Category: Biodiversity

Calculating tree habitat area

A key factor in calculating the value of a habitat under the new Statutory Biodiversity Metric is to work out its size. In the case of trees, their habitat size can be of critical importance to the calculation of their biodiversity valuation which will, in turn, help to preserve our precious tree habitats.

As a result, individual trees in an urban setting are often the most important habitat present.

Unfortunately, developers may be tempted to allocate all trees on their planned development site (especially those in groups) to Woodland and forest habitats rather than to Individual trees habitat, because this gives the trees and the site a lower biodiversity value. An example of this is the proposed development by Goram Homes of Hengrove Park in Bristol, as shown in the image above and discussed below. The whole site was granted outline planning permission in October 2019.

Unfortunately, the Statutory Biodiversity Metric User Guide (the Metric) definitions of these habitats are ambiguous and can make such allocations hard to counter, even in settings that are clearly urban.

The User Guide defines two broad tree habitats types whose areas are measured in hectares (ha):

  • Woodland and forest
  • Individual trees

Defining woodland and forest habitat

There are conflicting definitions of ‘woodland’. The Metric User Guide does not define Woodland and forest habitat.

However, the UK Habitat Classification, UKHab, upon which the Metric is based, defines it as ‘Land with 25% or more cover of trees that are five metres or more in height.’ This definition doesn’t mention the minimum land area required.

The UK National Forest Inventory 2015 (NFI) defines woodland as:

‘a minimum area of 0.5 hectares under stands of trees with, or with the potential to achieve, tree crown cover of more than 20% of the ground. Areas of young trees, which have the potential to achieve a canopy cover of more than 20%, will also be interpreted as woodland and mapped. The minimum width for woodland is 20 m, although where woodlands are connected by a narrow neck of woodland less than 20 m wide, the break may be disregarded if less than 20 m in extent.‘

Bristol City Council also uses area to define woodland in its 2008 Biodiversity Action Plan (Chapter 5, page 65), which states that ‘this action plan covers all woodlands over 0.5 hectares in extent found in Bristol.‘ As this is a local policy, and aligned with the NFI definition, we have adopted it.

The habitat area of Woodland and forest habitats is based on its total measured canopy area.

Defining individual trees habitat

The Metric User Guide (page 53) advises when to record Individual trees habitat:

Individual trees are classed as ‘urban’ or ‘rural’. You should consider the degree of ‘urbanisation’ of habitats around the tree and assign the best fit for the location. 

Use the broad habitat type ‘Individual trees’ to record: 

  • individual rural trees 
  • individual urban trees 
  • lines, blocks or groups of trees found within and around the perimeter of urban land.

In all circumstances ‘Individual trees’ should be used to record ancient and veteran trees, regardless of location. This could include ancient and veteran trees within hedgerows, ‘rural’ lines of trees and woodlands. 

Other clarifications

Do not use the hedgerow module classifications ‘line of trees’ and ‘ecologically valuable line of trees’ to record linear formations of trees in the urban environment.

These classifications should only be used for rural lines of trees.
Trees within overgrown non-native and ornamental hedges (for example, leylandii) should not be classified as individual trees, or as lines of trees. Record these as nonnative ornamental hedges within the hedgerow module.

Trees recorded as individual trees that will be removed for any purpose, including development, disease, or safety must be recorded in your baseline and recorded as lost.

Do not otherwise record individual trees if they occur within a habitat type characterised by the presence of trees, unless specified within the section on ‘recording individual trees at baseline’, which covers: 

  • trees within private gardens 
  • removal of trees within hedgerows 
  • removal of trees within rural lines of trees 
  • removal of trees within orchards and wood-pasture and parkland 

Recording individual trees at baseline 

The biodiversity metric uses set values to represent the area of individual trees depending on their diameter at breast height. 

This value is a representation of canopy biomass, and is based on the root protection area formula, derived from BS 5837:2012. 

Table 14 sets out class sizes of trees and their area equivalent. 

The User Guide then deals with specific instances where individual tree habitats also need to be recorded:

Recording baseline trees within private gardens

A private garden is a garden within the curtilage of a privately owned or tenanted dwelling house. Private gardens can contain important features for biodiversity, including mature trees and hedgerows. 

record all medium, large and very large trees within private gardens as individual trees 

Recording baseline trees within hedgerows and lines of trees 

  • if any medium, large or very large trees within a hedgerow or ‘rural’ line of trees are being removed, record these in the area baseline as individual trees 
  • the removal of trees may influence the linear value of hedgerows and ‘rural’ lines of trees within the hedgerow module 

Recording baseline trees within orchards and wood-pasture and parkland 

  • if any medium, large and very large trees within these habitats are being removed, record these in the area baseline as individual trees 
  • this does not change the way in which you would record the area of orchard or wood-pasture and parkland area habitat (see ‘Recording habitat mosaics’ section) 

Calculating the number of post-development trees required

Post-development size classes 

When planting trees post-development size class is determined by the size of the tree at site-planting. When using the tree helper: 

  • record newly planted individual trees as ‘small’, unless ‘medium’ size or above at the time of site-planting 
  • record trees planted with a DBH less than 7.5 cm as ‘small’ 

You should not: 

  • record natural size increases of retained trees within post-development sheets 
  • record natural size increases of planted trees within post-development sheets 

The post-development private garden has no public access, and biodiversity net gains cannot be legally secured. As these gains cannot be secured you should only record created private gardens as either: 

  • ‘urban – vegetated garden’; or 
  • ‘urban – unvegetated garden’ 

You should not: 

  • record the creation of any other new habitats within private gardens 
  • record enhancement of any habitat within private gardens 

However, habitats which are recorded in the baseline and remain within a private garden may be recorded as retained. 

A garden within the curtilage of a privately owned or tenanted dwelling house.  The post-development private garden has no public access, and biodiversity net gains cannot be legally secured. 

How these rules affect the tree habitat area calculation

As a result, this approach places a higher value on the areas of all but the largest Individual trees habitats than those that form part of a Woodland and forest habitat. The following graph shows this, with RPA, derived from DBH, representing the canopy biomass of both broad habitats.

We analysed 12 recent planning applications involving 2,116 trees, 612 (29%) of which were identified for removal. Using the Metric User Guide, they have a combined habitat area of 20.2 ha. If they are measured by canopy area it would only be 8.3 ha (41% of their habitat size). If their combined Root Protection Areas (RPA) were used, they would cover only 8.6 ha (43% of their habitat size).

On the basis that all these trees are in poor condition and have no strategic significance, we calculate that, with the minimum 10% biodiversity net gain now required, a total of 1,925 new trees would need to be planted to replace the habitat lost by the removal of these 612 tree – a ratio of just over 3:1.

To illustrate how this can affect actual applications, here’s a pending application by Goram Homes at Hengrove Park in Bristol. The area edged in red is the development site.

This next image shows how the applicant’s ecologists have defined each habitat: the woodland and forest habitats are shown as a diamond pattern on a dark green background. They cover 2.44 ha.

They’ve also identified 0.19 ha of Individual trees habitat, that is, trees not growing within the woodland and forest habitats.

Area 8 in the south of the site (0.99 ha) might legitimately be designated Woodland and forest habitat, but, given that all the remaining trees are growing separately and in groups in an urban park, they should be designated Urban Individual tree habitat.

The site is complex, with many trees growing in groups but, by excluding the trees in area 8 and treating all the other onsite trees as Urban Individual tree habitat, we calculate that their habitat area is 6.42 ha. This is 4.78 ha more than the applicant’s calculation. This difference will clearly have a significant impact on the final biodiversity net gain calculation, valuing these habitats at 34.07 area habitat units (or 2,989 Small size trees) as opposed to the applicant’s 12.11 (or 1,063 Small size trees).

To give the applicant their due, they at least attempted to calculate the tree habitat area. We have seen other applications where the trees were simply ignored, or were classified as another habitat – bramble scrub in one case, or the method by which the Individual trees habitat area was calculated bore no relationship to the evidence submitted. We must remain ever vigilant against such tricks.

This blog was amended on 09 May 2024 to include the comments of the Nature Conservation Officer dated 03 May 2024 about whether the trees on the Hengrove Park development site are Individual trees habitat or Woodland and forest habitat. The officer writes:

This blog was amended on 14 February 2024 following the obligation for most planning applications to achieve at least 10% biodiversity gain becoming obligatory on 12 February 2024.

Further changes have been made following updates published in July and August 2024, in particular the clarification of the application of BNG in private gardens.

Other Blogs in the series

Calculating habitat units

The trading rules explained

Why we need a new Bristol Tree Replacement Standard

We believe the time has come to revise the Bristol Tree Replacement Standard (BTRS), to reflect our changing understanding of the vital importance of urban trees to Bristol and how they contribute to biodiversity gain.

The current BTRS Standard, adopted nearly a decade ago in July 2014, provides a mechanism for calculating the number of replacements needed for any trees that are removed for developments. It was ground-breaking in its time as it, typically, required more than 1:1 replacement of trees lost to development.

Since then, Defra has published the statutory version of the Biodiversity Metric (SM) (on 29 November 2023), which became mandatory on 12 February 2024. In addition, Bristol has adopted Climate and Ecological Emergency Declarations, so an updated BTRS would be an important part of implementing these declarations. It would require all new developments, subject to some exceptions, to achieve a Biodiversity Net Gain (BNG) of at least 10%. Where deveelopments are exempt, BTRS will still apply.

Although Councillors rejected our proposals for a new Standard at their meeting on 31 October last, we’ve revisited our July 2023 proposals and recast our calculations. These proposals, set out below, provide a mechanism for complying with the new requirements and align the BTRS with the BNG provisions of the EA 2021.

The purpose of the BTRS is that it should only ever be a last resort and not the default choice – which, unfortunately, it has become. When considering any development involving established trees, the presumption should always be that trees will be retained. If this is not possible, then the impact of the proposed development must be mitigated. Only if this is impossible, should compensation for their loss be considered. This is the meaning of the Mitigation Hierarchy, as set out in paragraph 180 a) of the National Planning Policy Framework, which states:

If significant harm to biodiversity resulting from a development cannot be avoided (through locating on an alternative site with less harmful impacts), adequately mitigated, or, as a last resort, compensated for, then planning permission should be refused.

This is reflected in the Bristol Core Strategy, policy BCS9 (page 29),which states that:

Individual green assets should be retained wherever possible and integrated into new developments.

This is repeated in the proposed replacement for BCS9 – Policy BG1: Green infrastructure and biodiversity in new development (page 124) – which ‘aims to ensure that green and blue infrastructure and provision for nature is incorporated into new development’ so that, among other things:

The provision of green infrastructure in new development should … Retain and incorporate important existing green infrastructure such as trees (Policy BG4 ‘Trees’), hedgerows and water features …

It is a shame that the requirement is only an aspiration, not an obligation.

Background

Under the new proposed policy – BG4: Trees (page 131) – trees lost to development will be replaced using this table:

Table 1 The proposed BG4 tree replacement table.

However, when the balance of EA 2021 takes effect, the current version of the BTRS will not, in most cases, be enough to achieve the 10% BNG minimum required for nearly all developments. A new Section 90A and Schedule 7A will be added to the Town and Country Planning Act 1990 and will set out the level of BNG required.

Paragraph: 001 Reference ID: 74-001-20240214 of the Biodiversity net gain guidance states:

Under the statutory framework for biodiversity net gain, subject to some exceptions, every grant of planning permission is deemed to have been granted subject to the condition that the biodiversity gain objective is met (“the biodiversity gain condition”). This objective is for development to deliver at least a 10% increase in biodiversity value relative to the pre-development biodiversity value of the onsite habitat. …

Many development proposals will aim to achieve more than the minimum 10% gain voluntarily. Others may not but will still need to achieve much more in order to comply with the SM trading rules (page 140). This is based on the habitat type lost and its distinctiveness. In the case of Individual tree habitats – Urban or Rural –­ losses must be replaced within the same broad habitat (i.e. more Individual trees) or with a habitat of a higher distinctiveness.

However, for the sake of certainty, we propose only using the minimum 10% BNG required.

Our proposed new BG4 (BTRS) model

We propose that the table in BG4 be amended to reflect the requirements of the EA 2021 and SM and that the BG4 table (Table 1 above) be replaced with Table 2 below:

Table 2 Our proposed BG4 tree replacement requirement

The Replacement Trees Required number is based on the habitat area of each of the four SM tree category sizes (Table 13 below), divided by the area habitat of one BNG 4.0 Small category tree (see section 3 below) plus a 10% net gain. This is rounded up to the nearest whole number (since you can’t plant a fraction of a tree).

