Not dead and still dreaming

This blog I have not updated in some years. However, it isn’t being neglected. I simply don’t have much to say at the moment that hasn’t already been said, or I’m otherwise learning other aspects of life from everything that I do. I hope that people still find use in the material here and I hope that in the future this also applies; I hope to publish some more material, after some years.

I am, for those that are wondering, still working in the arboricultural sphere. In particular, I am working in the planning and development sphere. This sphere is sometimes a rather rabid beast and there’s a lot to digest. It’s interesting, as I’ve found I’ve learned more about matters not directly associated with arboriculture, though in learning about other matters I have improved vastly in more holistic terms and that has helped me do better in the arboricultural sphere.

That, I think, is the point of this quick post: don’t go through your arboricultural career with puritanical blinkers on, for I think that it’ll be the undoing of things. It may well be the undoing of you, too – sooner or later, anyway.

In saying that, I mean it when I say that the planning and development sphere is rabid. It will devour everything, if it’s not kept in line. Some things are worth devouring and other things are not; and people will disagree until the world ends on what those things are.

Have fun!

Not dead and still dreaming

Trees and construction logistics – foreseeing the challenges

There is a growing desire, as enshrined in planning policy at all spatial scales within England, to retain trees that are considered to be of a ‘good’ value (just look at the emerging London Plan). The value of trees may be determined through the lens of BS5837 or instead on the grounds of public visibility, and the contribution of the trees to the public realm and the character of the local landscape (especially, if within a Conservation Area). Therefore, what usually happens is that such trees are considered for retention as a baseline principle, subject perhaps to some losses (with mitigation) to enable the placement of new built structures comprising buildings and surfaces (and services that feed into and out from these places). The situation is of course much more nuanced than this, with an expectation at least within the design team that a project has to be economically viable for it to be worth pursuing – if trees stunt any ability to generate a profit after deductions, it’s not worth investing the time and more importantly the money. With this in mind, it’s also usually the case that a larger massing (often from a large floorspace) can provide a greater return on investment, as land values are quite high in some areas of England (London and the home counties, especially). Thus, what we may end up seeing is a maximised floorspace up to the areas where trees of value constrain further expansions. Let’s now run with this, into the following discussion.

From the above information, we can assume that the site has been designed for redevelopment in a manner that maximises use and utility (thus return on investment – i.e. profit, in some measure), up to the point where the trees stop further development. Great, you’d think, when looking at this on paper – the designs accommodate the trees and there’s a juxtaposition that in overarchingly harmonious. Maybe, it’s a little tight, but it’s all viable stuff in principle. A decision notice granting consent is therein issued, with the consultees being in agreement, subject to various conditions including the requirement for some pre-commencement arboricultural information (a method statement, at this stage, based on the RIBA Stage 4 details).

This is where things can sometimes get interesting.

Enter the main contractor (or consultants producing construction method statements) and a lens for construction traffic routeing (including material management and storage). The build process needs to be in accordance with health and safety laws, best practice, and planning policy – it needs to be logistically safe and fully functional. Now, it may be the case that areas directly under trees are where heavy vehicles need to be positioned or tracked through, be it for delivery, removal of materials, or any other reason. Perhaps a large piling rig needs to pass through into the site, below a tree (and maybe a piling mat set up where a tree is rooting) – or maybe a tower crane needs to be set up and tree crowns are directly impeding the effective use of the tower crane. Further, don’t forget about scaffolding and site huts, including welfare facilities (notably, on larger sites). These details were probably not considered before, as there wasn’t an investigation into construction logistics – instead, the focus was on securing good designs to fit the brief of the client and fit with the relevant expectations of planning policies and guidance documents.

surface tree roots construction site
If you need to track vehicles through this area, were the surface roots ever considered appropriately? They may have a major impact upon how a temporary access road could be constructed.

Now, it may be that the trees that previously and on paper looked like they could easily be retained are under pressure. It may be that some high value trees (e.g. Category A, as per BS5837) are needing to be pruned or even felled. It’s unlikely that the Local Planning Authority is going to appreciate this turn of events, as it could reek of a bait-and-switch act. Moreover, the decision could have been a contentious one that was called into committee and was a subject of hot debate. In any event, it is optically undesirable and can make matters all too bittersweet. Perhaps, pre-commencement conditions cannot get discharged, with the project becoming a non-starter after a lot of work has been invested – this is unlikely but could happen.

In short, without trying to drive things home by repeating myself, it really needs to be the case that the viability of a development (in terms of construction viability) is explored, at the earliest stage possible. Logistics of getting something built are key and it needs to be considered early on – effectively, before a full (or householder, outline, or hybrid) planning application is submitted. The details may not all be there; however, there needs to be enough confidence that areas where retained trees are to constrain access during development are not going to be placed under pressure, due to changes when conditions are being dealt with.

Think ahead and aim to stress test construction logistics at an early stage and how these sit up against retained trees – ask the right questions of the right people, progressing with as few unknowns as possible (at least, in an ideal world). Some detail is better than none.

Trees and construction logistics – foreseeing the challenges

BS 5837:2012 – is it fit to protect trees in the current legal context?

The views below and mine and mine alone. I am a professional within the industry of arboriculture and I do not pretend to know everything or even close. Therefore, please do not take my words as if they are fact. You’re entitled to disagree, to any degree – insofar as the disagreement is reasonable. The hyperlinks are for ease to you the reader (please click them). You may find a previous blog post or two of mine useful, as context, before reading this. I have kept the below as brief as I consider possible that will still provide the necessary degree of context and explanation to demonstrate my opinions.

