BS 5837 categorisation and ash dieback in England

BS 5837:2012 Trees in relation to design, demolition and construction – Recommendations is the British Standard (hereafter referred to as ‘BS 5837‘) that is used in the context of trees and development. Specifically, the scope of BS 5837 is to provide “recommendations and guidance for arboriculturists, architects, builders, engineers, and landscape architects“. Moreover, BS 5837 is  “expected to be of interest to land managers, contractors, planners, statutory undertakers, surveyors, and all others interested in harmony between trees and development in its broadest sense“. Thus, it is important to frame that BS 5837 is not just for the arboriculturist and therefore where the arboriculturist uses this document they must do so in mind of what the scope of BS 5837 is.

A key role of BS 5837 is to categorise trees in to different ‘value’ groups (A, B, C, and U) that reflect how important it is to consider a particular tree in the context of any planning and development matters (as a rule of thumb, A and B category trees should be considered more strongly for retention and accommodation within new designs). This is achieved through a tree survey, which “should be undertaken by an arboriculturist to record information about the trees on or adjacent to a site” in “a transparent, understandable and systematic way“. In order for a tree to fall in to any given category, “it should fall within the scope of that category’s definition“. These categories are determined through various factors though one vital factor is how long the tree likely has left in the landscape in its current context, which BS 5837 terms as a tree’s “estimated remaining life expectancy“. The remaining life expectancy is banded per category to the following:

  • high-quality (A-category) – at least 40 years life expectancy;
  • moderate-quality (B-category) – at least 20 years life expectancy;
  • low-quality (C-category) – at least 10 years life expectancy; and
  • trees unsuitable for retention (U-category) – no longer than 10 years life expectancy.

BS 5837 states that “the presence of any serious disease or tree-related hazards should be taken into account“, when assessing a tree and determining the category that it is to be placed in. In situations where a “disease is likely to be fatal or irremediableBS 5837 suggests that “it might be appropriate for the trees concerned to be categorized as U, even if they otherwise have considerable value“.

With regard to ash dieback, this therefore poses the following questions, which are addressed in sequence below:

  • how do we establish the presence of ash dieback on ash trees of all ages?;
  • how widespread is ash dieback?;
  • is ash dieback fatal?;
  • is ash dieback remediable?; and
  • does ash dieback have an impact upon BS 5837 categorisation?

Establishing the presence of ash dieback

The symptoms of ash dieback are most observable in young trees and on coppice regrowth – symptoms are covered in sufficient detail within the Observatree guide and Tree Council guide. Summarily, symptoms include browning/blackening foliage, wilting foliage, diamond-shapes lesions along shoots, and patchy crown dieback. In larger trees, identification is not so straightforward (in some cases) though symptoms generally arise as the crown becomes patchy with live growth (or generally becomes quite thin) and epicormic shoots are produced from larger branches – see the Tree Council guide. For larger trees in particular, it may therefore be quite difficult to identify the symptoms of ash dieback particularly in earlier stages of infection – as may it be difficult to parse symptoms of general crown thinning from other causes such as honey fungus. However, because ash dieback is generally not asymptomatic – and England may have seen symptoms of ash dieback as far back as the mid-2000s and not put the dieback down to this cause – as a rule of thumb it is reasonable to conclude that a patchy or thinning crown may be at least partially caused by ash dieback (and particularly, given its sheer distribution across England – see below).

ash tree some lower dieback
Is the dieback within the lower left crown of this mature ash tree associated with ash dieback or arising from another reason? Symptoms on mature trees are harder to identify.

In urban areas, ash dieback may potentially be less prevalent, because leaf litter is removed during the winter that would otherwise harbour fruiting bodies that produce spore for later infection the following year, and also because of a drier and windier microclimate that does not ‘suit’ the ash dieback pathogen so effectively.

The extent of ash dieback in England

The Forestry Commission has an online map (see the below image) that shows at which areas of England ash dieback has been recorded, which suggests that ash dieback is across approximately 70% of the country. In this respect, it is widespread – it is likely more widespread than recorded, given the map is reliant upon reports being submitted rather than by ongoing ‘active’ surveying to determine presence. Furthermore, the ‘density’ of ash dieback is not recorded and therefore some areas of England will be heavily-infected whilst others potentially not so much. Areas of England with more ash woodlands are more likely to have the capacity to be more extensively infected, by virtue of a greater ‘resource’ for the ash dieback pathogen to exploit.

asi dieback map 2019
Ash dieback in England as of July 2019 (yellow squares are where ash dieback has not been officially recorded).