The reasoning for our proposal is set out below:

1. Applying the Biodiversity Metric to Urban trees

The most recent Statutory Biodiversity Metric User Guide defines trees as Individual trees habitats as follows:

When to record individual trees

Use the broad habitat type ‘Individual trees’ to record trees where:

  • they are found as an individual or as part of a group;
  • are over 7.5cm in diameter at breast height (DBH).

Individual trees should also be recorded where they meet the definition of an irreplaceable habitat but would not otherwise be recorded.

Do not otherwise record individual trees if they occur within an area habitat type characterised by the presence of trees, examples of these are:

  • woodlands
  • orchards
  • wood-pasture and parkland

Individual trees are classed as ‘urban’ or ‘rural’. You should consider the degree of ‘urbanisation’ of habitats around the tree and assign the best fit for the location.

2. Calculating Individual trees habitat

Table 13 in the SM User Guide is used to calculate the ‘area equivalent’ of individual trees:

The biodiversity metric uses set values to represent the area of trees depending on their diameter at breast height. This value is a representation of canopy biomass, and is based on the root protection area formula, derived from BS 5837:2012.

You should report the number of individual trees within your project and input tree count into the ‘tree helper’ within the biodiversity metric tool to generate area values for data input. For multi-stemmed trees, use the DBH of the largest stem. You should:

  • account for each individual tree within a group or block of trees.
  • record the habitat underneath the tree canopy separately.
  • not reduce any area generated by the tree helper.
  • not deduct the area of individual trees from other habitats.
  • make clear in the user comments how many trees contribute towards the total area.

Recording trees within private gardens

You should assess most individual trees that are recorded in private gardens. You should record:

  • any medium, large and very large trees as individual trees
  • any small trees that are ancient or veteran

Recording trees within hedgerows

You should assess most individual trees that are recorded within hedgerows. You should record:

  • any medium, large and very large trees as individual trees
  • any small trees unless they are ancient or veteran.

You must assess the linear value of hedgerows within the hedgerow module separately.

Individual Tree habitats have medium distinctiveness and so, under Rule 1 of SBNG, ‘Losses must be replaced by area habitat units of either medium band habitats within the same broad habitat type or, any habitat from a higher band from any broad habitat type.’

3. Forecasting the post-development area of Individual trees

The SBNG User Guide provides this guidance:

You should use the tree helper to calculate the area for created trees.

You should categorise most newly planted individual trees as ‘small’, unless the tree is medium sized or above at the time of planting.

You should not factor in the age of nursery stock when using the ‘creation in advance’ function. The ‘creation in advance’ function should only be used where trees are planted in advance of the development (for example, as screening or as structural landscaping).

Exceptions

You cannot count:

  • newly planted trees within private gardens
  • natural size increases of baseline trees
  • trees planted as part of hedgerow creation or enhancement as individual trees.

Our calculations are based on Small category replacement trees being planted as per the SM guidance.

4. The likely impact of this policy change

We have analysed tree data for 1,038 surveyed trees taken from a sample of BS:5837 2012 tree surveys submitted in support of previous planning applications. Most of the trees in this sample, 60.5%, fall within the SM Small tree category, 32.9% are within the Medium tree category, 5.4% are in the Large tree category with the balance, 1.3%, being categorised as Very Large.

Table 4 below sets out the likely impact of the proposed changes to BG4. It assumes that all these trees were removed (though that was not the case for all the planning applications we sampled) and replaced with SM Small category trees:

Table 4 Proposed BG4 impact analysis.

The spreadsheet setting out the basis of our calculations can be downloaded here – RPA Table Statutory BNG 13 table Comparison.

The saga of the SNCI at Yew Tree Farm continues

What does ‘harmful impact’ mean?

When the Development Control Committee last met to discuss the Council’s application to extend the cemetery at South Bristol into the SNCI at Yew Tree Farm on 6 September 2023, we were disturbed to hear the Chief Planner’s interpretation of the meaning of ‘harmful impact’, as set out in the Local Plan policy, DM19 – ‘Development which would have a harmful impact on the nature conservation value of a Site of Nature Conservation Interest will not be permitted.’

We even wrote an open letter to the Chief Planner asking them to explain their reasoning. We published this as a blog: It seems that SNCIs are nothing special – an open letter to Bristol’s Chief planner.

Update

Unbeknown to us, it seems that the Council’s Nature Conservation officer did comment on the Chief Planner’s earlier advice to the Committee:

This document was disclosed on 1 December 2023 as a result of our Freedom of Information request of 01 November 2023. This appears to have formed the basis of the answers given below.

Update Ends

As we received no reply at the time, we took the opportunity to ask again when the Committee reconvened to make its decision on 29 November 2023. We asked two questions – see page 9 of the Public Forum. As the responses still didn’t really satisfy, we asked two supplementary questions:

  1. When you say, ‘the site’, what do you mean? Is it within the redline boundary or something else such as within the SNCI’s boundary?
  2. You say ‘The crucial additional clarification to highlight, is that to be in alignment with this policy it is NOT the overall biodiversity gain that is determinative. There rather needs to be an assessment that establishes whether there is harm with reference to the specific characteristics that make the site special.’

Does this mean that the replacement of one habitat which forms part of the ‘specific characteristics that make the site special’ – such as a replacing the Grassland Habitat that forms part of the current SNCI designation with a Lake Habitat that does not form part of the current SNCI designation, or that the provision of offsite mitigation measures to compensate for onsite habitat losses (in this case -6.44%) – would not be acceptable?

These were the replies:

To question 1

By ‘the site’, it’s the site as set out in the application document, so it’s the SNCI as contained in the application document the area in the redline boundary.

To question 2 (as it is quite complex, we have reproduced it verbatim)

You need to take the application as a whole and where it is demonstrated as that with regard to the features, particularly the grassland, that there is no impact ultimately or, if anything, a slight enhanced impact.

We intervened to ask – So you are saying that the substitution of the grassland habitat for a lake habitat…?

I am not saying that at all, I am saying that the grassland, actually that there is more grassland and that’s what the ecology report also says – more grassland of the type for which the SNCI is designated will be there through this application than before… within the redline boundary.

Here is the recording of this exchange – https://www.youtube.com/watch?v=-8EvqLA8-Lg (08:05 minutes from the start to 11:30 minutes).

What’s a redline boundary?

Anyone who wants to develop land must produce a location plan of the area proposed for development, delineated by a red line – the so-called ‘redline boundary’.[1]

When planning permission is granted, only the area within the redline boundary may be developed (though ancillary works may take place elsewhere).

Here is the location plan for the South Bristol Cemetery Extension application:

The South Bristol Cemetery Extension location plan (North is at the top)

The redline boundary here is quite complex because it’s made up of two burial areas, in the north and south, and an attenuation pond to collect runoff from the northern burial ground through a series of drains (the southern burial ground runoff will drain straight into Colliter’s Brook to the west). The area within the blue line is also owned by the Council and so is under their control.

The redline boundary is also important when it comes to calculating the biodiversity value (BNG) of the development site. All the habitats within the redline boundary are treated as ‘onsite’, while those outside the boundary are treated as ‘offsite’.

So, when we are told that ‘… there is more grassland and that’s what the ecology report also says – more grassland of the type for which the SNCI is designated will be there through this application than before… within the redline boundary,’ it’s just the onsite area that’s being referred to. This is important, as we show below.

The headline results shown in the most recent BNG 3.1 calculation relied on by the Council[2] show that 6.44% of the baseline onsite area biodiversity will be lost as a result of the development (see Figure 1).

Figure 1: The BNG 3.1 Headline BNG results.

Figure 2 shows the net losses of the onsite grassland habitat:

Figure 2: Grassland Area & Habitat Unit analysis (HUs)

Under the BNG Trading Rules, Medium Distinctiveness grassland habitats may only be replaced with the other Medium Distinctiveness grassland habitats or with habitats of a Higher Distinctiveness. So, in order to achieve the net 2.93% BNG which the Council claims will result from the development, it will be necessary to compensate for these losses by crediting 3.25 Habitat Units of High Distinctiveness Lakes habitat by creating the attenuation pond.

This is not what we are told is happening and it certainly cannot be said that: ‘… there is more grassland … of the type for which the SNCI is designated … within the redline boundary,’ This is plainly untrue and, even on the Chief Planner’s definition (which we do not accept), it is clear that this application will ‘have a harmful impact on the nature conservation value of a Site of Nature Conservation Interest.’ This is contrary to DM19.

What’s more, even if the proposed offsite habitat mitigations were taken into account, there’d still be a net loss of -0.47 HU of Medium Distinctiveness grassland habitat (see Figure 4 below).

Figure 4: Net Medium Distinctiveness habitat losses

There’s one other serious flaw in the application, which was not brought to the attention of the Development Control Committee at its meeting. There’s a shortfall of -0.11 Habitat Units of the High Distinctiveness habitat, Species-rich native hedgerow with trees. Lost High Distinctiveness habitats may only be replaced like-for-like. This has not happened. As a result, the application is in breach of the BNG Trading Rules and should not have been approved.

We have brought this to the attention of the Council and the LPA.

These are just some of the important reasons why we say that the Development Control Committee was wrong to grant this flawed application.

A copy of this blog can be downloaded here – The saga of the SNCI at Yew Tree Farm continues

[1] It seems that no one thought of those with red/green colour blindness, who might find it difficult to see this red line.

[2] 22_05714_FB-SOUTH_BRISTOL_CEMETERY_BNG__25TH_AUGUST_23_-3540800

The proposed Local Plan is not yet ready for further consultation let alone independent examination – an Open Letter to Councillors

Dear Councillors

The Mayor has now published the next iteration of the proposed new Local Plan (LP). This will be brought before you at Full Council on 31 October next. The Mayor recommends (item 8) that, under Regulation 19 of the Town and Country Planning (Local Planning) (England) Regulations 2012, the draft LP will be formally published in order for representations to be made and then submitted to the Secretary of State for examination.

The sustainability appraisal documents are published on the Local Plan Review web page.

In our opinion the proposed LP is not yet ready for further consultation, let alone independent examination, for the following reasons:

  1. It does not contain enough detailed information about the sites in the adopted LP to allow for a proper consultation or independent examination.
  2. Protection for green spaces has been reduced, contrary to adopted Council policy. 
  3. Despite the recent Ecological and Climate Emergency Declarations, this draft provides fewer environmental protections than the adopted LP.
  4. Comments on earlier drafts appear largely to have been ignored, rendering the consultation process flawed.

Our response in detail

Section 20 (2) of the Planning and Compulsory Purchase Act 2004 states that the authority must not submit the proposed LP unless they think the document is ready for independent examination. In our view, the proposed LP is not yet ready for further consultation, let alone independent examination. Our reasons are set out in detail below:

A proper consultation has not been conducted. In a 2001 judgement Lord Woolf defined a proper consultation as containing four elements.[1] The final element is that ‘the product of consultation must be conscientiously taken into account when the ultimate decision is taken’. You have not responded to our carefully considered comments on both the 2019 and the 2022 consultations on earlier drafts of the LP and there is no evidence that the Local Plan Working Party even discussed them. We do not know how many other organisations who submitted comments were also ignored, because these have not been published.