Scope of this post

This post does not dissect or scrutinise the technical elements of BS 5837:2012 Trees in relation to design, demolition and construction – Recommendations. This post does analyse whether this standard is fit for purpose, in the current legal context.

A background to British Standards

British Standards (hereafter ‘standards’) are published by the BSI Group, an organisation that operates under Royal Charter with the defined purpose of producing and publishing standards and providing related services for the ultimate betterment of industry – specifically, “to provide a reliable basis on which common expectations can be shared regarding specific characteristics of a product, service or process“. The key is therefore in the name: standardisation (which facilitates replicability). Adherence to a standard is voluntary.

Focussing on standards, the Royal Charter permits the BSI Group to “set up, sell and distribute standards of quality for goods, services, and management systems and prepare and promote the general adoption of British and international standards and schedules“. Periodically, published standards must be revised, altered, and amended “as experience and circumstances may require“.

BSI Group royal charter snip
The header and introduction of the Royal Charter for the BSI Group.

How standards are made

Standards are developed in collaboration between representatives of the relevant organisations associated with the scope and objective of the standard, adhering to – and being the only standard that must be adhered to (i.e. adherence is not voluntary) – the standard for producing new standards (BS 0:2016 A standard for standards – Principles of standardization).

Development is the responsibility of the relevant technical committee (e.g. B/213 for trees), which will normally oversee more than one standard. The technical committee is comprised of relevant organisations, companies, and individuals that fairly represent the range of professions affected by the standards it oversees. Within the technical committee, there are working groups that will focus on more detailed elements (e.g. the development of a single standard or aspects of it). The technical committee will therefore develop a draft standard that they will then publish for public consultation (a ‘draft for public comment’ or ‘DPC’), which will last for a minimum of 60 days.

BS 5837 Technical committee trees B213 construction
The B/213 technical committee page on the BSI Group website.

Following consultation, the feedback will be collated, analysed by the technical committee, and modifications made to the standard so that it is then published – or, in some cases, work is halted, due to significant concerns over the contents of the draft standard. The decision is down to the consensus of the technical committee

Typically, the production of a standard takes 18 months. Throughout, the operations of the technical committee must be transparent and defensible. The result should be a standard that is desirable, practical, and authoritative, provides no exclusive benefit to any party, serves the needs of the relevant community, and is consistent with regulations and legal principles established at the time of publication.

How standards are reviewed

Published standards must be maintained, in order to remain relevant (and for the BSI Group to adhere to its responsibilities as provided by Royal Charter). Therefore, all standards must be reviewed at least every five years, which can be initiated by the BSI Group or the technical committee. The review is undertaken by the technical committee responsible for the standard. Public consultation remains necessary, for the review of a standard. In some cases, the review may result in the standard being withdrawn.

Trees and construction – the standard

The standard for trees in the context of construction (encompassing all RIBA stages) is BS 5837:2012 Trees in relation to design, demolition and construction – Recommendations (hereafter ‘BS 5837’). This standard replaced the 2005 version. Its purpose is to provide “recommendations relating to tree care, with a view to achieving a harmonious and sustainable relationship between new construction/existing structures and their surrounding trees“. To this end, it sets out the recommended process through which trees should be managed in the context of construction, which includes outside of the planning process (e.g. if building something under permitted development). However, it is principally used as part of the planning process where trees are being managed on or adjacent to construction sites where planning permission is required.

bs58372012 front page
The front cover of BS 5837.

BS 5837 – legal developments since its publication

Since the time of the publication of BS 5837 in April 2012, there have been many important legal developments. These include but may not be limited to:

Implications for BS 5837 relevancy

Of these three legal developments, the NPPF is the crucial development, because it states at paragraph 47 that planning applications must (in 99% of cases) be determined in accordance with the relevant planning policies, which includes at national, regional, and neighbourhood level, in addition to the core that is determining applications against local planning policies.

BS 5837, in its role within the planning process (excluding permitted developments) as a standard document, does not make reference to this crucial element (e.g. there is no reference in the Figure 1 flowchart to consulting planning policy). The first major pit-stop for managing trees in the context of development is relevant planning policy that will give a valuable indication of what is and is not acceptable, in planning terms. Therefore, BS 5837 is not fully fit for purpose, in this regard.

The NPPF also states at paragraph 54 that “local planning authorities should consider whether otherwise unacceptable development could be made acceptable through the use of conditions or planning obligations“, and at paragraph 55 “should be kept to a minimum and only imposed where they are necessary, relevant to planning and to the development to be permitted, enforceable, precise and reasonable in all other respects“. For this reason, the Town and Country Planning (Pre-commencement Conditions) Regulations 2018 refined the approach to conditions insofar as local planning authorities must confirm the details of pre-commencement conditions with the applicant prior to imposing the conditions upon them. Specifically, the manner in which Arboricultural Impact Assessments and Arboricultural Method Statements apply within the current framework of BS 5837 now may routinely differ in practice from what is recommended in theory.

These reasons collectively have a significant material effect upon the manner in which trees should be managed and considered, as part of the planning process. BS 5837 in its current form does not consider these effects and therefore it does not “provide a reliable basis on which common expectations can be shared regarding specific characteristics of a product, service or process“.

local plan policy trees excerpt redbridge
An excerpt of a local planning policy that requires trees to be protected should they be retained.

So what?

BS 5837 was reviewed in 2017, according to the BSI Group. In this context, it is not now until 2022 that it must be reviewed. Indeed, it can be reviewed before then though this will arise probably only following concern being raised over an insufficient scope.

My opinion is that BS 5837 cannot ‘wait’ another five years before being reviewed, becuase assuming it takes 18 months to produce a new version of BS 5837 then it will not be until 2023/4 that BS 5837 accounts for the significant changes in the planning process. In this context, BS 5837 will not “provide a reliable basis” for over half a decade from now during which time development is still ongoing and ideally should be speeding up to meet government housing targets.

