I was down in the New Forest these past few days on a holiday and paid a visit to this exquisite hollowing yew tree at the Saint Nicholas’ Church in Brockenhurst, which has been carbon dated to determine its age – this process suggests that the yew tree is over 1,000 years old. Given the slow rate of growth for yew trees, in addition to its sheer size in particularly stem girth, this age is seemingly fair. If you’re in the vicinity, please pay a visit to this tree, which has stood as a timeless relic across eras that would humble even the loftiest of minds should man have been graced with this enduring ability. Those seeking numinous experience needn’t look inside the adjacent walls but directly at this yew.
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 irremediable” BS 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).
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.
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.
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.
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.
As an arboricultural consultant, this article is written through the lens of a professional working in this context (i.e. my professional opinion). That being said, there is a clear cross-over to the public sector aspect of arboriculture (i.e. the tree officer). This article is also only a cursory glance at the issue so please follow-up with your own research. It goes without saying that the recommendations of BS 5837:2012 apply in the context of development and therefore this document is not directly referenced below.
Legal policy context
The UK government’s position on planning and development in England is kept ‘relevant’ through the National Planning Policy Framework (NPPF), which was most recently revised in 2018 and updated to a minor extent in February 2019. The purpose of the NPPF is to guide Local Planning Authorities (LPAs) to ensure that the manner in which they manage development within their jurisdiction is sustainable. Sustainability – in the NPPF – is defined broadly as “meeting the needs of the present without compromising the ability of future generations to meet their own needs” and is broken down in to three key parts: economic, social, and environmental (usually where trees come in) sustainability. To effectively pursue this trifold ‘archetype’, the NPPF specifies that “planning policies and decisions should play an active role in guiding development towards sustainable solutions, but in doing so should take local circumstances into account, to reflect the character, needs and opportunities of each area“.
In order for LPAs to achieve this, they must therefore produce a Local Development Plan (LDP) as per the legal requirement of the Planning and Compulsory Purchase Act 2004 (sometimes as both a Core Strategy document and a Development Management Policies document – usually coming into action at the same time but not always and with the former covering strategic policies and the latter non-strategic policies), which provides planning policies at the local level and guides local development (the NPPF specifies that planning and development “should be genuinely plan-led“). The Localism Act 2011facilitated in the descension of local policy also to the neighbourhood level with Neighbourhood Plans (NPs) (see also this link and this link), which is relevant but needn’t be discussed in detail right now (NPs are considered non-strategic and relevant in the planning context but are not yet widespread across England – use them where they exist). The NPPF specifies that the LDP must be supported by Supplementary Planning Documents (SPDs – defined in the NPPF as documents that “add further detail to the policies in the development plan“), where they are required and appropriate, in addition to Conservation Area (CA) appraisals and management documents. SPDs in particular must not conflict with the current LDP else they are generally redundant, as per the The Town and Country Planning (Local Planning) (England) Regulations 2012.
Within all of these documents, there will almost always be a reference to trees. This may come as a specific tree policy or as a green infrastructure policy (or anything in between). Sometimes, policies may relate only partly to trees but still be relevant – for example, a policy that specifies that landscape character should be properly assessed and protected within any proposed development.
Arboriculture and planning policy
With all of the above policy context, it is important to understand what the role of the arboriculturist is in the context of development management leading up to the point where the proposed development can legally begin to be built (following the discharge of pre-commencement conditions).
On the private side, the role in this process is as a consultant who works with the design team from RIBA Stages 1 through 5 (sometimes from 0) and usually – but depending on the scale of the project – with the project manager, planning consultant, architect, landscape architect, structural engineer, M&E engineer, and main contractor (in preparing for tender). The arboriculturist must, during this process, be aware from the offset of the relevant planning policy and documents from the LDP and any other related documents (e.g. SPDs, CA appraisal, and NP in some cases). The arboriculturist must therefore undertake a desktop study, to identify all relevant policies and documents and extract important details. For example, if we were looking at a proposed development in the Mill Hill area of the London Borough of Barnet, it would be necessary to consider the following documents at the local level:
The Core Strategy and Development Management Policiesdocuments;
The Mill Hill CA Character Appraisal Statementdocument.