  1. When the 2019 document, New Protection for Open Space, was published for consultation, a schedule with maps was produced so that consultees could see which sites were being proposed and with what designation – Local Green Space (LGS) or Reserved Open Space (ROS). No such document has been produced in this version, which means that there is no easy way for consultees to see what has been changed, added or removed – save for slavishly working though the only document showing the new designations set out in 08.3 Appendix A3 Policies Map. Whilst this may be sufficient for those interested only in the information at ward level, it is nigh on impossible for those with a city-wide interest.
  2. An interactive GIS map of the proposed Bristol Local Plan Policies Map should be made available to facilitate examination. The pdf version provided has 38 layers in the Key and many sites have multiple designations, which makes it very difficult to interpret. The current Local Plan Policies map does this.
  3. Whilst the document Appendix 3 Assessing the effects of the Publication Version Policies, cross-references, to a limited extent, how some proposed new policies relate to policies in the adopted LP, there is no equivalent schedule for the adopted policies which will be removed – Core, Site Allocation and Development Management Policies (SADMP) and ancillary Supplementary Planning Documents (SPDs) etc. – nor any comprehensive cross-tabulation showing which of the adopted LP policies have been transferred to the proposed LP and which have not.
  4. No schedule has been prepared showing those sites protected under the adopted LP and whether they will be protected under the proposed LP. For example, SADMP DM17 currently provides protection for sites designated as Important Open Spaces, Unidentified Open Spaces and Urban landscapes. It appears that DM17 will be removed but that these current protections will not be adopted in the proposed LP. We have mapped 523 Important Open Space sites covering over 2,000 hectares. As far as we can see, some 1,000 hectares of these and all Unidentified Open Spaces and Urban landscapes, will no longer have any protection. If this is the case, then the proposals should make this clear. Our recent article, Will Councillors Honour Their Promise To Protect Bristol’s Green Spaces? addresses our wider concerns.
  5. SNCIs are currently given protection from development under SADMP DM19. This states that ‘Development which would have a harmful impact on the nature conservation value of a Site of Nature Conservation Interest will not be permitted’. It is proposed that DM19 will be removed in its entirety. Under proposed new policy BG2: Nature conservation and recovery, this protection has been changed to read: ‘Development which would have a significantly harmful impact on local wildlife and geological sites, comprising Sites of Nature Conservation Interest (SNCIs) and Regionally Important Geological Sites (RIGS) as shown on the Policies Map, will not be permitted.’ This is a dilution of the current protection enjoyed by SNCIs (and RIGS); the phrase ‘significantly harmful’ is a subjective judgement and undermines the current protection provided, especially when the Chief Planning Officer has recently advised Councillors that damage to an SNCI which is offset by onsite mitigations under the Biodiversity Metric is not harm.
  6. Whilst we are very pleased to see that our campaign to have all those Sites of Nature Conservation Interest (SNCIs) which were allocated for development in 2014 (save for BSA1305 – why?) has succeeded and have had their Site Allocations removed, we are concerned to note that not all of the 108 sites (not 85 as is wrongly suggested) have also been designated as LGS – some are ROS and some have no designation at all. No explanation has been given for this.
  7. No schedule of the sites identified in BG2 has been produced. As we have pointed out, there are 108 SNCIs, not the 85 stated in Appendix 1: Sustainability Appraisal Updated Scoping Report 2023 A1-4 (at page 26). A schedule of all these sites will enable consultees to identify and locate them.
  8. In September 2021 the council unanimously resolved to protect the Green Belt and Bristol’s green spaces. Despite this, around 30 of the 96 sites proposed for residential development are green spaces (nearly 40 hectares) and three areas in our urban Green Belt are proposed to be removed from the Green Belt for development. No new green or open spaces are proposed.
  9. Proposed policy BG4: Trees is deeply flawed. As currently drafted it will allow developers to offset tree losses by using habitats that are not allowed under BNG 4.0. If allowed this will result in the hollowing out of Bristol’s trees and frustrate the One City plan to increase tree canopy (see Annex A below).
  10. The proposal that replacement trees ‘should be located as close as possible to the development site’ will still allow developers remove trees to build, because all they need to do is pay compensation for their replacement with no concern for where they are to be planted. This will result in trees and their biodiversity being lost from those areas under greatest development pressure, with any offsite compensation being exported to already green suburbs and creating even greater tree inequalities.
  11. It is proposed that development which would result in the loss of ancient woodland, or ancient or veteran trees, will not be permitted, but neither Bristol’s known veteran trees nor its 11 ancient woodlands are mapped or expressly protected on the Bristol Local Plan Policies Map.
  12. No express protection is given for other urban woodlands that are not ancient (woods that have not existed continuously since 1600), are not in a conservation area or are not protected with a TPO.

Our request

Bristol City Council has recently declared both Climate and Ecological Emergencies and resolved to protect our green spaces. The Environment Act 2021 with its still-to-be-published regulations (which will be fully implemented in 2024 together with a proposed new version of the National Planning Policy Framework) will provide even greater environmental protections and the next iteration of the One City Plan aspires to achieve a significant increase of tree canopy. Yet, against all this, the proposed new Local Plan will result in reduced protection for the environment when compared with the current, adopted Local Plan.

In light of this, we ask you to reject the Mayor’s recommendation until the above crucial issues have been addressed and insist that Bristol’s nature does not continue to suffer yet more decades of decline but is properly protected.

The Bristol Tree Forum

24 October 2023

Annex A – Email to BCC Specialist Planning Policy Officer 21 October 2023

Dear Michael,

I see that the latest iteration of the proposed Local Plan has been published. We are examining it and will comment in due course, but we have to express serious concerns about the proposed new wording of Policy BG4: Trees. 

We are disappointed that our proposal for BTRS has not been adopted, but we are also very concerned that this paragraph in particular, will provide developers with an opportunity to avoid replacing lost trees at all: ‘Where the tree compensation standard is not already met in full by biodiversity net gain requirements (policy BG3 ‘Achieving biodiversity gains’), for instance because biodiversity net gain requirements do not apply to the development or because biodiversity gains are provided through a different habitat type, development will still be expected to meet the tree compensation standard on-site or off-site through an appropriate legal agreement.‘

As you know, most trees in an urban environment will be classified as broad Individual tree habitat under BNG 4.0. This broad habitat has only two sub-types – rural and urban – and can only be replaced with the same broad habitat type (Individual tree) or by a more distinctive, High or Very High habitat. This means that other Medium (e.g. most woodland habitats) or Low distinctiveness habitats cannot be used without breaking the BNG 4.0 trading rules – as BG4 currently suggests it can. These High or Very High distinctiveness habitat types are rare, especially in the urban space. 

In this case, developers (who will not have the space to create all the Individual tree habitat that BNG 4.0 will demand**) will offer these or Individual tree habitats elsewhere and, because there are no such sites in Bristol, will offset the BNG losses out of the city, resulting in the hollowing out of Bristol’s trees and frustrating the One City plan to increase tree canopy.

We suggest that the proposed wording could also make BG4 unworkable because it is contrary to the BNG 4.0 rules and guidance. We suggest that you delete the words ‘or because biodiversity gains are provided through a different habitat type.’

Can you clarify whether the current Bristol Tree Replacement Standard SPD will remain, please. Is there a list of proposed deprecated policies and SPDs etc. available?

** For example, one small single dwelling development we are looking at which would require five BTRS trees to be planted to replace the three lost, will require 148 BNG 4.0 Small category trees to be planted to achieve a net gain of just 10%. There is not enough room on the site to plant the five BTRS trees, let alone 148.

Subsequent email to BCC Local Plan Team Manager 26 October 2023

Dear Colin,

I am sure you have seen our request to councillors in advance of next week’s Extraordinary Full Council meeting to adopt the Mayor’s recommendation to allow the draft Local Plan to progress to Regulation 19/20 consultation and then to independent examination. If not, I attach a copy.

We Bristolians are as much entitled to know which of their places (and a Local Plan is surely all about place) will not be protected under a new Local Plan as they are to know which will be. Yet, as far as we can see, this information has not been published with the papers laid before Councillors. Please correct me if I am wrong.

For example, we are aware that, under the 2019 document, New Protection for Open Space, it was proposed that Important Open Spaces, currently protected under SADMP DM17, would be replaced with new policies for Local Green Space (LGS) and Reserved Open Space (ROS) (para 2.13). It was obvious then that this would result in a large number of sites, currently protected under this part of DM17, losing this protection because they were not going to be designated as either LGS or ROS nor given any other protections. You will recall that it took us quite some time to get a list of these deprecated sites which we then listed in Appendix A of our response to that consultation. We have no idea whether our representations were considered. From what we have seen, it appears that, if they were, then they were ignored.

It also appears that those other places also given protection under DM17 – Unidentified Open Spacesand Urban landscapes – will also no longer be protected under the new plan, though this has not been expressly stated as far as we can see. It may well be that other place protections have also been quietly dropped and not replaced, but we cannot tell.
This is why we are calling for the following schedules (preferably geolocated) to be published before the next stage of the consultation begins:

  1. All proposed LGS/ROS designations.
  2. All sites (places) currently protected under the adopted Local Plan and how they will be protected (or not) under the new LP.
  3. Currently adopted policies which will be removed – Core, Site Allocation and Development Management Policies (SADMP) and ancillarySupplementary Planning Documents (SPDs) etc. – cross-tabulated to show which of these policies have been transferred to the proposed LP and which have not.
  4. All sites proposed to be protected under BG2.
  5. All known veteran and ancient trees and woodland within the city boundaries.

If this information is not provided then it will be impossible for those who wish to respond to the consultation to make an informed decision whether or not to accept what is being proposed and the whole consultation process will, we suggest, be fatally flawed.

I have also heard it suggested that, should Councillors not approve the Mayor’s recommendation then the current adopted Local Plan will lapse and allow developers to proceed as they wish. You know as well as I do that this is not correct. It may well be that, on appeal, developers may argue that Paragraph 11d of the NPPF applies because the Local Plan is out-of-date (Homes England argued this in the recent Brislington Meadows appeal), but this is a very different matter from what I understand has been suggested. Hopefully you will ensure that Councillors are not misled if this is repeated.

I look forward to hearing from you.

[1] R v North & East Devon Health Authority, ex parte Coughlan [2001] QB  213, [2000] 3 All ER 850, 97 LGR 703

It seems that SNCIs are nothing special – an open letter to Bristol’s Chief planner

Dear Simone,

We were very disturbed to hear your advice to Councillor Pearce at last night’s Development Control Committee B meeting to consider the expansion of South Bristol Cemetery on to land used by Yew Tree Farm, a Site of Nature Conservation Interest (SNCI). You advised Councillor Pearce that the definition of ‘harm’ under SADMP DM19 was based on the net (not gross) harm caused after mitigation had been considered.

You seemed to be using Biodiversity Net Gain (BNG) as the proxy for harm, so that the reported net gain of nearly 3% was sufficient to conclude that there was no ‘harmful impact’ as defined by DM19.

Bristol Local Planning Policy DM19 plainly states that ‘Development which would have a harmful impact on the nature conservation value of a Site of Nature Conservation Interest will not be permitted.’ It could not be clearer.

If your interpretation of this is correct (and we say it cannot be), it will effectively nullify any policy protection for SNCIs or indeed, any other existing green infrastructure and all SNCIs could be developed in a free-for-all. We set out our reasoning below.

The Mitigation Hierarchy

The Mitigation Hierarchy, as enshrined at paragraph 180 a) of the NPPF, states:

When determining planning applications, local planning authorities should apply the following principles:

a) if significant harm to biodiversity resulting from a development cannot be avoided (through locating on an alternative site with less harmful impacts), adequately mitigated, or, as a last resort, compensated for, then planning permission should be refused;

On your interpretation, the first element of this cascading test, avoidance, will never have to be applied. Instead, you need only consider the second element, mitigation, for it is only then that ‘harm’ can be assessed. This cannot be the intention of this provision.

Green spaces protection

The effect of your approach is to make a nonsense of the prohibition against causing any ‘harmful impact’ to an SNCI as conferred by DM19. It effectively nullifies the special protection given to these sites. Here is the full DM19 policy wording:

On your interpretation, the whole section relating to Sites of Nature Conservation Interest may as well be deleted, as it adds nothing to the more general policy set out above.

The section relating to wildlife corridors is also rendered meaningless if there can now be no net ‘harmful impact’.

The same conclusion must also apply to the protection of Urban Landscapes under SADMP DM17, another feature which ‘contributes to nature conservation in Bristol’, on your interpretation. Your interpretation might also be extended to the other Existing Green Infrastructure identified in DM17.

Achieving BNG means there is no ‘harm’

When the Environment Act 2021’s requirement for all developments to achieve at least 10% biodiversity net gain takes effect later this year, it must follow that schemes which achieve this will have caused no ‘harm’ under your definition.

How then should this be interpreted if the net gain can only be achieved through offsite mitigation (as will often be the case)? Even in this scenario, it seems that there can never be any circumstance where an SNCI can suffer a harmful impact because it must always be mitigated by the requirement to achieve at least 10% BNG. It is even possible to imagine that the SNCI status of the target site will be lost as a result of the development, yet, as you see it, this will not be ‘harm’.

