11 Sep BS5837 Draft Review
Following the publication of the draft standard last month, we wanted to provide a complete review of the document for our colleagues and clients that may have experience of working with arboriculturists on development projects.
There is obviously no certainty as to which bits of the draft standard will be retained, amended or omitted. However, this review should provide an summary of key changes, as well as our insight into what may, or may not work in practice.
The Headlines
In this section we are going to share some the stand out headlines from the draft. Initial thoughts/context provided in italics below the respective bullets:
- Standard is to be a ‘Code of Practice’
This could provide more weight in terms of decision making than in previous editions.
- New tree categorisation
– Category V (Veteran)
– Category X (<10 years remaining)
– Category A-C (based on higher/lower attributes)
- No definition of a veteran tree included.
- Assumption every tree is Category B unless there is a justification for upgrading (Category A) or downgrading (Category C) (justification must be provided in the tree schedule)
- Diameter at Breast Height (DBH) measured at 1.3m (previously 1.5m).
This will match forestry conventions.
- Mature Category A trees to have 15x stem diameter RPA (capped at 22.5m)
- Dead trees to have RPA assigned
This would relate to mechanically active rooting area when tree was alive.
- Remaining life expectancy of trees altered. Rather than +40 years, this has been replaced with +30 years
Assume this is to tie in with BNG 30 year calculation, but no rationale provided.
- Introduction of Root Opportunity Area (ROA) for younger trees.
This relates to the area beyond RPA which could be utilised for future rooting.
- Tree Survey Plan (or commonly referred to as Tree Constraints Plan) is to be replaced with an Arboricultural Constraints and Opportunities Plan (ACOP).
Future tree canopy growth is expected to be illustrated on this plan.
- Canopy cover assessments will be standard for all development sites.
No specific methodology or repeatable model provided.
- Arboricultural Impact Report (AIR) to be provided.
Formerly Arboricultural Impact Assessment.
- New hierarchy for protection of trees. This would follow the ‘avoid, mitigate, compensate’ approach.
- More detailed approach on assessing impacts on trees.
This considers site population, species, age and quality. Details on assessing both positive and negative impacts on trees from a development proposal.
- Possible to modify RPA to accommodate proposed structures.
Provides more pragmatic approach to potential RPA incursions and inform decision on impact to rooting environments.
- No mention of Biodiversity Net Gain (BNG).
This was to be expected. This standard covers the UK, BNG applies to England.
So, what do the headlines actually mean. In the next section we don’t quite explore ‘The Good, The Bad and The Ugly’. But more look at some of the stand-out improvements, areas of frustration as well as the ‘head scratchers’.
The Good
There is much more emphasis on assessing the impacts from development on the existing tree stock, and the AIA will be referred to as the Arboricultural Impact Report (AIR). Not only is it much more comprehensive in terms of what should be assessed, but it also looks at trees in both the individual sense (what are the potential impacts both above and below ground) but also considers the tree population on site. There is a very helpful annexe which defines the Significance of the Impacts on Arboricultural Features, and this considers both positive and negative impacts. This is a very helpful addition as it provides a defined way of rationalising tree loss or retention in the same way that the LVIA will look at the broader affect of development on the landscape.
Canopy cover assessments are likely to become “standard” and an assessment will also be required on how development affects the population in terms of age, diversity and species selection. While this is a welcome change, this is also frustrating – this is explored below.
We suspect this is partially trying to get some alignment with BNG as one of the other changes is to the categorisation system. Veterans have their own classification and the RPA will be uncapped at 15x stem diameter – this is the same as the current status. All trees with less then 10 years remaining contribution will now be Category X. After that, the default will be that every tree on site is a Cat B, unless there is a justification to upgrade to Category A or downgrade to Category C. One requirement to the upgrade criteria will be that a tree has to have at least 30 years remaining contribution. This is why we think they are trying to align this with BNG.