BS 5837:2012 – is it fit to protect trees in the current legal context?

Planning conditions in arboriculture

Trees are living organisms that often get tied-up in with new planning and development projects. The design of the proposed development and how it relates to the trees on and adjacent to the application site aside, if the Local Planning Authority (LPA) considers the proposed development acceptable in policy terms then they will grant consent (or recommend consent, in cases where the application goes to committee). Consent comes in the form of a decision notice, which sets out the limitations of the proposed development and what further elements must be satisfied before the development is completed and handed-over to the applicant. Depending on the type of planning application, the decision notice may grant consent in full (including householder applications) or in outline. Decision notices for full (and householder) applications come with planning conditions attached, which have to be discharged at the relevant stages of the development process (e.g. pre-commencement, pre-brick and beam, pre-occupation) or otherwise control the proposed development in accordance with already-submitted documents (i.e. performance conditions), and outline applications come with reserved matters that for all intents and purposes draw-out the planning process and reduce up-front expense costs to the applicant. When applications for reserved matters are submitted, the LPA can on the back of this consent the application subject to relevant and appropriate planning conditions. For example (and explained more below), an LPA may request as a reserved matter an Arboricultural Impact Assessment (AIA), which following approval as a reserved matter the LPA can then condition an Arboricultural Method Statement (AMS).

At this point, it is necessary to understand what the difference is between an AIA and an AMS. An AIA is a report that makes a judgement with regard to the impacts to the trees from the proposed development and the means of mitigating these impacts, and typically is provided at the planning stage prior to a decision notice being issued to the applicant; this is because an AIA gives context to and in hopefully many cases justifies the proposed development (subject to appropriate design and adherence to policy), and therefore hopefully informs the arboricultural consultee at the LPA and the case officer that the proposed development should be considered for consent in the context of trees exclusively (i.e. provided once RIBA Stage 3 has been completed, which is often when a full planning application is submitted to the LPA). An AIA will usually provide a draft tree protection plan (TPP), and also an ‘outline’ (‘heads of terms’) AMS that likely works on an element of assumption that may sometimes be sufficient – the TPP at least will detail the means of mitigation discussed within the AIA.

RIBA Plan of Work
The RIBA Plan of Work 2013 (source).

An AMS, on the other hand, is a document that provides a clear and detailed methodology for RIBA Stage 5, which is the construction phase of the project, and uses the technical designs of RIBA Stage 4 to specify appropriate construction methodology to protect the retained trees within the application site and those ones adjacent that will be affected by the development. In this context, an AMS will include the relevant details including construction traffic management, material storage, foundation design, service installation, and landscaping, and specify the manner in which these operations will be undertaken and the tree protected so that they don’t die or have a shortened life expectancy due to damage during the construction phase, including by providing a detailed TPP and specifying a clerk of works schedule. A detailed AMS can only be provided as a planning condition, because before this stage the AMS will have to speculate on certain matters, which is generally not acceptable and doesn’t work effectively when tendering a development out for construction. More detail on both types of report can be found by reading BS 5837:2012.

In order to appropriately protect trees during the development process, which is required by law, the LPA must pay careful attention to the use of planning conditions and reserved matters. These must always be relevant in context, as must they always be worded appropriately. A failure to word conditions and reserved matters in this manner means that they are not fit for purpose. In order to understand how the LPA will best use conditions to control development and protect trees, let us entertain a few examples that relate to full planning applications.

Cellweb access and trees
Ground protection installed adjacent to trees to allow for vehicular access through a wooded area, which will be specified in full detail as part of a detailed AMS but covered at least by way of suggestion in an AIA to manage tree impacts.

Scenario 1: AIA provided at full planning; AMS required as pre-commencement (very frequent)

Imagine that you have a large plot of land that currently has three residential dwellinghouses with a shared communal garden, and you want to demolish all three dwellinghouses and construct a duo of larger dwellinghouses on separate plots. There are many trees throughout the site, including adjacent to the existing dwellinghouses, and you want to retain most of the trees to incorporate into the two new plots (removing only the small ornamental trees and those that are in clear structural and/or physiological decline). You submit an AIA with the planning application, which demonstrates that the proposed development can be achieved, and is acceptable in policy and arboricultural terms. A draft TPP is provided alongside an outline AMS that shows that the retained trees can be protected during demolition and construction work, though you have yet to develop technical designs. The LPA logically recognises this and attaches a pre-commencement detailed AMS, which must be discharged before any work can begin on site, to ensure that the retained trees are sufficiently protected.

Scenario 2: AIA provided at full planning with outline AMS; performance condition attached (frequent)

Imagine that you have a vacant plot of land that has two large oak trees at the top right corner. You want to build a house at the bottom left corner, far away from both oak trees. The LPA has rightly requested an AIA to support the full planning application, because they must appropriately consider trees in the context of planning and development. The AIA demonstrates that the proposed development will in no way impact upon the two oak trees, and is therefore acceptable in policy and arboricultural terms. The AIA provides an outline AMS including a TPP that allows easily enough room for construction activities whilst sufficiently protecting both oak trees. The LPA logically recognises this and attaches a performance condition to the decision notice requiring full adherence to the AIA including the TPP.