You are in effect stating that no SNCI in Bristol now has any greater protection than any ‘other habitat, species or features, which contribute to nature conservation in Bristol’ and the whole special status of SNCIs has become meaningless.

This cannot be what was intended when SNCIs were created and given special protection under the Local Plan.

​We urge you to reconsider your advice.

Our statement to the Planning Committee can be read here.

A copy of this letter may be downloaded here

Our proposal for a new Bristol Tree Replacement Standard revisited

Following discussions with the Council about our recent proposal to revise BTRS, we have drafted a new version which we believe will strengthen tree protection across the city even further if it is adopted into the proposed new Local Plan.

Revisions are shown in red.

The latest version of the Biodiversity Metric (BNG 4.0), just published by Natural England,[1] is likely to become mandatory when the balance of the Environment Act 2021 comes into force later this year. We have revisited our June 2022 proposals and reviewed our calculations.  We have met with Bristol City Council Officers and discussed possible alternatives with them.  Here is the revised version.

The Bristol Tree Replacement Standard[2] (BTRS), adopted a decade ago, provides a mechanism for calculating the number of replacements for any trees that are removed for developments. It was ground-breaking in its time as it, typically, required more than 1:1 replacement of trees lost to development and within one mile of the development.

The presumption when considering any development involving established trees should always be that trees will be retained. The application of BTRS should only ever be a last resort. Providing funds in exchange for trees that are removed on development sites should not be the default choice which it seems to have become. In addition, in many instances, the locations of the promised replacement trees are not specified and the trees are never planted. As a result, section 106 tree replacement funds continue to accumulate to the 2023 figure of approximately £800K. This figure as barely changed over the years we have been monitoring it.

The starting point for any decision on whether to remove trees (or any other green asset for that matter) is the Mitigation Hierarchy. Paragraph 180 a) of the National Planning Policy Framework sets it out as follows:

If significant harm to biodiversity resulting from a development cannot be avoided (through locating on an alternative site with less harmful impacts), adequately mitigated, or, as a last resort, compensated for, then planning permission should be refused.[3]

BTRS is and should always be ‘a last resort’. This is reflected in the Bristol Core Strategy, policy BCS9 adopts this approach and states that:

Individual green assets should be retained wherever possible and integrated into new developments.[4]

However, with the development of a new Local Plan for Bristol, we believe that the time has come for BTRS to be revised to reflect our changing understanding of the vital importance of urban trees to Bristol in the years since the final part (SADMP[5]) of the Local Plan was adopted in 2014.

In addition, Bristol has adopted Climate and Ecological Emergency Declarations so a new BTRS will be an important part of implementing these declarations. Nationally, the Environment Act 2021[6] (EA 2021) will come force later this year. This will require nearly all developments to achieve a Biodiversity Net Gain (BNG) of at least 10%. Our proposal provides a mechanism for complying with this new requirement and so aligns BTRS with the BNG provisions of the EA 2021.

Background

Under current policy – BCS9 and DM17[7] – trees lost to development must be replaced using this table:

Table 1 The Current DM17/BTRS replacement tree table.

However, when the balance of EA 2021 takes effect, the current version of BTRS will not, in most cases, be sufficient to achieve the 10% BNG minimum that will be required for nearly all developments. A new section 90A will be added to the Town and Country Planning Act 1990 and  set out the level of BNG required (see Schedule 14 of EA 2021[8]).

The Local Government Association says of BNG that it:

…delivers measurable improvements for biodiversity by creating or enhancing habitats in association with development. Biodiversity net gain can be achieved on-site, off-site or through a combination of on-site and off-site measures.[9] GOV.UK says of the Biodiversity Metric that: where a development has an impact on biodiversity, it will ensure that the development is delivered in a way which helps to restore any biodiversity loss and seeks to deliver thriving natural spaces for local communities.[10]

This aligns perfectly with Bristol’s recent declarations of climate and ecological emergencies and with the aspirations of the Ecological Emergency Action Plan,[11] which recognises that a BNG of at least 10% net gain will become mandatory for housing and development and acknowledges that:

These strategies [the Local Nature Recovery Strategies] will guide smooth and effective delivery of Biodiversity Net…

Our proposed new BTRS model

We propose that the Bristol Tree Replacement Standard be amended to reflect the requirements of the EA 2021 and BNG 4.0 and that the BTRS table (Table 1 above) be replaced with Table 2 below:

The Replacement Trees Required number is based on the habitat area of each of the three BNG 4.0 tree category sizes (Table 8-1 below) divided by the area habitat of one BNG 4.0 Small category tree (see section 3 below) plus a 10% net gain. This is rounded up to the nearest whole number – you can’t plant a fraction of a tree.

Replacing lost woodland

The current BTRS model does not deal effectively with the loss of woodland where it is impractical to measure individual tree sizes. Under BNG 4.0 these habitats are treated as Woodland and forest habitat and their habitat area is measured by the area they cover. We propose using the same method and adding 10% to allow for biodiversity net gain. The ratio will be 1 to 1.1 so that a woodland of, say, one hectare must be replaced with one which is 1.1 hectares.

The definition of a woodland is as set out in the UK Habitat Classification[12], w Woodland and forest: ‘Land with more than 25% cover of trees more than 5m in height.’

Replacing lost trees with hedgerows

The aim of BTRS is to replace lost tree habitat and canopy. Planting hedgerows cannot do this.

Whilst the planting of hedgerows is always to be encouraged, especially native species hedgerows, proposals to replace trees lost to development with hedging is very rarely a suitable solution and will not be permitted unless the developer is able to show that there are exceptional reasons for doing so. The planning arboricultural officer will need to agree the exceptional circumstances.

If a replacement hedgerow is permitted, this cannot be credited towards any BNG 4.0 calculation relating to trees. Hedgerows are a different habitat type, being linear as opposed to area based as trees are.

Making the BTRS calculation transparent

Often, years pass before trees lost to development are replaced and often the lost trees are only shown in the subsequent s106 agreement[13] as a single sum which was calculated years before and indexed to allow passage of time since the adoption of the Supplementary Planning Document (SPD) in January 2013.[14]

We would like to see a schedule set out in the S106 agreement and/or as a planning condition which itemises:

  1. The number and identity (using Id used in the BS5837:2012 survey) of each tree to be removed.
  2. The number and species of the trees to be planted on the development site.
  3. The number and species of the trees to be planted on public land.
  4. Which offsite trees are in to be planted open ground and which in hard standing.
  5. The agreed location and species of each offsite replacement tree which and should be within one mile radius of the lost tree.
  6. Trees planted under BTRS should not replace lost public trees, such as street trees removed in the normal course of tree management.
  7. Like for like replacement.  Compensation for the loss of large-form trees should result in large-form trees being planted.
  8. Require that replacement trees or trees damaged as a result of the development that die within five years of planting will be replaced at the developer’s expense – This is the standard condition for trees planted on a development site.

The reasoning for our proposals is set out below:

  1. Applying the Biodiversity Metric to Urban trees

The most recent Biodiversity Metric[15] (BNG 4.0) published by Natural England this April, defines trees in urban spaces as Individual trees called Urban tree habitats. The User Guide states that:

Individual trees may be classed as ‘urban’ or ‘rural’. Typically, urban trees will be bound by (or near) hardstanding and rural trees are likely to be found in open countryside. The assessor should consider the degree of ‘urbanisation’ of habitats around the tree and assign the best fit for the location.

Individual trees may also be found in groups or stands (with overlapping canopies) within and around the perimeter of urban land. This includes those along urban streets, highways, railways and canals, and also former field boundary trees incorporated into developments. For example, if groups of trees within the urban environment do not match the descriptions for woodland, they may be assessed as a block of individual urban trees.

  1. Calculating Individual trees habitat

Table 8-1 in the BNG 4.0 user guide is used to calculate the ‘area equivalent’ of individual trees:

Note that the tree’s stem diameter will still need to be ascertained using BS:5837 2012,[16] and that any tree with a stem diameter (DBH) 7 mm or more and of whatever quality (even a dead tree, which offers its own habitat benefits) is included. Under the current DM17/BTRS requirement, trees with a DBH smaller than 150 mm are excluded, as are BS:5837 2012 category “U” trees. This will no longer be the case.

The Rule 3 of the BNG User guide makes it clear that like-for-like replacement is most often required, so that lost Individual trees (which have Medium distinctiveness) are to be replaced by Individual trees rather than by other habitat types of the same distinctiveness.[17]

  1. Forecasting the post-development habitat area of new Individual trees

The BNG 4.0 User Guide provides this guidance:

8.3.13. Size classes for newly planted trees should be classified by a projected size relevant to the project timeframe.

  • most newly planted street trees should be categorised as ‘small’
  • evidence is required to justify the input of larger size classes

8.3.14. When estimating the size of planted trees consideration should be given to growth rate, which is determined by a wide range of factors, including tree vigour, geography, soil conditions, sunlight, precipitation levels and temperature.

8.3.15. Do not record natural size increases of pre-existing baseline trees within post-development calculations.

Our calculations are based on ‘small’ category replacement trees being planted.

  1. Retain the community benefits of green assets

The current requirement that any off-site tree replacements are within a one-mile radius of the site should be retained. Were tree replacements to be allowed at any distance from the site, the local community that has lost trees due to a development would likely not benefit from their replacements. If trees lost in tree-deprived areas were to be replaced in areas with more available space and often more trees, the result would be greater inequality in tree cover. Currently, the most socially and economically deprived areas in the city centre have the lowest tree cover, and as these areas are also under the most pressure from developments, to lose this localism in tree replacement would lead to a further deprivation in tree cover for these communities.

  1. The likely impact of this policy change

We have analysed tree data for 1,038 surveyed trees taken from a sample of BS:5837 2012 tree surveys submitted in support of previous planning applications. Most of the trees in this sample, 61%, fall within the BNG 4.0 Small range, 38% are within the Medium range, with the balance, 1%, being categorised as Large.

Table 4 below sets out the likely impact of the proposed changes to BTRS. It assumes that all these trees were removed (though that was not the case for all the planning applications we sampled):

The spreadsheet setting out the basis of our calculations can be downloaded here – RPA Table BNG 4.0 8-1 table Comparison.

Our proposed changes to DM17 and BTRS are set out in Appendices 1 and 2.

Appendix 1 – Our proposed changes to DM17: Development Involving Existing Green Infrastructure…

Trees

All new development will integrate important existing trees[18].

Development resulting in the loss or deterioration of irreplaceable habitats (such as ancient woodland and ancient or veteran trees) will be refused, unless there are wholly exceptional reasons and a suitable compensation strategy exists.[19]

Where tree loss or damage is unavoidable to allow for sustainable development, replacement trees of an appropriate species will be provided in accordance with the tree replacement requirements set out in the Council’s Planning Obligations – Supplementary Planning Document. …

Trees

2.17.6 Due to their characteristics and value, Aged and Veteran trees are considered to be of relatively greater importance than other trees and even trees of a similar species. Aged trees, by definition, have developed characteristics associated with great age and often have particular landscape and townscape value. Veteran trees are considered to have particularly important nature conservation value. Both will often have significant visual amenity, and potentially historic and cultural importance. As such, their loss or harm will not be permitted, and the design and layout of development will be expected to integrate them into development.

2.17.7 Trees are considered valuable multifunctional green infrastructure assets. This policy seeks to protect the most valuable trees and in line with the Core Strategy approach to green infrastructure assets, mitigate for the loss of other important trees by securing replacement trees on-site or in the public realm. The tree compensation standard set out in this policy provides a suitable mechanism to determine the appropriate level of mitigation where loss of trees is proposed as part of development.

2.17.8 The council’s Planning Obligations Supplementary Planning Document sets out the circumstances when off-site tree provision will be necessary. Where trees are to be provided off-site, planning obligations will be sought to provide the appropriate number of replacement trees, utilising the approach set out in the Supplementary Planning Document.

Appendix 2 – Our proposed changes to BTRS

Trees – Policy Background

The justification for requiring obligations in respect of new or compensatory tree planting is set out in the Environment Act 2021, Policies BCS9 and BCS11 of the Council’s Core Strategy and in DM 17 of the Council’s Site Allocations and Development Management Policies.[20]

Trigger for Obligation

Obligations in respect of trees will be required where there is an obligation under the Environment Act 2021 to compensate for the loss of biodiversity when Urban tree habitat is lost as a result of development.