The other proposed changed (but one they have specifically asked for feedback on) is that Mature Cat A trees will have the RPA calculated at 15x stem, with a cap of 22.5m. We think this will be one of the more controversial areas, as we know that the constraints on developable areas within sites is becoming ever more prevalent. Obviously the premise is a good one as we need to ensure adequate protection for the highest quality trees. However, how this interacts with other environmental and planning constraints will be an interesting development and competing needs for space could make a lot of development sites unviable.
This standard gives much more emphasis to the importance of soils which we feel is an excellent addition. This is probably not something that is going to be reflected very differently through the reports, but the fact that there is much more detail within the Annexes means that we have something by which we can push for better data up front. This not only will help in considering the impacts on existing trees but will also help with the consideration of how to protect soils in areas for new planting, something we think is very rarely considered or factored into tree protection plans at present.
An odd, but good new inclusion, is consideration of the RPA of dead trees. Given that we know that dead trees have a number of positive benefits, the most obvious being habitat and carbon sequestration, there are times where retaining a dead tree is a good thing. The new proposal is that the RPA of a dead tree is calculated as being “sufficient to enclose the former mechanically active root plate”. And there is the frustration and/or head scratcher. No guidance is provided on how this should be calculated and given that almost every development scheme is likely to result in an increase in “traffic” across the site, this becomes a health & safety issue. Therefore there is an expectation calculate what the mechanically sound root plate of the tree was and then to state that the tree can be left in situ based on that. We suspect that this will meet significant resistance from design teams given the complete uncertainty on stability.
Another good change is the introduction of a Hierarchy of Controls to Protect Trees. This follows a simple stepped process of, eliminate the need for controls, reduce the activity to reduce the potential harm, provide engineered solutions, use work methods to reduce the potential for harm or use enforcement controls. We think this is a good thing as it helps the narrative of explaining the decision making process, rather than just jumping straight to the most convenient solution.
In addition to this is the possible inclusion of a ‘permit to work’ system, whereby any person undertaking work activity within or directly around the Root Protection Area could be implemented to either control activities or verify competence of others (i.e. sub-contractors). This gives greater protection for trees on site as this should only be granted to a trained and competent arboriculturist/arborist. It will be intriguing to see if this is adopted and brought into planning conditions.
The Frustrating
One of the most frustrating parts of this revised standard is that no definition of a veteran tree is provided. While we acknowledge that this is being developed by other parties, the lack of inclusion in this standard, where these irreplaceable habitats are assigned such significance (and within National and Local planning policy) seems very short-sighted.
We feel that the introduction of canopy cover assessments has been one of the successes of the sector in recent years as they are quite easy to explain and understand, and fairly simple to bring into a design team. However, the idea that this should be standard for every site is very frustrating and we think there should be some kind of threshold. Maybe this is something that could/should be addressed within local plan and Supplementary Planning Guidance (SPG) but householder applications are exempt under BNG, so we are unsure why there is not something similar for canopy cover.
More frustratingly, given the standard has no problems with providing a formula for calculating the RPA or providing exact definitions of what needs to be measured for canopy spread and tree heights, the document offers no guidance on how to do a canopy cover assessment other than to acknowledge that there is almost no data available in the UK on tree growth rates. Having accepted that, the guidance then says that when determining future growth, would need to take into consideration tree form so that we don’t over estimate the rate of growth. Fastigiate trees (for example) grow upwards rather than outwards, so this needs to be factored in. The guidance actually goes onto state (for example) that shoot extension growth on trees with a branching angle of 75° should be multiplied by a factor of 0.3. While this is a great example, it does not provide a definitive figure for shoot extension growth, nor how this changes for each tree species, growing environment or other factors.
As a company we are in a fortunate position that having undertaken a large number of projects in Oxford (where Local Planning Policy requires a canopy cover assessment), that we have developed our own model. However, without a standard approach every local authority could have different requirements. This could perhaps be seen as a good opportunity for arboriculturists to develop bespoke models, but we feel this would undermine the integrity of this part of the British Standard.
We also have reservations about how canopy cover assessment will be considered in conjunction with more holistic systems such as the Urban Greening Factor, which is already required in London, but is increasingly being seen in other Local Plans. Furthermore, as “valuation” systems such as i-Tree also improve and become more functional, these will provide better outcomes than simply guessing by how much trees grow. While the introduction of some sort of wider assessment than just the “quality” of a tree is a good thing, it is frustrating that the document does not offer alternatives that may have longer term potential.