Within these documents, the arboriculturist will need to identify those relevant planning policies and details that will have an impact upon the scope and eventual nature proposed development. Focussing on the LDP documents only, one will be able to see that strategic policies CS5 and CS7 are relevant in this case, in addition to the non-strategic policies DM01 and DM06 (the latter because Mill Hill is a CA). Specific details from the SPD and CA appraisal can then be selected from the texts. It is the role of the arboriculturist to ensure that they operate to these policies and details and advise the design team of this – particularly, the project manager and planning consultant, who will drive the proposed development designs forward in this respect and keep the project ‘on course’ and the trees suitably considered and accounted for (i.e. no clearfell operations where there are many good trees). Ultimately, the arboriculturist should include within their supporting statement (an Arboricultural Impact Assessment or ‘AIA‘) accompanying a planning application these policies and details and demonstrate how the proposed development complies with them.
On the public side of the industry, the tree officer at the LPA will assess the proposed development in line with the relevant planning policy from the LDP and details from the relevant SPDs etc. The tree officer will usually operate within the Development Management department of the LPA and report directly to the case officer handling the planning application – the tree officer is, in this capacity, a consultee. Specifically, the tree officer will review the proposed development plans provided by the architect and review any relevant supporting documents (such as a Design & Access Statement but most importantly the AIA – where this is lacking and considered relevant, the tree officer will almost always request one through the case officer). It is therefore important that the AIA discusses planning policy and other relevant planning details, because this demonstrates to the tree officer that the proposed development complies with policy and that it should be supported and granted consent.
Ideally, the arboriculturist on the private side and public side will directly liaise, where there is reason to do so. For example, the consultant may be contacted by the tree officer to discuss a specific tree and the juxtaposition of a building elevation, or to discuss hard surfacing proposed within close proximity to a tree.
Why planning policy is important
It has been previously established that adhering to planning policy is important. However, it is important to understand why, in context to the above discussion.
Principally, planning policy is the measure through which the NPPF specifies that proposed developments should be managed and determined by the LPA. For example, if the proposed development accords with all relevant planning policies, the NPPF specifies that the proposed development should almost always be granted consent. By extension, it is important for any proposed development to accord with relevant planning policy prior to the proposed development going in as a planning application, in order to ensure the highest probability of a Decision Notice being issued granting consent (and subject to conditions or reserved matters for full and outline applications respectively – it is more complicated for hybrid applications). Planning policy details still apply following consent where conditions etc apply and this is notably the case with complex developments including basement construction and large estate developments.
It is also important for the arboriculturist to utilise planning policy to its fullest extent, because this assists in elevating the industry towards a higher professional end. Specifically, by integrating planning policy and arboriculture the arboriculturist becomes a very valuable member of the design team and not simply limited to the fringes (or at least reduce the likelihood of being perceived that way). After all, arboriculture is a ‘young’ industry and there is clear scope to progress the industry so that it is valued in a similar fashion to architecture – undoubtedly, trees are key components of all ecosystems including urban and there is much crossover between arboriculture and landscape architecture.
As a cautionary note, it is not always the case that all planning policies can be satisfied. For example, a policy specifying that all trees of good quality will be retained and protected during any development operations may be outweighed by an area formally designated for significant development within the LDP (such as for a large multi-storey car park, to support economic growth within a town centre). In such a case, it is important that both the private and public arboricultural professional understand the constraints posed by the hierarchical nature of some planning policy (i.e. strategic may outweigh non-strategic), which might involve discussions with the planning consultant and case officer respectively. In ambiguous cases, it is most critical that the (private) consultant arboriculturist advises the client of the need to strongly consider a pre-application discussion with the LPA to avoid unnecessary future conflict at the planning stage – indeed, the NPPF explicitly suggests this in paragraph 41: “the more issues that can be resolved at pre-application stage, including the need to deliver improvements in infrastructure and affordable housing, the greater the benefits [and] for their role in the planning system to be effective and positive, statutory planning consultees will need to take the same early, pro-active approach, and provide advice in a timely manner throughout the development process“.
To effectively summarise the above, it can be stated that:
the consultant arboriculturist within the design team must fully consider appropriate local planning policy and feed this policy detail to the design team and cover it within the AIA;
the tree officer at the LPA must assess any proposed development in line with relevant planning policy and details arising from SPDs etc;
by emphasising the importance of planning policy both on the private and public side as regards arboriculture (through referring to it throughout the development process leading up to a Decision Notice), the industry will be considered in a more valuable light by all other professionals at that time and hopefully in the future; and
therefore the arboricultural professional will be considered on a similar level to the architect and landscape architect (more broadly the entire range of professions involved in planning and development), which means trees are more likely to be protected during development operations and by extension feature more readily in developed areas and for longer periods.