Any offsite Urban tree habitat creation will take place in sites which are either on open ground or in areas of hard standing such as pavements and are located as close as possible to the site of the lost tree.

Where planting will take place directly into open ground, the contribution will be lower than where the planting is in an area of hard standing. This is because of the need to plant trees located in areas of hard standing in an engineered tree pit.

All tree planting on public land will be undertaken by the council to ensure a consistent approach and level of quality, and to reduce the likelihood of new tree stock failing to survive.

The trees planted will not replace lost public trees, such as street trees or trees in parks, removed in the normal course of tree management.

Level of Contribution

The contribution covers the cost of providing the tree pit (where appropriate), purchasing, planting, protecting, establishing and initially maintaining the new tree. The level of contribution per tree is as follows:

  • Tree in open ground (no tree pit required) £765.21
  • Tree in hard standing (tree pit required) £3,318.88[21]

The ‘open ground’ figure will apply where a development results in the loss of Council-owned trees planted in open ground. In these cases, the Council will undertake replacement tree planting in the nearest appropriate area of public open space.

In all other cases, the level of offsite compensation required will be based on the nature (in open ground or in hard standing) of the specific site which must be identified by the developer and is approved by the Council during the planning approval process. In the absence of any such agreement, the level of contribution will be for a tree in hard standing.

The calculation of the habitat required to compensate for loss of Urban trees is set out in Table 8-1 of the Biodiversity Metric (BNG), published by Natural England. This may be updated as newer versions of BNG become mandatory under the Environment Act 2021.

The following table will be used when calculating the level of contribution required by this obligation:

Were it is not reasonably practicable to ascertain the number or size of trees growing in a woodland, the level of compensation required will be 110% of the area covered by the trees, so that, for example, a woodland of one hectare will be replaced with one which is 1.1 hectares. The definition of a woodland is as set out in the UK Habitat Classification, w Woodland and forest: ‘Land with more than 25% cover of trees more than 5m in height.’[22]

The level of contribution required for planting trees in a woodland setting will be £[TBD] per 100 square metres.

Proposals to replace trees lost to development with hedging is very rarely a suitable solution and will not be permitted unless the developer is able to show that there are exceptional reasons for doing so and the planning arboricultural officer has agreed.

Planning obligations will contain the following:

  1. The number and identity (using Id used in the BS5837:2012 survey) of each tree to be removed.
  2. The number and species of the trees to be planted on the development site.
  3. The number and species of the trees to be planted on public land.
  4. Which offsite trees are in to be planted open ground and which in hard standing.
  5. The agreed location and species of each offsite replacement tree which and should be within one mile radius of the lost tree.
  6. Trees planted under BTRS should not replace lost public trees, such as street trees removed in the normal course of tree management.
  7. Like for like replacement.  Compensation for the loss of large-form trees should result in large-form trees being planted.
  8. Require that replacement trees or trees damaged as a result of the development that die within five years of planting will be replaced at the developer’s expense.

[1]https://publications.naturalengland.org.uk/publication/6049804846366720

[2]https://www.bristol.gov.uk/files/documents/81-spd-final-doc-dec2012/file – Page 20.

[3]https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1005759/NPPF_July_2021.pdf

[4]https://www.bristol.gov.uk/files/documents/64-core-strategy-web-pdf-low-res-with-links/file at page 74.

[5]https://www.bristol.gov.uk/files/documents/2235-site-allocations-bd5605/file

[6]https://www.legislation.gov.uk/ukpga/2021/30/contents/enacted

[7]https://www.bristol.gov.uk/files/documents/5718-cd5-2-brislington-meadows-site-allocations-and-development-management-policies/file page 36.

[8]https://www.legislation.gov.uk/ukpga/2021/30/schedule/14/enacted

[9]https://www.local.gov.uk/pas/topics/environment/biodiversity-net-gain.

[10]https://www.gov.uk/government/news/biodiversity-30-metric-launched-in-new-sustainable-development-toolkit.

[11]https://www.bristol.gov.uk/documents/20182/5572361/Ecological_Emergency_Action_Plan.pdf/2e98b357-5e7c-d926-3a52-bf602e01d44c?t=1630497102530.

[12] https://ukhab.org/

[13]https://www.legislation.gov.uk/ukpga/1990/8/section/106

[14]https://www.bristol.gov.uk/files/documents/81-spd-final-doc-dec2012/file

[15] https://nepubprod.appspot.com/publication/6049804846366720

[16]https://knowledge.bsigroup.com/products/trees-in-relation-to-design-demolition-and-construction-recommendations/standard

[17] Table 3-2 Trading rules (Rule 3) to compensate for losses. Any habitat from a higher distinctiveness band (from any broad habitat type) may also be used.  [18] Need to define what ‘important means.

[19] This is based on NPPF para. 180 c). We have inserted ‘will’ instead of ‘should’.

[20] These references may need to be changed to reflect any replacement policies adopted with the new Local Plan.

[21] These values should be updated to the current rates applicable at the time of adoption. The current indexed rates as of June 2023 are £1,171.79 & £5,082.29 respectively.

[22]https://ukhab.org/

Farewell to the Meadows

A small grove massacred to the last ash,
An oak with heart-rot, give away the show:
This great society is going to smash;
They cannot fool us with how fast they go,
How much they cost each other and the gods.
A culture is no better than its woods.

W.H. Auden from ‘Bucolics, II: Woods’

Nearly six weeks ago, on 17 April, our hopes of preserving our beloved Brislington Meadows were dashed. Homes England has been allowed to continue with its plans to use the land for housing. The almost universal cry of ‘No!’ from across the city has fallen on deaf ears; Homes England will carry on regardless.

But we haven’t given up. We have all – The Bristol Tree Forum, Greater Brislington Together and Save Brislington Meadows Group – been searching high and low to find a way to stop this, even at the eleventh hour. And we’ve succeeded! We’ve found serious omissions in the planning inspector’s decision which, we believe, give us grounds to have it overturned.

Here’s a summary of the reasons why we think the decision should be set aside. They are a bit technical, but they are important:

  1. The Inspector’s Decision has entirely missed the fact that part of the site – part of the proposed vehicle access at the north-west corner to Broomhill Road, with a strip of housing development there (the only viable point of access onto the development site) – is designated in the adopted Site Allocations and Development Management Policies (SADM) as ‘Important Open Space: Belroyal Avenue, Brislington’.
  2. SADM policy DM17 states: ‘Development on part, or all, of an Important Open Space as designated on the Policies Map will not be permitted unless the development is ancillary to the open space use.’  The failure to have regard to this clear conflict with policy was a breach of s.38(6) and s.70 of the Planning and Compulsory Purchase Act 2004. It’s notable that this part of the site is outside of the Site Allocation, discussed below, which the Inspector placed so much weight on.
  3. What’s more, this same part of the development was confirmed by the Council as a public open space called Belroyal Avenue Open Space in its 2008 Bristol Parks and Green Space Strategy. Because of this and its historic use for recreation, the site is protected by a statutory trust under s.10 of the Open Spaces Act 1906.  However, when this land was sold to Homes England in March 2020, the Council failed to meet the requirements of s.123(1) and (2A) of the Local Government 1972. As a result, the site remains subject to the statutory trust, held for the enjoyment of the public, and may not be developed. The principle of the statutory trust was recently confirmed by the Supreme Court in the case of Day v. Shropshire. Even though this case was not brought before the planning inspector (it was published only three days before our three-week planning appeal ended), the legal principle at the heart of it was a material consideration that should have been taken into account in the Inspector’s 17 April decision. This is especially so, given the earlier 1 November 2016 Cabinet decision (item 12) to ignore the 2012 decision of the Greater Brislington Partnership not to declare this land surplus to their Green Space requirements and decide that the land should be sold anyway. Site ‘1’ on map N5954e  – which was available when the Cabinet met in November 2016 – clearly shows the Belroyal Avenue Open Space as owned by the Council and subject to its 2008 Bristol Parks and Green Space Strategy designation.
  4. The Brislington Meadows Site Allocation policy, BSA1201 (at page 154), states that ‘the development should retain or incorporate important trees and hedgerows within the development which will be identified by a tree survey.’ The Inspector identified a number of ‘relatively important trees for the purposes of BSA1201’ which would be lost. To allow this must be a breach of the BSA1202 requirement. Despite this, they then found compliance with the policy. This is irrational, as is the fact that they judged that ‘broadly speaking, the most important hedgerows would see the most retention’. This must mean that some of the other most important hedgerows will be lost. This is also in conflict with BSA1201. 
  5. Compliance with BSA1201 is also used to reduce significantly the weight accorded to the breach of DM17 in respect of the requirement to integrate important existing trees. This gives another ground of challenge in relation to the Inspector misinterpreting the criterion in BSA1201 and/or irrationally failing to acknowledge that the loss of important trees and hedgerows constitutes a breach of BSA1201, being compounded by a consequential reduction in weight accorded to the conflict with DM17.

For all these reasons, we’ve a strong case to make to overturn the inspector’s decision. Time is running out, though – we only have until this coming Friday, 26 May, to issue proceedings. It is tight, but we could do it. But we’ll need to find at least £50,000 to bring and argue our case.

No doubt Homes England would be determined to fight us all the way and, whilst they seem to have access to almost limitless public funds and can afford the most expensive lawyers, we don’t. We’re just a group of local volunteers doing the best we can to save this precious green space. We don’t have much money – certainly not £50,000! Any money we can raise will depend on the generosity of the public. This is a big ask, especially as times are hard and money is tight. Also, should we lose (and we could), Homes England will want their costs paid as well. This is just too much of a risk.

We’ve written to the Council asking if they plan to challenge the decision and have said why we think they have a case. Sadly, we’ve had no answer. We suspect they’ll be reluctant to do so and expose themselves, yet again, to criticism for their mistakes and misjudgements. This is perhaps especially true given that, as well as losing the appeal, the Inspector has also ordered them (actually, us tax payers) to pay a large part of Homes England’s appeal costs.

Here is a copy of this article.

Analysing the Armada Way Biodiversity Net Gain Assessment

Our Friends at Straw (https://strawplymouth.com/), have been fighting hard to save the few remaining trees on Armada Way, Plymouth so we decided we would try to understand the true impact of the soon-to-be-obligatory Biodiversity Net Gain requirements – how they have been used compared with how they ought to be used – so that the trees that still grow there are properly valued and the loss of their companions can be compensated.

Our conclusion? Over 1,300 extra new trees will need to be planted to replace what has been lost and achieve the 20% net gain promised by the Council.

We have been looking in detail at the Armada Way Biodiversity Net Gain assessment report and calculations, which were provided as part of Plymouth City Council’s ‘Meaningful Community Engagement’. We referred to the Biodiversity Net Gain Metric 3.1 User Guide and additional guidance. This document details our thoughts on the assessment. In summary, we believe the assessment has several flaws that led to the baseline (existing) habitat being undervalued. We also found there was a major flaw in the Metric itself, which led to a significant overestimation of new tree (post-intervention) habitat area. The new Metric (4.0), which supersedes the one used in this assessment, has rectified this issue. If the new Metric were to be used, well over 1000 new trees would need to be planted on site to compensate for losses of healthy, mature trees and achieve the required Biodiversity Net Gain.

Background to Biodiversity Net Gain

Plymouth City Council (PCC) along with many other local authorities, were early adopters of Biodiversity Net Gain (BNG), which is due to become mandatory in November 2023 as part of the Environment Act 2021. While planning permission for nearly all new developments will require a biodiversity increase of 10% or more, Plymouth City Council have decided that they will require at least 20% BNG for this scheme.

The BNG Assessment for the Armada Way development claimed that a 25.09% BNG would be delivered on site. Biodiversity Metric 3.1 was used for the assessment, which was undertaken by the Environment Partnership (TEP) using YGS tree survey data; with baseline habitat ‘walkover survey’ data provided by a local ecologist. The Biodiversity Metric is a tool used by ecologists to measure changes in biodiversity on a development site. Biodiversity Metric

3.1 has now been superseded by Biodiversity Metric 4.0, which was released on 24 March 2023 (more on this later).

In a BNG Assessment, the biodiversity value of a development site, pre and post ‘intervention’, is measured. Post-development biodiversity value can be increased through the enhancement of baseline habitats and/or the creation of new ones either on or offsite.