Scenario 3: AIA provided at full planning with a detailed AMS; performance condition attached and pre-occupation condition attached (very infrequent)

Imagine you have a large plot of land that currently has a private boarding school throughout its grounds, and you want to demolish all of the school buildings and construct a new school in its place, because the existing buildings are all in poor condition. There are trees throughout the site though generally away from the existing buildings, and those adjacent to the buildings are to be removed to facilitate the development. You do not want to deal with the myriad details arising from pre-commencement conditions for such a large project that may take many months to fully discharge, and want to simply start building as soon as you get the decision notice through granting consent. You are very confident that you will obtain planning consent, because the LPA has supported the proposed development throughout the planning process including at pre-application, and there is significant community support for the proposed development following completed public consultations. Therefore, you have completed many of the technical details of RIBA Stage 4 already, and provide these details at full planning with an AIA and a detailed AMS demonstrating that the proposed development is both acceptable in policy and arboricultural terms, and can be completed and the retained trees appropriately protected. The LPA logically recognises this and attaches a performance condition to the decision notice requiring full adherence to the detailed AMS, and also attaches a pre-occupation planning condition requiring that the clerk of works detailed within the AMS be submitted as proof that the AMS was adhered to throughout and that the trees were sufficiently protected (e.g. through submitting a series of reports that accompanied each clerk of works visit).

Now let’s take a look at some scenarios that should not occur at full planning, else run the risk of trees being inappropriately considered and conditions being insufficient to protect the trees, for clear and evident reasons:

Scenario 4: No AIA at planning; AIA required as pre-commencement

You are the landowner for Scenario 1 (above). You do not submit an AIA to the LPA at full planning, because you want to see if you can obtain planning consent without this expense. The LPA reviews the proposed development details, and grants consent with a pre-commencement condition requiring that an AIA be provided. There is no mention anywhere of an AMS as a pre-commencement condition.

Scenario 5: Detailed AMS requested by LPA at planning; intent to attach performance condition

You are the landowner for Scenario 1. You submit an AIA to the LPA at full planning. The LPA however request a detailed AMS or they will refuse the application. You have only developed detail to RIBA Stage 3, and have yet to design to technical detail the project, because you don’t want to spend that money before obtaining planning consent. You ask your arboricultural consultant to provide a detailed AMS, which they try to do but cannot fully complete. The LPA accepts this and attaches a performance condition to the decision notice that requires full adherence to the details of the ‘detailed’ AMS.

Scenario 6: No AIA at planning; no arboricultural conditions attached

You are the landowner for Scenario 1. You do not submit an AIA to the LPA at full planning, because you want to see if you can obtain planning consent without this expense. The LPA reviews the proposed development details, and grants consent without any arboricultural conditions attached. You can therefore begin works at the site once all other pre-commencement conditions have been discharged, and without any tree protection measures.

Now, let us understand what appropriately-worded planning conditions might look like. Based on the above scenarios, we have three appropriate types of appropriate planning condition:

  • pre-commencement AMS: No works will take place at the application site, until a detailed AMS has been submitted to the LPA and approved in writing. The AMS will include all relevant details to protect the retained trees, including a detailed TPP. Relevant details may include but are not limited to construction methods, construction traffic management, demolition methods, finished levels, ground protection, landscaping methods and materials, material storage, service runs, and tree protection barrier fencing. The AMS will also include details of a clerk of works schedule that specifies arboricultural supervision at appropriate stages of the development process. Any variations to the details of the AMS must only be undertaken after the proposed variations have been agreed in writing by the LPA.
  • performance condition: The proposed development shall be completed in full adherence to the arboricultural details submitted to the LPA (documents and plans specified here). Any variations to the details of the documents and plans must only be undertaken after the proposed variations have been agreed in writing by the LPA.
  • pre-occupation condition: Prior to the occupation of the site, the LPA will be provided with clear and obvious proof that the details of the AMS have been adhered to, including the clerk of works supervision schedule. Proof will be demonstrated through the submission of a series of brief reports or single larger report, which summarise the details of each clerk of works visit, including where relevant photographic evidence of adherence to the AMS and TPP.

With regard to outline applications, the process as detailed above broadly applies, though with the following deviations. Specifically, the LPA may require an AIA as a reserved matter, and subsequent to the approval of this condition a detailed AMS. However, where an outline application will impact upon trees, which may include tree removal or site new built structures adjacent to trees to be retained, the LPA is (in accordance with planning law) permitted to request an AIA to support the outline application to demonstrate that it is acceptable in policy and arboricultural terms. In such an eventuality, it will instead be necessary for the decision notice for the outline application to include a reserved matter requiring a detailed AMS be approved prior to any works starting on site.

Trees and construction
Details of tree protection measures will be specified within a detailed AMS.

In situations where a large site is to be completed as a phased development, the LPA must co-ordinate with the applicant, in order to ascertain the most effective use and wording of planning conditions and reserved matters, because there is a real risk of the phased nature of the development staggering the development of technical designs that will impact upon trees. This matter is too complex and varied to cover here, though the above principles should be applied, to achieve the desirable end as regards trees and the protection of.

From the above, it should be clear how planning conditions relate to arboricultural matters, in the planning process. However, if you have any queries, please leave a comment below.



Planning conditions in arboriculture

The ancient 1,000+ years old yew at Brockenhurst

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.

I’ll let the pictures do the talking.


The ancient 1,000+ years old yew at Brockenhurst

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

Local planning policy in arboriculture

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 2011 facilitated 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.

Planning policy will often specify that trees of a good quality (such as some of these mature oaks) should be retained and protected within the context of 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 Policies documents;
  • The Green Infrastructure SPD;
  • The Mill Hill CA Character Appraisal Statement document.

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.

To address planning policy sufficiently, the arboriculturist must ensure that the design team is aware of the need to protect good trees in the context of proposed development (such as this mature copper beech tree that is adjacent to an access route).

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“.

