Kveldssanger]

11/04/16. Fact #189.

 

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.

 

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).

 

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.

 

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.

 

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.

 

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.

[Kveldssanger]

13/04/16. Fact #190.

The below relates to UK case law associated with the management of 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 expected 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 depending on the peculiar characteristics of the particular defendant, 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 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.

Edited April 13, 2016 by Kveldssanger

[Kveldssanger]

On holiday next week, so won't be posting anything. Got an ATF meet with Saturday as well, and pretty busy for the rest of the week, so if there's next to nothing for ~10 days it's not because I've sodded off and am never to return.

[Gary Prentice]

"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 expected more frequently."

 

Inspected or expected?

 

I really good, useful read. I'm going to save this for future reference.

[Kveldssanger]

Haha nice spot!

 

I enjoyed looking into it all to be honest (did it for my course), and thought I'd share it as I know a good many people think about case law and get lost in its surface-level complexity. Plus, accessibility to case law info isn't that easy. The LTOA have some good bits, though unless you buy a Mynors book then it's probably not straightforward at all.