Biodiversity value is calculated in Habitat Units (HUs). These are derived from the habitat area, the habitat’s ‘distinctiveness’ and its condition, as well as any ‘strategic significance’ applicable to the site of the habitat. The time taken to achieve the target condition of newly created habitats is also taken into account, as well as the difficulty involved in creating the habitat. A 30-year maintenance and monitoring plan must be included to ensure newly created habitats will survive and reach the desired condition within the period.

Requests from STRAW for further information and evidence

STRAW tell us that they had several concerns and queries regarding the Armada Way BNG Assessment so wrote to the assessor asking for clarification on several points. They also wrote to the ecologist who carried out the baseline habitat walkover survey. Despite a follow-up email, they never received a reply from the assessor. The Armada Way development project manager at PCC, assured STRAW and Plymouth Tree Partnership that the BNG Assessment had been ‘triple checked’ and peer-reviewed and promised that the peer review would be shared. Despite repeated requests, this information was never shared.

Concerns about the Armada Way assessment

Significant overestimation of post-intervention habitat area

One of STRAW’s main concerns around the BNG Assessment for Armada Way, was the significantly greater habitat area ascribed to post development (new) trees than to the existing trees, due to an issue with the Metric itself. We accept that the PCC’s ecologist followed Metric 3.1 correctly in using actual Root Protection Area (RPA) calculations for the existing trees and in using the ‘Urban Tree Helper’ tool for new trees to be planted. However, the two methods are not comparable. If the existing tree measurements had been put into the Urban Tree Helper tool, instead of a habitat area of 0.67 hectares (ha), they would have had a habitat area of over 3 ha. This significant flaw in Metric 3.1 has now been addressed in Metric 4.0, which now requires existing tree habitat measurements to be entered into the tool as well as post-intervention (new tree) measurements.

Note: Root protection area (RPA) is used as a proxy for tree habitat area. RPA is calculated using a standard equation provided in BS5837:2012 – Trees in relation to design, demolition and construction. Recommendations, using the ‘diameter at breast height’ (DBH) measurement, where the diameter of the trunk is measured 1.5m above ground.

Although we believe PCC’s ecologist followed the 3.1 User Guide correctly for habitat area calculations, (i.e. using actual root habitat area for the baseline trees and the Urban Tree Helper tool for new trees) we believe they should have noted the significant flaws in the use of different methods for the baseline and post-intervention calculations, which result in the lowest possible habitat area for baseline trees and a significantly exaggerated habitat area for newly planted ones.

Issues with use of the Urban Tree Helper tool only for the new trees

The BNG Urban Tree Helper tool assigns trees to ‘Small’, ‘Medium’ and ‘Large’ categories, according to their DBH. When considering the categories for new trees to be planted, the ‘Small’ category is for trees with a DBH of between 7cm and 30cm after 30 years and the ‘Medium’ category for trees with a DBH of >30 – <=90cm after 30 years. Large category trees are those with a DBH of 90 cm or over.

The 33 new trees (to be planted) were assessed as reaching the ‘Medium’ category after 30 years. Having consulted tree experts and looked at the available literature, we do not believe that these 33 trees should have been assigned to the Medium category. They should have been assigned to the Small category. What is really interesting is that the Medium category trees – those with a DBH of between >30 and <=90cm – are all given a ‘metric area equivalent’ of a tree with a 90cm DBH, so that even a 31 cm DBH tree has the equivalent habitat area of a 90cm DBH tree. This obviously means a significant overestimation in the habitat area for a tree of 31cm DBH. Our researches have found that, if these trees did reach the medium category after 30 years, which is very unlikely, they would only just enter this category – i.e. they would be at the very low end of the range.

Importantly, an existing tree with a 30cm DBH, using Metric 3.1, would be given a habitat area based on 30cm – 0.0041 ha whereas a newly planted tree – still expected to reach a DBH of 30cm after 30 years, would be assigned a habitat area of 0.0366 ha. This is clearly ridiculous given that they are both the same size trees.

Using Metric 3.1, if there were 100 existing trees due to be felled, that all had DBHs of 30cm, the total habitat area would be 0.41 ha. If these were to be replaced by 100 trees that were expected to achieve a DBH of 30cm after 30 years, they would be credited with a habitat area of 3.66 ha even though they would probably never grow so large. This demonstrates this important failing of Metric 3.1 and the huge overestimation of the habitat value of newly created tree habitat area as part of the proposed scheme. Fortunately, this has been addressed in Metric 4.0, with the requirement for existing trees also be assessed using the Urban Tree Helper tool – though this is now too late for the Armada Way trees. Metric 4.0 also addresses the issue of newly planted trees being wrongly assigned to the Medium category and thereby achieving a significantly exaggerated habitat area. All new trees must now be assigned to the Small category unless there is strong evidence to support assigning them to higher categories.

We entered baseline tree data into the Urban Tree Helper tool. The Results show a much higher baseline habitat area (over 3 ha) when following the Metric 4.0 User Guide. Post- intervention habitat area is 1.6 ha; a net loss.

Other concerns with the BNG assessment

Not including all existing trees in the baseline calculations

Apart from this obvious flaw that significantly overestimates new habitat area, we believe the assessment also has several other flaws. There were 11 ‘Category U’ trees that should have been included in the baseline habitat area calculations but were left out. These are trees that were considered unsuitable for retention under BS5837:2012. They should however, according to the Metric 3.1 User Guide, have been included in the baseline calculations. This would have given a greater total habitat area for the baseline urban trees on the site.

Trees growing in groups were also disregarded and counted as just one tree. Had they all been assessed then this would also have increased the baseline habitat area (as detailed in Section 7 of User Guide 3.1).

Misleading statements on damage to the built environment

The BNG assessment stated that ‘the current tree stock is understood to be generally inappropriate to their urban setting, with several causing damage to the built environment’. No evidence was referenced to underpin this statement in relation to Armada Way, and it is not relevant to a BNG Assessment save for assessing their condition. The assessor appears to have used data from a larger tree survey of the area (487 trees), not just Armada Way, and there seems to be no clear evidence in the reports that damage had occurred in Armada Way from the existing (now mostly destroyed) tree stock.

Strategic significance

The BNG Metric includes a consideration of ‘strategic significance’ where local plans and strategies are taken into account. We believe there is room for doubt over the strategic significance being set as ‘low’. The Plymouth and South Devon Joint Local Plan, the Plymouth Plan for Trees and the Plymouth Policy Area Open Space Assessment were possibly relevant and if included might have justified setting the baseline ‘strategic significance’ at ‘medium’ or ‘high’. The Plymouth and South Devon Joint Local Plan refers to the mitigation hierarchy: ‘AVOID LOSS – retain suitable existing arboricultural features on site wherever possible’. DEV26.5 in the plan states that ‘applying Biodiversity Net Gain is not an alternative to the application of the mitigation hierarchy and it would be unacceptable practice for a developer to compensate without first seeking to avoid and mitigate’. Whether or not this should have been considered under ‘strategic significance’, it should have been an important consideration for the project. There appears to be no evidence of any attempt, at any stage of the design process, to retain existing healthy, mature trees.

The Plymouth Plan for Trees (2018) also does not appear to have been considered. The following principles should arguably have been taken into consideration:

  • Plymouth’s trees and woods should be celebrated. Arguably an urban forest in the heart of the city, which happens to be a designated public green space and in a ward with well below average tree cover, should have been valued and celebrated and included in the design from the early stages. Tree canopy cover in the St Peter and the Waterfront ward is 9.3% (forestresearch.gov.uk). The national average for council wards is 16%. The minimum council wards should aim for is 20% (Woodland Trust, 2023). New trees, even if greater in number, would have a much-reduced canopy cover, even in the longer term.
  • Use all available planning and forestry legislation and powers to safeguard Plymouth’s trees. It would be expected that the Plymouth and South Devon Joint Local Plan (adopted in 2019) would have informed the process, particularly: ‘AVOID LOSS – retain suitable existing arboricultural features on-site where-ever possible’.
  • Increase overall canopy cover in the city. The Armada Way proposal appears to reduce canopy cover, despite tree numbers increasing. Tree canopy cover in the St Peter and the Waterfront ward (that Armada Way lies within) is 9.3% (forestresearch.gov.uk). The national average for council wards is 16%. The minimum council wards should aim for is 20% (Woodland Trust 2023).

The Plymouth Policy Area Open Space Assessment identified Armada Way as a Green Corridor. This does not seem to have been considered and could potentially have increased the ‘strategic significance’ score in the BNG assessment. If these plans and strategies were considered and ruled out as having ‘strategic significance’, the reasoning should have been explained in the assessment, or at least provided upon request, to meet BNG transparency principles.

Habitat condition

There may also have been an undervaluation of the condition of tree habitat and ‘other habitats’ in the assessment. The lichen, bryophyte and other epiphyte communities associated with the existing trees (now gone) does not seem to have been adequately considered. There is the potential for bat roosts, and we have photographs of bryophytes, lichen communities and fungi associated with the trees. There were also a number of nests in the trees that were felled (not active ones, but evidence of the suitability of the trees for nesting). These factors form part of the condition assessment of Urban trees.

The condition of grass habitats and hedge features were set to low, even though they had recently been significantly cut back, and according to the User Guide, they should have been given higher scores as a precautionary measure if recently altered/ cut back. We noted at least 10 different plant species making up the grass habitats, possibly over 15 species. If these had been considered, as they should have been, the condition score would have been higher. When we queried this with the ecologist who undertook the habitat walkover survey, they said they visited the site for an hour to look at the grassland which had just been mown and that he was not involved in the BNG assessment. We also understand that the survey was also carried out during the intense heatwave of 2022.

Lack of maintenance budget

It should be noted that the stated BNG (25.09%), which we believe we have shown here to be incorrect, was to be achieved after 30 years. No maintenance budget had been identified beyond the project construction period (approx. 2 years). Further maintenance funding was to be sought from future (unknown) levies on residential development projects, i.e. funding for 28 years of maintenance had not been secured. With the significant uncertainties over changing climatic conditions over this period, and temperature increases highly likely, we would expect a low confidence in survival potential of newly planted trees. This does not comply with the new provisions of the Environment Act 2021 which require that post intervention habitats be maintained for at least 30 years.

From our detailed analysis, we have advised STRAW that if PCC used the same baseline data for urban tree habitat with the baseline urban tree habitat updated to 3.1137 hectares, which is the area calculated using Metric 4.0, PCC would need to plant at least 1,384 ‘Small’ category trees to achieve at least the 20% net gain promised. On this basis, we calculate that the plans as detailed in the TEP BNG assessment will instead deliver a 62.10% net loss of habitat.

Here is a copy of our BNG 4.0 calculation.

Here is a copy of our Urban tree habitat area calculation.

Conclusions and recommendations

We conclude that, ignoring the other issues identified above, even if PCC’s calculations were compliant with the Metric 3.1 User Guide, the information detailed here highlights the actual losses on the ground. These losses should be of concern to PCC and should be communicated to the public. It is too late to amend the plan to include the 110 healthy mature trees that have now been felled. The remaining trees could however be included. Significantly more trees would need to be planted to compensate for these losses than the number currently proposed, and even more to achieve the 20% biodiversity net gain aspired to. We believe that both the assessment and the methodology were flawed and that this is an important case study that should inform other projects.

In our view, it would be prudent now for PCC to undertake a new BNG assessment (particularly as plans have since been amended and most of the trees surveyed have now been felled) using Metric 4.0 and to publicly share the findings, especially given the previous lack of transparency on the ecological assessments.

We sincerely hope that they will do this and meet their obligation to make good the losses which Plymouth has suffered.

Our proposal for a new Bristol Tree Replacement Standard using Biodiversity Metric 4.0

The latest version of the Biodiversity Metric (BNG 4.0), just published by Natural England, is likely to become mandatory when the balance of the Environment Act 2021 comes into force later this year. We have revisited our June 2022 proposals and reviewed our calculations. Here is the revised version.

The Bristol Tree Replacement Standard (BTRS), adopted a decade ago, provides a mechanism for calculating the number of replacements for any trees that are removed for developments. It was ground-breaking in its time as it, typically, required more than 1:1 replacement of trees lost to development.

The presumption when considering any development involving established trees should always be that trees will be retained. The application of BTRS should only ever be a last resort. It should not be the default choice which it seems to have become.