The Head Scratchers
While they are proposing the Mature Category A trees have a larger RPA, they are also proposing a new definition of what is meant by a ‘mature’ tree. Most arboriculturists would say trees have several life stages, from newly planted, young, semi-mature, early mature, mature, over mature (or post mature) and ancient. This BS proposes that mature and over mature are the same thing. We think this is to do with veteran trees, as a veteran is not a life stage, but a condition or status as a result of features. Over mature trees would generally have lots of veteran features so by getting rid of that age class, it “forces” the surveyor to either say a tree is mature or a veteran. We are not convinced by this approach as it seems to be trying to make veteran status a life stage and this could result in some dispute between tree officers and consultants.
The other change which is odd is a new concept called the Rooting Opportunity Area (ROA). This is where there are trees that are not yet at maturity and therefore still have growth potential. We would therefore need to calculate the ROA (as a larger area to the RPA) to show what space (in future) the tree would need to reach its full potential. However, no guidance is offered on how the ROA should be calculated and therefore this is another area that we see as a bone of contention, both between tree officer and consultants, and consultants and developers.
One other change that we are not sure whether is a ‘Head Scratcher’ or a ‘Good’ thing is that there is a section on Modifications to the RPA to Accommodate Proposed Activities. The reason we think this is a head scratcher is because the definition of the RPA remains unchanged, in that it is the minimum area around a tree which contains sufficient roots and soil volume to sustain a tree. The protection of the RPA is a priority in design terms and should remain sacrosanct and there must be no encroachment. And yet, this guidance suggests that where encroachment of the RPA could have an adverse impact on the tree, mitigation measures must be applied and that could include an extension to the RPA. In one sense, we think this is ‘Good’ as it is the application of common sense. However, we still think it is a ‘Head Scratcher’ as it totally contradicts the premise of the RPA being sacrosanct and it allows for greater subjectivity and could become an area of contention between Tree Officers and Consultants as to the ‘actual’ impact to the tree.
Other Changes
Tree Survey/Constraints Plans will now be referred to as a Arboricultural Constraints and Opportunities Plan (ACOP). This is meant to allow us to set out more completely where arboricultural opportunities lie for future growth. The concept here is good but we are less clear about how easy it will be to set this out on a two dimensional plan. Furthermore, it is an expectation that potential canopy cover growth will be illustrated on the ACOP. In principle this is a welcome change as it should lead to overall better design, with greater separation from trees, however this may lead to additional tree loss due to the perceived interaction from nearby trees. It will also require significantly more desktop time in order to present a coherent data set to the design team, this will obviously increase costs too.
Tree protection plans will be required to give a 1m buffer outside the RPA boundary to ensure that the RPA is properly protected (notwithstanding the comments above about being allowed to encroach the RPA). The guidance is also more specific about the level of detail required on the Tree Protection Plan (TPP).
More emphasis has been given in the section on method statements about the importance of monitoring trees on development sites. This is a welcome change, given that this is often the part of the process where huge amounts of good design work can be undone through a misunderstanding of tree protection measures. Regardless of this change, unless local authorities are able to provide robust planning conditions, or have the resources available for enforcement, whether this leads to better protection of trees on construction sites will remain to be seen.
Conclusion
In conclusion from an initial review of the standard is that this is broadly a robust document that has some significant changes that are generally going to be a good thing. It is a welcome change for arboriculturists, with more emphasis on involvement during the design process. However, there will be more detailed analysis required at feasibility stage in order to prepare preliminary canopy growth indicators. The document also provides a more robust system for assessing impacts, but fails to provide a repeatable process or model for canopy cover assessments. There are also conflicting parameters for Root Protection Areas and potential encroachment or modification. There are obviously going to be areas of contention and there are going to be a lot of consultation comments for the Committee to sift through, but overall the revision appears to provide a more detailed suite of recommendations for how trees should be considered in design and construction terms.