By appropriately considering planning policy relevant to trees, specimens such as this high-quality oak tree stand the best chance of a prosperous future and therein for the betterment of the proposed development.

Take-home messages

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.
Local planning policy in arboriculture

Trees and subsidence – situations involving trees with a Tree Preservation Order (TPO)

Tree-related subsidence is quite a common occurrence on shrinkable soils (e.g. clay) and I have addressed this some time ago already (to a limited specificity). In cases where the tree(s) in question is a low-value tree and / or there is no degree of statutory protection afforded to the tree, the landowner may very well consider it appropriate to remove the tree or otherwise prune the tree to address water uptake (see the Hortlink study and additional Building Research Establishment publications, for more information). In a situation where the tree(s) implicated in cases of subsidence where the evidential tests have been satisfied ([1)] has actual damage occurred that is consistent with subsidence damage?; [2] have roots encroached under the foundations that are damaged?, and; [3] was the damage from adjacent tree[s] foreseeable?) and the tree(s) is subject to a Tree Preservation Order, it may very well be necessary to consider the management of the situation in a more holistic lens. Specifically, this is because of the precedent set by the case Perrin & Anor v Northampton Borough Council & Ors [2007] EWCA Civ 1353 (19 December 2007).

As a precursor before continuing, it is important to recognise that the case being discussed below must never be considered in isolation – therein, nor should the decision be made in blunt isolation. Subsidence damage as caused by a tree subject to a Tree Preservation Order is an issue that demands cross-profession liaison; specifically, between at least the arboriculturist, planning officer / manager, and insurance officer / manager for the Local Planning Authority (i.e. Local Council), in addition to the tree owner and their insurer – plus the homeowner with a damaged house and their home insurer – where the tree is a third-party tree). The economics and logistics of any given situation are key and the role of the arboriculturist is not to operate as a lone professional but to engage and be willing to compromise where there is an evident requirement for this.

Trees and subsidence - step cracking by windows
A property in Essex with subsidence cracking around the upper storey window as caused by adjacent oak trees subject to a TPO (evidential tests satisfied).

Returning to the case, the situation involved a privately-owned oak tree subject to a TPO that had been determined to be causing subsidence damage to an adjacent property. The claimants applied for permission from the Local Planning Authority (LPA) to remove the oak tree, which was refused on the grounds that it was “not necessary to fell the tree, not because felling would not abate the nuisance (it obviously would), but because other engineering works, such as the underpinning of the claimants’ house, could be carried out instead, thereby curing the problem and preserving the tree” – in more detail, the LPA stated that “the tree proposed for removal is a tall, broad-spreading and healthy specimen [that] stands in the garden of a house overlooking the open valley that defines the western edge of the old core of the village of Great Billing [and] forms a handsome and prominent feature in views along and over the valley, part of which is a public park, and is also an amenity to the village and its conservation area (within which the [tree stands]), particularly in views from the direction of the Parish Church of St Andrew [so] consequently the removal of the tree would have a significantly detrimental impact on the amenity of a wide area and its enjoyment by the public“. The claimants subsequently appealed to the Secretary of State and the appeal was dismissed. This resulted in the court case and the resultant appeal that is the subject of this discussion. The claimants argued that the then current TPO protection did not apply to the oak tree, because Section 198(6)(b) of the Town and Country Planning Act 1990 did not apply (as abating the nuisance of subsidence was a legal exemption to the TPO).

The summarised original ruling – known as Perrin & Anor v Northampton Borough Council & Ors [2006] EWHC 2331 (TCC) (26 September 2006) – by the Technology and Construction Court (TCC) was that “the possibility that other engineering works could be carried out is irrelevant” (with the caveat that only “in the vast majority of cases, the fact that alternative engineering schemes are available would indeed be irrelevant“), because “the TPO will not apply to whatever cutting down, uprooting, topping or lopping of the tree is necessary to abate or prevent that nuisance” that was in this case subsidence damage caused by the oak tree. The judge determined that this conclusion was a “workable solution to the problem posed [and] also fair” (because it “would be unworkable if a member of the public had to weigh up all the factors listed [in paragraph [51] of his judgment] before coming to a clear view as to whether or not works to the tree were necessary“). Thus, the judge agreed with the claimants as regards the scope of Section 198(6)(b) that it was necessary to remove the oak tree to abate the nuisance and prevent further damage. Northampton Borough Council disagreed and took this to the Court of Appeal.

Trees and subsidence - pruning trees
In some instances, managing trees on a cyclical pruning cycle will help manage the water uptake of the tree and therein reduce the extent of soil desiccation and thus shrinkage.

The judges at the Court of Appeal went to some length to pull apart the meaning of Section 198(6)(b) and the TCC judge’s interpretation – particularly, whether the removal of the oak tree was necessary, in context to this Section (“this appeal turns on whether the judge was correct in his conclusion that the word ‘necessary’, in the context of section 198(6)(b) of the 1990 Act“). Critically, the judges concluded that the TCC judge was incorrect in his conclusion, because the judge had failed to appreciate that it was indeed appropriate to consider engineering solutions in the extent as to what is necessary in abating and preventing further nuisance when applying the exemptions of Section 198(6)(b). Very importantly, the judges also determined that “it is pertinent to have in mind that nothing in section 198(6)(b) of the 1990 Act authorises B to go onto A’s land for the purpose of preventing or abating a nuisance“, which means that even if the claimants had opted to apply the exemption of the Section they would be trespassing if they did indeed proceed to fell the oak tree not within their land. Legally and logistically, the original judge’s conclusion is thus flawed, for this reason plus others noted in the judgement.