The starting point for any decision on whether to remove trees (or any other green asset for that matter) is the Mitigation Hierarchy. Paragraph 180 a) of the National Planning Policy Framework sets it out as follows:

If significant harm to biodiversity resulting from a development cannot be avoided (through locating on an alternative site with less harmful impacts), adequately mitigated, or, as a last resort, compensated for, then planning permission should be refused.[1]

BTRS is and should always be ‘a last resort’. This is reflected in the Bristol Core Strategy, policy BCS9 adopts this approach and states that:

Individual green assets should be retained wherever possible and integrated into new developments.[2]

However, with the development of a new Local Plan for Bristol, we believe that the time has come for BTRS to be revised to reflect our changing understanding of the vital importance of urban trees to Bristol in the years since the final part (SADMP) of the Local Plan was adopted in 2014.

In addition, Bristol has adopted Climate and Ecological Emergency Declarations so a new BTRS will be an important part of implementing these declarations. Nationally, the Environment Act 2021 (EA 2021) will come force later this year. This will require nearly all developments to achieve a Biodiversity Net Gain (BNG) of at least 10%. Our proposal provides a mechanism for complying with this new requirement and so aligns BTRS with the BNG provisions of the EA 2021.

Background

Under current policy – BCS9 and DM17[3] – trees lost to development must be replaced using this table:

Table 1 The Current DM17/BTRS replacement tree table.

However, when the balance of EA 2021 takes effect, the current version of BTRS will not, in most cases, be sufficient to achieve the 10% BNG minimum that will be required for nearly all developments. A new section 90A will be added to the Town and Country Planning Act 1990 and  set out the level of BNG required (see Schedule 14 of EA 2021).

The Local Government Association says of BNG that it:

…delivers measurable improvements for biodiversity by creating or enhancing habitats in association with development. Biodiversity net gain can be achieved on-site, off-site or through a combination of on-site and off-site measures.[4]

GOV.UK says of the Biodiversity Metric that:

where a development has an impact on biodiversity, it will ensure that the development is delivered in a way which helps to restore any biodiversity loss and seeks to deliver thriving natural spaces for local communities.[5]

This aligns perfectly with Bristol’s recent declarations of climate and ecological emergencies and with the aspirations of the Ecological Emergency Action Plan,[6] which recognises that a BNG of at least 10% net gain will become mandatory for housing and development and acknowledges that:

These strategies [the Local Nature Recovery Strategies] will guide smooth and effective delivery of Biodiversity Net…

Our proposed new BTRS model

We propose that the Bristol Tree Replacement Standard be amended to reflect the requirements of the EA 2021 and BNG 4.0 and that the BTRS table (Table 1 above) be replaced with Table 2 below:

The Replacement Trees Required number is based on the habitat area of each of the three BNG 4.0 tree category sizes (Table 8-1 below) divided by the area habitat of one BNG 4.0 Small category tree (see section 3 below) plus a 10% net gain. This is rounded up to the nearest whole number – you can’t plant a fraction of a tree.

The reasoning for our proposal is set out below.

Applying the Biodiversity Metric to Urban trees

The most recent Biodiversity Metric (BNG 4.0) published by Natural England this April, defines trees in urban spaces as Individual trees called Urban tree habitats. The User Guide states that:

Individual trees may be classed as ‘urban’ or ‘rural’. Typically, urban trees will be bound by (or near) hardstanding and rural trees are likely to be found in open countryside. The assessor should consider the degree of ‘urbanisation’ of habitats around the tree and assign the best fit for the location.

Individual trees may also be found in groups or stands (with overlapping canopies) within and around the perimeter of urban land. This includes those along urban streets, highways, railways and canals, and also former field boundary trees incorporated into developments. For example, if groups of trees within the urban environment do not match the descriptions for woodland, they may be assessed as a block of individual urban trees.

Calculating Individual trees habitat

Table 8-1 in the BNG 4.0 user guide is used to calculate the ‘area equivalent’ of individual trees:

Note that the tree’s stem diameter will still need to be ascertained using BS:5837 2012,[7] and that any tree with a stem diameter (DBH) 7 mm or more and of whatever quality (even a dead tree, which offers its own habitat benefits) is included. Under the current DM17/BTRS requirement, trees with a DBH smaller than 150 mm are excluded, as are BS:5837 2012 category “U” trees. This will no longer be the case.

The Rule 3 of the BNG User guide makes it clear that like-for-like replacement is most often required, so that lost Individual trees (which have Medium distinctiveness) are to be replaced by Individual trees rather than by other habitat types of the same distinctiveness.[8]

Forecasting the post-development habitat area of new Individual trees

The BNG 4.0 User Guide provides this guidance:

8.3.13. Size classes for newly planted trees should be classified by a projected size relevant to the project timeframe.

• most newly planted street trees should be categorised as ‘small’

• evidence is required to justify the input of larger size classes

8.3.14. When estimating the size of planted trees consideration should be given to growth rate, which is determined by a wide range of factors, including tree vigour, geography, soil conditions, sunlight, precipitation levels and temperature.

8.3.15. Do not record natural size increases of pre-existing baseline trees within post-development calculations.

Our calculations are based on ‘small’ category replacement trees being planted.

The likely impact of this policy change

We have analysed tree data for 1,038 surveyed trees taken from a sample of BS:5837 2012 tree surveys submitted in support of previous planning applications. Most of the trees in this sample, 61%, fall within the BNG 4.0 Small range, 38% are within the Medium range, with the balance, 1%, being categorised as Large.

Table 4 below sets out the likely impact of the proposed changes to BTRS. It assumes that all these trees were removed (though that was not the case for all the planning applications we sampled):

The spreadsheet setting out the basis of our calculations can be downloaded here – RPA Table BNG 4.0 8-1 table Comparison.

Our proposed changes to BTRS are set out in Appendix 1.

A copy of this article is available here.

Appendix 1 – Our proposed changes to BTRS

See the Planning Obligations Supplementary Planning Document at page 20.

Trees – Policy Background

The justification for requiring obligations in respect of new or compensatory tree planting is set out in the Environment Act 2021, Policies BCS9 and BCS11 of the Council’s Core Strategy and in DM 17 of the Council’s Site Allocations and Development Management Policies.[9]

Trigger for Obligation

Obligations in respect of trees will be required where there is an obligation under the Environment Act 2021 to compensate for the loss of biodiversity when Urban tree habitat is lost as a result of development.

Any offsite Urban tree habitat creation will take place in sites which are either on open ground or in areas of hard standing such as pavements and are located as close as possible to the site of the lost tree.

Where planting will take place directly into open ground, the contribution will be lower than where the planting is in an area of hard standing. This is because of the need to plant trees located in areas of hard standing in an engineered tree pit.

All tree planting on public land will be undertaken by the council to ensure a consistent approach and level of quality, and to reduce the likelihood of new tree stock failing to survive.

Level of Contribution

The contribution covers the cost of providing the tree pit (where appropriate), purchasing, planting, protecting, establishing and initially maintaining the new tree. The level of contribution per tree is as follows:

  • Tree in open ground (no tree pit required) £765.21
  • Tree in hard standing (tree pit required) £3,318.88[10]

The ‘open ground’ figure will apply where a development results in the loss of Council-owned trees planted in open ground. In these cases, the Council will undertake replacement tree planting in the nearest appropriate area of public open space.

In all other cases, the level of offsite compensation required will be based on the nature (in open ground or in hard standing) of the specific site which will has been identified by the developer and is approved by the Council during the planning approval process. In the absence of any such agreement, the level of contribution will be for a tree in hard standing.

The calculation of the habitat required to compensate for loss of Urban trees is set out in Table 8-1 of the Biodiversity Metric (BNG), published by Natural England. This may be updated as newer versions of BNG become mandatory under the Environment Act 2021.

The following table will be used when calculating the level of contribution required by this obligation:

[1] https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1005759/NPPF_July_2021.pdf

[2] https://www.bristol.gov.uk/files/documents/64-core-strategy-web-pdf-low-res-with-links/file at page 74.

[3] https://www.bristol.gov.uk/files/documents/5718-cd5-2-brislington-meadows-site-allocations-and-development-management-policies/file page 36.

[4] https://www.local.gov.uk/pas/topics/environment/biodiversity-net-gain.

[5] https://www.gov.uk/government/news/biodiversity-30-metric-launched-in-new-sustainable-development-toolkit.

[6] https://www.bristol.gov.uk/documents/20182/5572361/Ecological_Emergency_Action_Plan.pdf/2e98b357-5e7c-d926-3a52-bf602e01d44c?t=1630497102530.

[7] https://knowledge.bsigroup.com/products/trees-in-relation-to-design-demolition-and-construction-recommendations/standard

[8] Table 3-2 Trading rules (Rule 3) to compensate for losses. Any habitat from a higher distinctiveness band (from any broad habitat type) may also be used.

[9] These references may need to be changed to reflect any replacement policies adopted with the new Local Plan.

[10] These values should be updated to the current rates applicable at the time of adoption. The current indexed rates as of May 2023 are £1,143.15 & £4,958.07 respectively.

Biodiversity Metric 4.0: what’s it all about?

On 24 March 2023 Natural England published Biodiversity Metric 4.0. This revised metric will revolutionise the way we value urban tree habitats, making it clearer than ever that they are a very important habitat.

It is anticipated that BNG 4.0 [1] will be given statutory force when the biodiversity elements of the 2021 Environment Act [2] take effect later this year (see Measuring biodiversity net gain – Publication of Biodiversity Metric 4.0). All new planning applications issued after 24 March, where a Biodiversity Net Gain (BNG) calculation is required, will be required to use it.

Unlike several neighbouring local authorities (e.g., BANES & South Gloucestershire County Council), which have already adopted Supplementary Planning Documents to protect their biodiversity, Bristol City Council has decided not to require this as part of current planning applications until the rest of the EA 2021 comes into force. The failure to do this will have a negative ecological and social impact for the many current planning applications. In the meantime, only developers will benefit.

Given Bristol’s declaration of an ecological emergency in 2020, BNG 4.0 must now be implemented in Bristol. This is a key environment measure which could be adopted at no cost to the council.

The NPPF basis for achieving biodiversity net gain

Paragraph 180 a) of the National Policy Planning Framework [3] (NPPF) echoes the overarching Mitigation Hierarchy principles and obliges local planning authorities to refuse planning permission:

if significant harm to biodiversity resulting from a development cannot be avoided (through locating on an alternative site with less harmful impacts), adequately mitigated, or, as a last resort, compensated for…

Paragraph 179 c), states that plans should:

…pursue opportunities for securing measurable net gains for biodiversity.

BNG 4.0 has been designed to give effect to these two core planning goals.

Pending planning applications

Natural England advises that:

‘Users of previous versions of the Biodiversity Metric should continue to use that metric (unless requested to do otherwise by their client or consenting body) for the duration of the project it is being used for. This is because users may find that certain biodiversity unit values generated in biodiversity metric 4.0 will differ from those generated by earlier versions.‘

Given that the approach to valuing urban trees has fundamentally changed, we urge all ‘consenting bodies’ (LPAs for most of us) to require developers to adopt this new methodology, for Individual trees habitats at least.

We have always argued that the old Urban tree habitat area calculation methodology used in BNG 3.0 is flawed and unworkable, and we advocated for the use of the calculation method given in BNG 3.1, if only for Urban tree habitat area calculations. With the advent of BNG 4.0, we plan now to argue instead for the BNG 4.0 Individual trees habitat methodology to be used.

The BNG 4.0 Guide

Here is a link to the BNG 4.0 User Guide, which was published with BNG 4.0 (the quotes in italics below are taken from it). We set out below the salient points that cover most trees growing in an urban setting.

What is Individual trees habitat?

BNG 4.0 has made a substantial change to the way trees growing in the urban space will be valued and introduces a new broad habitat category called Individual trees (to replace the Urban tree habitat category first published with BNG 3.0):

8.3.1. The broad habitat type ‘Individual trees’ may be used where a tree (or a group of trees) over 7.5 cm in diameter at breast height (DBH) does not meet or contribute towards the definition of another broad habitat type.

8.3.2. Individual trees should not be recorded separately where they occur within habitat types characterised by the presence of trees, such as orchards, lines of trees or wood-pasture and parkland, but can be recorded where they do not form part of a primary habitat description.

8.3.3. Ancient and veteran trees are irreplaceable habitats and the broad habitat ‘Individual trees’ must not be used to record these.