The effects of ash dieback

Research across Europe suggests that the mortality rate of ash does vary. In no countries has ash dieback resulted in the total mortality of ash; however, it has caused mortality of woodland ash at rates of up to 70% in woodland stands and 82% in young regeneration. Consequently, it is projected that mortality rates for woodland ash of between 50-75% will become evident across Europe, by around 2040-2045. Symptoms of ash dieback on infected trees will likely persist for years prior and are generally progressive though may involve periods of ‘recovery’, given mortality is not instant but a gradual process that might take even a decade (or closer to two). The government does however suggest mortality rates of up to 90% are possible. Moreover, it is likely that no ash tree is ‘immune’ though mature trees will likely have a greater resilience to the effects of ash dieback. There is no widespread empirical data for the mortality rate of urban and open-grown landscape ash trees though this rate will likely be lower. In urban locations, evidence from Ukraine suggests that younger trees are most susceptible.

The remediability of ash dieback

The capacity to remediate the adverse effects of ash dieback on infected ash trees is currently very limited. Pruning measures may be able to control localised infections on larger trees by simply removing these infected parts of the tree; however, this in itself does not aid the tree in resisting future infection and this approach has its own impacts including the increased risk of fungal decay to the ash tree. Treatment options (including preventative) for younger ash trees are being developed and tests do demonstrate reasonable efficacy though trials are very much on-going and the situation currently remains rather bleak.

The impact upon BS 5837 categorisation

Research available suggests that ash will be subject to mortality rates of 50-75%, by 2040-2045 – this mortality rate could however be much higher although this is not possible to substantiate for England. In this respect, at least half of all ash will be expected to die within the next 30 years. Because the effects of ash dieback are evident for usually years before mortality (as much as 15 years), it is therefore reasonable to estimate that at least 50% of all ash will be showing symptoms within a shorter time-frame. For ash showing symptoms already, mortality will likely be sooner than those not showing symptoms.

With regard to the impact this has upon the arboriculturist assessing ash trees in the context of planning and development, where ash trees are displaying symptoms of physiological decline (as evidenced by dieback or patchy growth within the crown) it would be reasonable – even where these trees are mature – to assume they will be dead within 15 years. This would mean the ash tree would be within the C category (at least 10 years remaining) or U category (less than 10 years remaining). The lack of data on mature ash means that it’s not viable to assert that this statistic cannot apply to such ash trees, in spite of limited research suggesting that they may be more resilient.

The trickier question arises where ash trees are not showing symptoms of ash dieback. If we take the worst-case-scenario of 90% mortality and accept that no ash tree is fully ‘immune’ to the ash dieback pathogen – in accepting also that the arboriculturist may be assessing the tree in winter when the effects are harder to spot or in a year when the ash tree is ‘recovering’ – then it remains very difficult to foresee ash existing for any longer than 40 years within the landscape. In this respect, the capacity to give an ash tree the highest categorisation (A category) is not apparent, unless it’s an ancient tree or a tree that can be considered a veteran.

ash tree not in leaf
Where an ash tree is not in leaf, determining whether it is a B or C category may be difficult, if there are no significant symptoms of physiological stress or decline as evidenced by crown architecture.

Consequently, we are left with the moderate-quality category (B). This category may very well be the most appropriate category for mature ash trees in open-grown or urban situations with no current symptoms of ash dieback, whilst still appreciating that estimating the tree to have at least 40 years of life remaining is – in the current scientific context – probably over-zealous. This is, of course, assuming that the ash is not subject to other drivers that are ‘undermining’ its estimated life expectancy, which may include fungal decay by Inonotus hispidus (that is rather frequently observed on mature ash trees in urban areas across England) or observable root damage.

Concluding remarks

The current situation is complicated, because the arboriculturist requires more research as regards the effects of ash dieback on ash trees (particularly mature ash in urban areas and the countryside beyond the woodland setting) in order to form conclusions that are based less on assumption / anecdote (or ‘transferable’ research from ash woodlands) and more on robust scientific data. Until this point is reached, the arboriculturist has to make-do with the hand they have been drawn, which demands reasonability of assessment in accordance with BS 5837. It is unlikely that an arboriculturist will be able to justify the positioning of an ash within the highest (A) category and therefore it is the case that almost all ash trees will be placed within the moderate (B), low (C), and unretainable (U) value categories. Ash trees already displaying symptoms will almost always be most appropriately placed in the low (C) and unretainable (U) categories. Those ash trees that ride the blade between the B and C categories are more challenging, because this may have a significant impact upon whether the ash tree is considered ‘appropriate’ for inclusion within the context of planning and development.

In the intermediary period, arboriculturists must guide the design team to ‘future proof’ developed landscapes against current pest and disease threats and also future threats. In the context of ash trees, this may involve removing and replacing with a different species some specimens (for argument’s sake, say 50%) where they are dominant within a proposed development site – even if they currently do not show symptoms of ash dieback.

ash winter countryside dieback
Are these ash trees B or C category trees – or is the one on the left a B category and the one on the right C category? These are the decisions arboriculturists are required to make even when foliage is lacking.
BS 5837 categorisation and ash dieback in England

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