Moreover, the judges determined that “Parliament plainly intended that, in such a case and subject to obtaining the consent of, or compensation from, the local planning authority, A is left to bear his own loss: a risk which, in the ordinary way, he will cover by insurance. In those circumstances it is not self-evident why Parliament should have wished to encourage A to carry out operations to the tree in order to abate or prevent damage to B’s house, so relieving A from his liability in damages” – or, plainly-interpreted, a homeowner has insurance for matters including subsidence and therefore Section 198(6)(b) would not logically allow as an exemption the removal of a tree for the purposes of abating and preventing further subsidence damage. Indeed, “in that context it is to be borne in mind that the expectation that a refusal of consent [by the LPA and even afterwards by the tree owner] will give rise to a claim for compensation [by the homeowner with a damaged house] has been a feature of the legislation since tree preservation orders were first introduced” – or, in short, if the LPA refuse the application for the removal of the TPO’d tree then the affected party / parties can claim against the LPA for the costs incurred for employing an engineering solution such as underpinning (or the homeowner, if they then refuse to remove the tree in spite of consent by the LPA).

The judges rounded-up by asserting that, in their determination, it is “intended that a protected tree should remain protected unless there was a real need to lift that protection” and that that the phrase in Section 198(6)(b) of “so far as may be necessary for the prevention or abatement of a nuisance” be interpreted as “‘if and so far as may be necessary for the prevention or abatement of a nuisance’“. In fact, the judges “[found] it difficult to see how the expression can be read in any other way“, because “if it were appropriate to ask what is the minimum that needs to be done to the tree itself in order to prevent or abate the nuisance, should it be irrelevant to ask whether anything needs to be done to the tree itself[?]“. Bluntly, is it not irrelevant to consider engineering solutions (e.g. underpinning) in cases where the implicated tree is subject to a TPO, because one cannot proceed “without first asking whether anything needs to be done to the tree itself“. Clearly, in some cases, nothing needs to be done to the tree.

Trees and subsidence - mature unpruned trees
Trees on shrinkable soils near to built structures are not however always managed and in such cases the risk of subsidence will generally be higher in theory and in law (foreseeability).

As a means of context-proofing their determination, the judges appended their main determination with the statement as follows: “if prevention or abatement of a nuisance could be achieved either by operations to the tree itself or by works other than operations to the tree itself and the landowner has the resources to fund the former but not the latter, it may well be that the operations to the tree itself are necessary because the other works cannot, and will not in practice, be done” – simply, if a landowner cannot afford to subsidise the underpinning works to the affected’s property, it may be most economical to remove the tree and this is indeed acceptable. However, the judges “suspect that will rarely be a determinative factor; given that the costs are likely to fall not on the landowner but on his insurer (subject to whatever claim to compensation there may be)“. In addition, “in practice, it will be for the [tree owner] rather than the [homeowner with a damaged property] to decide whether to cut down the protected tree; or to pay damages in respect of the costs of remedial or preventative works” – or by extension the LPA insofar as they refuse to consent the removal of the implicated TPO’d tree.

As a very straightforward summary, the appeal case of Perrin & Anor v Northampton Borough Council & Ors [2007] EWCA Civ 1353 (19 December 2007) therein determined that where a tree subject to a TPO is implicated in a subsidence damage claim – and permission for removal is sought from the LPA – it cannot be assumed that removal will be consented. Indeed, the LPA can rightly refuse consent, citing the need for an engineering solution. In such an eventuality, the LPA will however be liable to subsidising the costs of the engineering solution – or, if the LPA grant consent for removal and the tree owner refuses, this responsibility passes on to the homeowner (or their own insurer). In this eventuality, it is pertinent for the LPA at least to consider the theoretical value of the tree(s) by assessing them in line with a valuation system such as Capital Asset Valuation of Amenity Trees (CAVAT), which can help equate the value of the trees against the cost of underpinning (including the associated activities to facilitate the underpinning).

Trees and subsidence – situations involving trees with a Tree Preservation Order (TPO)

A non-update update for 2019

As is very evident, this blog has been inactive since June 2017. This will probably remain that way for a good while and perhaps forevermore – right now, I am at a point in my life where I have to focus my time elsewhere. However, I logged in today to check the data for this blog and it is clear that it is still a widely-used resource; in fact, more widely used in 2018 and 2019 thus far than in previous years. Now, this blog has over 110,000 views from over 58,000 individuals. To that end, I am very pleased to know that there are people the world over who find this useful. I am particularly pleased that my ‘trees in religion’ mini-series has proven to be very popular and I hope that the research I undertook has assisted in other people gaining a better understanding of the mythos surrounding trees.

Keep learning and keep supporting traditional printers and book merchants.


A non-update update for 2019

Fungal succession and wood decay in living trees – a seminar report (Part II)

See Part I here.

The second part of this cluster of blog posts is one the first of the duo of talks presented by Lynne Boddy. Lynne is a well-known mycologist and researcher and thus, as regards wood-decay fungi, is a good authority from which we can all learn a substantial amount. For a fungi enthusiast such as myself, learning about fungi from one of the best is, in and of itself, very exciting. However, the information presented was equally as exciting, which I shall run through below.

As a slight aside, please watch out for her new book, which is currently being written and should be finished later this year, currently entitled Fungi in Trees. This book will be aimed at the arboriculturist. Lynne is also planning to re-write Fungal Decomposition of Wood, which was a magnum opus co-authored with Alan Rayner.