Even though all irreplaceable habitats fall outside BNG 4.0, they should still be recorded in the metric calculation. A special form for this has been built into the calculator and special rules apply.

Note: Paragraph 8.3.1 refers to trees ‘over 7.5 cm in diameter’ but table 8-1 below refers to trees that are ‘greater than 7 cm’. BS5837:2012 requires all trees 75 mm or over to be surveyed – at paragraph 4.2.4.

Broad habitat type Individual trees can be in either ‘urban’ or ‘rural’ habitats:

8.3.4. Individual trees may be classed as ‘urban’ or ‘rural’. Typically, urban trees will be bound by (or near) hardstanding and rural trees are likely to be found in open countryside. The assessor should consider the degree of ‘urbanisation’ of habitats around the tree and assign the best fit for the location.

8.3.5. Individual trees may also be found in groups or stands (with overlapping canopies) within and around the perimeter of urban land. This includes those along urban streets, highways, railways and canals, and also former field boundary trees incorporated into developments. For example, if groups of trees within the urban environment do not match the descriptions for woodland, they may be assessed as a block of individual urban trees.

Either way, they have the same Medium habitat distinctiveness, so the difference is perhaps academic.

Developers may seek to argue that some urban trees in groups or blocks are a woodland habitat or a ‘Hedgerow – line of trees’ habitat and not Individual trees habitat. BNG 4.0 and earlier versions use a different approach to calculating their habitat sizes. This approach is based on canopy area for woodland habitats and a linear measurement for ‘Hedgerow – line of trees’ habitats. However, it is the degree of ‘urbanisation’ that is key.

Trees in private gardens

Individual trees habitats within private gardens are also to be recorded in the baseline calculation, but should not form part of the post-development BNG calculation:

8.3.6. Established trees within gardens should be recorded in a site baseline.

8.3.7. Where private gardens are created, any tree planting within the created garden should not be included within post-development sheets of the metric. The habitat type ‘Urban – Vegetated garden’ should be used.

This is an important distinction and means we should be alive to any attempt to include newly created habitat in private gardens into post-development BNG calculations. The logic is that, as private space is outside the control of the developer, any post-development habitat management obligations they have cannot be applied to these spaces, and so should be excluded from the post-development calculation.

Measuring Individual trees habitat size

Habitat size is one of the key parameters used for calculating a habitat’s value – called Habitat Units (HUs). For baseline area habitats, the formula is based on four parameters:

HU = Area in hectares x Distinctiveness x Condition x Strategic significance.

Note: For linear habitats, length in kilometres is used instead of area.

The way BNG 4.0 measures the habitat area of Individual trees has reverted to the methodology used in BNG 3.0 but, thankfully, now uses a table that works!

The effect is far more generous than the one used in BNG 3.1 as it values all the trees in the bottom two categories, Small & Medium, at the top of their range. All Large category trees are given the same habitat value as a tree with a stem diameter (called DBH – diameter at breast height) of 130 cm. [4] Given that the vast majority of urban trees fall within this range – with DBHs of between 7cm and 130 cm – this has the effect of greatly enhancing their habitat value.

The following graph illustrates the effect on a range of DBHs from 7 cm to 160 cm; RPA refers to root protection area and the orange stepped lines are the BNG 4.0 habitat area values assigned to each DBH. [5]

This difference is significant. For example, in a recent application we were involved with, trees on the site that had a baseline Urban tree habitat area of 0.7056 ha using BNG 3.1 now have an Individual trees habitat area of 3.1137 ha when the BNG 4.0 methodology is applied. This makes their habitat unit value much greater than it was before.

Here is the BNG 4.0 Individual trees habitat area measurement methodology:

8.3.8. Once the size, number and condition of trees is known, assessors should generate an area equivalent value using the ‘Tree helper’ within the metric tool ‘Main menu’ (Figure 8-2). The ‘area equivalent’ is used to represent the area of Individual trees. This value is a representation of canopy biomass, and is based on the root protection area formula, derived from BS 5837:2012.

8.3.9. Table 8-1 sets out class sizes of trees and their area equivalent. For multi-stemmed trees the DBH of the largest stem in the cluster should be used to determine size class.

Note: The correct metric equivalent area of Large category trees is 0.0765, not 0.0764.

This same approach applies to Individual trees habitats in groups or blocks:

8.3.12. Assessors should account for the size class (Table 8-1) of each Individual trees within a group or block. The number of Individual trees present within a group or block should be entered into the tree helper to calculate area equivalent. Do not reduce any area generated by the tree helper even if tree canopies overlap.

Assessing baseline Individual trees habitat condition

As ‘condition’ is one of the parameters used for calculating the habitat’s value, each Individual trees habitat tree, group or block needs to be assessed against the following criteria. [6]

Condition Assessment Criteria
AThe tree is a native species (or at least 70% within the block are native species).
BThe tree canopy is predominantly continuous, with gaps in canopy cover making up <10% of total area and no individual gap being >5 m wide (Individual trees automatically pass this criterion).
CThe tree is mature (or more than 50% within the block are mature).
DThere is little or no evidence of an adverse impact on tree health by human activities (such as vandalism, herbicide or detrimental agricultural activity). And there is no current regular pruning regime, so the trees retain >75% of expected canopy for their age range and height.
ENatural ecological niches for vertebrates and invertebrates are present, such as presence of deadwood, cavities, ivy or loose bark.
FMore than 20% of the tree canopy area is oversailing vegetation beneath.
Number of criteria passed
Condition Assessment Result (out of 6 criteria)Condition Assessment Score
Passes 5 or 6 criteriaGood (3)
Passes 3 or 4 criteriaModerate (2)
Passes 2 or fewer criteriaPoor (1)
Note that ‘Fairly Good and Fairly Poor’ condition categories are not available for this broad habitat type.

In our experience, very few Individual Urban tree habitats will ever be assessed as in ‘Good’ condition and many will only ever achieve a ‘Poor’ score. Many urban trees are not native, [7] few survive to become mature, most are subject to some form of management or show ‘evidence of an adverse impact on tree health by human activities’, and most trees in a public space will never be allowed to develop ‘natural ecological niches’ as these often also present a public safety risk.

The same challenges will also apply when attempting to assess the future condition of post-development Individual Urban tree habitats after 30 years have passed (we discuss this below). In our view, every such tree should always be assessed as having a ‘Poor’ outcome given the uncertainties they face.

Assessing baseline Individual trees habitat strategic significance

Strategic significance is the fourth parameter used in calculating HUs. There are three categories – High, Medium and Low:

To qualify as ‘High’, the following evidence needs to be available:

5.4.3. Assessors must provide evidence by referencing relevant documents. If published, the relevant strategy is the Local Nature Recovery Strategy (LNRS). If an LNRS has not been published, the relevant consenting body or planning authority may specify alternative plans, policies or strategies to use.

5.4.4. Alternative plans, policies or strategies must specify suitable locations for habitat retention, habitat creation and or enhancements, and might, for example, be:

  • Local Plans and Neighbourhood Plans
  • Local Planning Authority Local Ecological Networks
  • Tree Strategies
  • Area of Outstanding Natural Beauty Management Plans
  • Biodiversity Action Plans
  • Species and protected sites conservation strategies
  • Woodland strategies
  • Green Infrastructure Strategies
  • River Basin Management Plans
  • Catchment Plans and Catchment Planning Systems
  • Shoreline management plans
  • Estuary Strategies

5.4.5. If no alternative is specified, agreement should be sought from the consenting body or Local Planning Authority when determining strategic significance.

In many cases, the proposed development site will fall within one of the criteria above (especially where the authority has adopted a well-designed tree strategy) and so should be given ‘High’ strategic significance.

If it does not then, given that trees nearly always provide ‘a linkage between other strategic locations’, we suggest that Individual trees habitats should always be assigned ‘Medium’ strategic significance.

It is notable that the Medium strategic significance dropdown option in the Metric calculator is still labelled ‘Location ecologically desirable but not in local strategy’. This suggests a wider definition than is perhaps suggested above.

Post-development Individual trees habitat creation. 

Post-development Individual trees habitat creation also uses the same parameters for the HU calculation discussed above, but with a time-to-target factor added. This is the time it will take the new habitat to reach its target condition. If the created Individual trees habitat condition will be Poor, the time-to-target period is ten years, if it will be Medium, it is 27 years, and if it will be Good, it will be 30+ years.

These periods can be increased or reduced in yearly increments if, somehow, habitat creation has been advanced or delayed.

These are then factored into the calculation to allow for the future habitat created using the 3.5% discount tables – so x 0.700 for ten years, x 0.382 for 27 years and x 0.320 for 30+ years.

The calculation also assesses the difficulty of creating the target habitat. For Individual trees habitats, this is pre-set to Low (score 1), so does not affect the eventual calculation.

Existing habitats can also be enhanced on or off site or created off site. We do not discuss this here.

Post-development Individual trees habitat area forecasting 

This assumes that any new tree planted will grow into a Small category tree at the end of the ‘project timeframe’. This is likely to be 30 years by default, as per Part 1 s.9 of Schedule 14 of the 2021 Environment Act. [8] This is the approach advised in the Guide:

8.3.13. Size classes for newly planted trees should be classified by a projected size relevant to the project timeframe.

  • most newly planted street trees should be categorised as ‘small’
  • evidence is required to justify the input of larger size classes.

8.3.14. When estimating the size of planted trees, consideration should be given to growth rate, which is determined by a wide range of factors, including tree vigour, geography, soil conditions, sunlight, precipitation levels and temperature.

8.3.15. Do not record natural size increases of pre-existing baseline trees within post-development calculations.

If a larger Individual trees habitat area projection is advanced, this will need to be justified.

The evidence of tree growth rates is patchy at best – see the About section in our Tree Canopy Prediction tool. To overcome this, we have adopted the simple rule-of-thumb approach commonly used by arboriculturists and assume that a tree’s girth grows by one inch (2.54 cm) a year. We then apply this to the standard tree sizes adopted in BS 3961-1 – Nursery Stock Specification to Trees and Shrubs [9] to calculate the eventual size of a tree 30 years after it has been planted. In all cases, save for semi-mature trees, the tree will be a BNG 4.0 Small category tree.

Here is the model we use:

The age of the tree being planted should not be ‘credited’ when calculating the time-to-target period. Sadly, BNG 4.0 does not take account of mortality rates, which are high for urban trees.

The Trading Rules

Individual trees habitats are given Medium distinctiveness in BNG 4.0 and so are subject to the Rule 3 Trading Rules:

3.2.1. Rule 3 is automatically applied by the metric and sets minimum habitat creation and enhancement requirements to compensate for specific habitat losses (up to the point of no net loss). These requirements are based on habitat type and distinctiveness, as set out in Table 3-2 (below).

In effect, any habitat losses may not be traded down. In this case, the broad habitat category is Individual trees. Given that there are very few habitats with high or very high distinctiveness that are likely to be either applicable or feasible, this will mean that Individual trees habitats will mostly need to be replaced like-for-like.

In our view, urban trees are too important to be substituted by any other, non-tree habitat.

The effect of these rules is that, not only will the proposed project have to achieve at least 10% biodiversity net gain when the Environment Act 2021 takes effect later in 2023, it will also need to comply with the Trading Rules. In some cases, this will mean that far more than the minimum 10% net gain will need to be achieved.

We look forward with interest to seeing how developers will ‘manage’ this new metric.

A copy of the article can be downloaded from here – Biodiversity Metric 4.0: what’s it all about?

[1] http://publications.naturalengland.org.uk/publication/6049804846366720

[2] https://www.legislation.gov.uk/ukpga/2021/30/contents/enacted

[3] https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1005759/NPPF_July_2021.pdf

[4] This is a girth of 4.08 metres.

[5] This is the spreadsheet it is based on – BNG 4.0 – Individual trees BNG Analysis.xlsx

[6] See Biodiversity Metric 4.0 – Technical Annex 1 – Condition Assessment Sheets and Methodology & Biodiversity Metric 4.0 – Technical Annex 2 – Technical Information

[7] See table 2 of the Woodland Condition Survey forms linked to https://woodlandwildlifetoolkit.sylva.org.uk/assess for the list of recognised native tree and shrub species.

[8] https://www.legislation.gov.uk/ukpga/2021/30/schedule/14/enacted

[9] https://www.thenbs.com/PublicationIndex/documents/details?Pub=BSI&DocID=16650