Fungi invading trees

Primarily, we must accept one core tenet of wood decay: the anatomy of wood has a massive impact upon mycelial networks that sojourn through the wood substrate and selectively metabolise woody cells and their deposits as they go. Indeed, in an ideal world, fungi would gun right for the ray parenchyma, which are incredibly nutritious living cells. However, these cells are very challenging to get to, by virtue of their ‘aliveness’ – living cells in good condition are not easily devoured. Further to this, the sapwood also offers a great for invading fungi, though again – because of the high moisture condition meaning the environment is largely anaerobic (fungi are aerobes and require oxygen to metabolise) – this part of the wood structure is not easily accessed. Of course, the vascular wilts have a better time invading sapwood that is functional, though many fungi will have to bide their time or arrive opportunistically onto and into dysfunctional sapwood if they are to have any means of success in acquiring the treasures within. Thus, is recognising that such living (i.e. conductive and functional) areas of wood are likely just beyond the reach of many fungi, wood-decayers will typically resort to the back-up food source: non-functional xylem vessels within the heartwood or ripewood.

The heart rotters

Now, in accepting this, we come on to perhaps the broadest cohort of wood-decay fungi in living and standing trees (fallen trees have their dysfunctional sapwood metabolised like ravens engulfing a fresh meal) we know of: the heart rotters. These fungi will enter the central wood (i.e. ‘the heart’) through exposed areas of this central column by either sufficiently deep root, stem or branch injury – ultimately, there generally needs to be a continuity of viable ‘heart’ substrate, if any significant degree of colonisation is to take place. Where continuity doesn’t exist, or the fungus finds itself limited to only a certain area, the only manner in which is will typically be able to continue existing is by (1) exiting and finding another host or (2) biding its time and waiting for currently functional sapwood to become incorporated into the heartwood or ripewood, in which it can then spread into – assuming the tree doesn’t lay down defensive barriers that cannot be breached, in order to protect its sapwood, which is in itself a pursuit undertaken to safeguard and hence sustain the high-moisture content of its sapwood (contrary to Shigo’s model, which infers compartmentalisation is largely there with the end in mind of prohibiting fungal succession into the wood). When we look at the ripewood of beech (Fagus sylvatica), we can observe this phenomenon very well – a rosy-coloured ripewood (red heart) with lots of separation lines between instances of decay successions. Ganoderma australe, Ganoderma pfeifferi and Ganoderma resinaceum will afford the most acutely observable examples of this, given their ability to breach such zones by metabolising the phenolic deposits laid down by the tree (this touches upon the idea of a fifth wall in the CODIT model, which will be discussed more later on).

An old Laetiporus sp. on yew (Taxus baccata).

By virtue of the heartwood or ripewood being the least inhospitable (heartwood, in particular, is still often disgustingly harsh, as regards its environment), we can observe immediately some species-specific associations. For instance, the dyer’s mazegill (Phaeolus schweinitzii) will frequent gymnospermous hosts, such as the cedars (Cedrus spp.) and pines (Pinus spp.), whilst chicken of the woods (Laetiporus spp. – often L. sulphureus, though by all means not always, as we will see) will be found often on oak (Quercus spp.), sweet chestnut (Castanea sativa) and yew (Taxus baccata) – it can, indeed, be found on other hosts, as well. On the face value of things, these three tree species have seemingly little in common, though all three have extractive-rich heartwood that this genus can metabolise effectively. In terms of why the genus is referred to and not the exact species, Lynne is of the stance that what we term ‘chicken of the woods’ and default to as L. sulphureus is actually a variety of different species each with their own specialisations – perhaps even down to a specific host tree species (notably for yew, where the chicken species is most pertinently viewed as being a different one).

From a tree management perspective, Lynne then addressed the importance of the heart rotters – what is their impact? Put simply, they change the way in which we view the tree from a safety perspective; as in, when fruiting bodies of heart rotters are identified, if the tree is standing and there exists a target, management considerations are routinely entertained and sometimes the tree is felled. Additionally to this, however, we have other things to appreciate:

  • heart rotters do a great job at recycling nutrients, which can then be re-assimilated by the tree when they are uptaken back through its roots (including adventitious aerial roots) or the mycorrhizal fungi the roots associate with
  • the wood qualities produced by heart rotters are ideal as habitat for saproxylic insects and nesting birds

The heart of this beech has been hollowed-out by decay fungi. In the process, before its failure, what conditions did this decay provide for insects and, crucially, what habitat does it provide now?

Latent colonisers

Considered the specialised opportunists, such fungi are present within the sapwood or bark, as propagules (thick-walled resting spores known as chlamydospores). Biding their time until conditions are right, wherein the sapwood becomes dysfunctional through means such as wounding or drought (causing embolism), they are perhaps most acutely observed in the years after drought years where they can trigger the formation of strip cankers and resultant reaction growth by the tree (see the below photo). Thus, this year, in the UK, is one to watch out for, as regards such fungi (it might also explain why Kretzschmaria deusta was so abundant this year, given Ascomycetes love dry conditions, which prevailed last summer).

B nummularia Eutypa spinosa beech canker strip
A strip canker in beech caused by the latent fungus Biscogniauxia nummularia that has induced reaction growth, which can be seen cross-sectionally in the bottom right image).

At this point, a delegate enquired as to whether Massaria disease of plane (Splanchnonema platani), as an Ascomycete, could be prevailing in urban conditions recently, because of dry conditions (such as London, where it was been very severe these past few years but prior to that un-noticed). Discussions continued and Frank Rinn interjected to add his thoughts:

  • massaria progresses quickly in dry conditions
  • recent dry summers have allowed for massaria to thus progress very rapidly (killing branches in as little as three months)
  • the lowering of water tables in cities for the construction of basement levels of buildings has meant that plane trees can no longer tap into groundwater supplies
  • mature plane trees afford the best conditions for massaria; notably lower lateral branches, which are shaded from the rest of the crown and thus may be most prone to stress
  • there have been reports since 1903 from Croatia where large plane trees have shed branches and the massaria fungus was termed the “branch-cleaning fungus”
  • conditions are collectively ideal for massaria to become prevalent, as they stand

Reverting back to latent fungi, Lynne then mentioned that she considers fungi to be latent across a broad variety of trees. For example, the coal fungus (Daldinia concentrica), whilst found most often on ash (Fraxinus excelsior) can be isolated from the sapwood of a great range of different broadleaved tree species in the UK. It is, indeed, only when specific conditions arise that are preferable for this fungus that it begins to create mycelial networks – such conditions might not arise in particular trees, or may arise only after conditions suitable for other fungi have arisen and thus D. concentrica then has no capacity to colonise the substrate. Hence, ash remains the core host of this species, in the current climate. However, for the jelly ear fungus (Auricularia auricula-judae), which is also a latent fungus within the vascular system, having been found largely solely on elder (Sambucus nigra) in the 1950s, it is now found on over 20 host species – this marks a huge increase in host range, prompted perhaps by changing climatic conditions.

Wall V

The CODIT model, offered to us by Shigo, details four walls – as can be seen here. As mentioned by Frank Rinn, Shigo himself was considering the possibility of a fifth wall, though this never ‘made it into’ the model. However, Lynne argues that there is the potential for a fifth one, which hearkens back to what was discussed above, as regards rot within the heart of beech wood).

Specifically, whilst the barrier zone (fourth wall) is a zone laid down at the time of wounding by the vascular cambium, the dynamic responses by the tree that occur in real-time as fungal decay advances constitutes a distinction from this initial barrier. Indeed, as decay advances, living cells within the heartwood or ripewood (they do exist; though through mechanisms not fully appreciated, but thought to be associated with the rays running radially through the wood), in addition to the functional sapwood, will, in order to protect the sapwood and keeps its high-moisture quality intact, will plug woody cells beyond the current zone of decay with extractives and phenolic compounds – this will occur within the sapwood most often, though may also be able to occur in the heartwood around the regions where pockets of living cells exist. This response resultantly produces incredibly dense zones of wood that afford the tree’s sapwood a means of protection, which it would otherwise lack, assuming the barrier zone (Wall IV) failed to contain fungal decay. Of course, if this fifth wall fails, another will form, and so on and so forth.

Ganoderma pfeifferi beeswax beech failure 3
See the myriad of demarcations across this cross-section of a beech that failed from decay by Ganoderma pfeifferi, which suggest a dynamic fifth wall being effective.

Perhaps we will see this idea discussed more in Lynne’s re-write of Fungal Decomposition of Wood.

Fungal succession

When a tree decays, the fungi that initiated the decay process will not end it. This is because, much like all other ecosystems, as an environment changes those organisms that are best-placed to utilise it change as well. In this sense, fungi are no different – they succeed into a dynamic and ever-altering substrate (wood). Such a phenomenon can be so readily observed when walking into any woodland, when comparing the fungi on standing trees and those in early stages of decay and those much more heavily decomposed. In instances where an entire tree falls and stays largely intact, succession can be most acutely observed, as Lynne detailed with a little help from Ted Green and some research students.

The tree in question, a mature beech, failed and was left, in sections, for ease of its movement into an accessible place, to decay. Along the beech, it was found, through analysis of the wood in the laboratory and by presence of fruiting bodies, different fungi were observed colonising different parts at different stages of decay (as shown below). Such an observation does seem readily apparent, though to have it confirmed through scientific means affords us with an understanding that is more concrete than merely the anecdotal. Indeed, whilst Trametes gibbosa was not isolated from this beech, the presence of Bjerkandera adusta infers that, at some point in the future, T. gibbosa will be found – it parasitises upon the mycelium B. adusta, before then colonising the wood substrate itself. We are, in a sense, therefore, witnessing fungal warfare.

beech fungi succession analysis
The different fungi found at different parts of the beech at different stages of the decay process (open in a new tab to see this in a slightly larger size).

Delving further into the notion of fungal warfare, what is essentially meant is chemical warfare. Fungi synthesise and secrete enzymes, which they use principally to degrade wood, though that can also be used to defend territory or attack other fungi. The result of any fungal battle can be one of four things:

  • deadlock, whereby neither fungi gains any ground against the competing fungus
  • replacement, whereby one fungus loses its territory entirely by the other
  • partial replacement, whereby one fungus loses of some its territory to the other
  • mutual replacement, whereby the fungi essentially ‘trade’ places with one another and neither gains any net ground

So how does one determine the outcome of any such skirmish, you ask? Unfortunately, there are so many variables in play that even pitting two fungi against one another in a laboratory is only going to give a slight allusion to what really occurs, though there does nonetheless exist a limited hierarchy of combativeness from which we can assume who the victor will be, under most circumstances (see here at 25:09 timestamp).

Of course, even in assessing this we still have so many caveats to throw in. For example, where moisture conditions are drier because the wood is more exposed, Ascomycetes (i.e. Hypoxylon spp.) will have a better time in securing more wood substrate, as they operate effectively under dry conditions. Indeed, the wood qualities of the substrate itself will even play a role – did the tree uptake pollutants during its life or is it exposed to such pollutants currently, for example. More crucially, if a dead piece of wood (or entire tree) is standing and has thus been subject to relatively dry and exposed conditions suddenly falls to the woodland floor, those fungi reigning when the tree was standing will likely succumb to wood-decay fungi adapted to higher moisture levels and cooler more stable conditions.

The next part of this series will be a brief one on bacteria in wood, as discussed again by Lynne Boddy. I hope to have that written up in the coming few days.

Fungal succession and wood decay in living trees – a seminar report (Part II)