To calculate the biodiversity value of a habitat, it’s important to assess its strategic significance. A habitat’s strategic significance takes into account both its type and its location. If strategic significance is high, then the habitat’s value will be uplifted by 15%. However, as this article explains, the opportunities for doing this are limited.
A habitat parcel is an area of habitat which is all of the same distinctiveness, condition and strategic significance. Strategic significance refers to the importance of a habitat parcel based on its location and type.
Each habitat parcel needs to be assessed both before, when the baseline habitat is surveyed, and after development, on or off site.
This flowchart sets out how to assess the strategic significance of a habitat parcel. It uses the tables, shown below, from the Statutory Biodiversity Metric User Guide.
When developers cannot meet their obligation to replace habitat lost within their development site, plus at least 10%, they may buy habitat units to offset this lost habitat. These habitat units are available in ‘biodiversity gain sites’.
This article was updated on 23 February 2025 to take account of the development of our new site which dynamically analyses the Biodiversity Gain Register and collates and summarises the published data:
To date, 46 of these biodiversity gain sites (BGS) have been registered in England. They provide:
1,376.7 hectares (ha) of baseline area habitat.
32.76 kilometres (km) of baseline hedgerow habitat.
11.37 km of baseline watercourse habitat.
The BGS sites cover 1,770.41 ha, though not all of this area is used for habitat improvement. 1,420.83 ha baseline habitats are made available for offsetting habitat loss caused by development elsewhere where this lost habitat cannot be replaced on the development site itself.
Distribution of biodiversity gain sites in England
19 of the 46 BGS sites are controlled by RSK Biocensus Limited as the Responsible Body but are mostly owned by Environment Bank. One other is controlled by Harry Ferguson Holdings (based on the Isle of Wight) as the Responsible Body, with the remaining sites under the control of various Local Planning Authorities (LPA) as the Responsible Body. We assume that the LPA sites have been created in order to deal with those local developments which require offsite mitigation. Nonetheless, these sites are also selling habitat to developers which require offsite mitigation but are outside the LPA boundary.
We also ask who is policing these sites to ensure that was has bee promised is being delivered? This must especially be the case for LPA sites given that the LPA cannot monitor itself?
In Bristol the LPA has delegated this function to neighbouring authorities using s.101 Local Government Act 1972 (the power for councils to delegate functions to other local authorities). – See 30 Sept 2024 Economy and Skills Committee notes – from paragraph 9. It will be interesting to see how this turns out. However, despite this, no BGS sites have yet been registered in the city, so it is hard to see how this initiative will be delivered where offsite mitigation is required.
The habitat improvement potential
These sites provide a total of 4,819.3 area baseline habitat units (HUs), 325.59 hedgerow baseline HUs and 105.6 watercourse HUs – we have assumed that all the sites have low strategic significance and that the watercourse habitats are free of encroachment.
We have been able to calculate the improved habitat units being created but not the improved habitat units being enhanced. This is because the parameters upon which these baseline habitats are being improved have not been identified.
The take up to date
So far, 31 of these 46 BGS sites have been used by 59 LPAs to allocate lost habitat caused by 85 developments. The majority of habitats are Other neutral grassland and the remainder are Lowland meadows, Traditional orchards, Floodplain wetland mosaic and CFGM, Mixed scrub, Woodland and forest and Hedgerow habitat.
To encourage developers to choose sites as close as possible to the habitat loss, they don’t need to pay a ‘spatial risk’ penalty if the biodiversity gain site is within the same Local Planning Authority (LPA) as the development. However, if the biodiversity gain site is outside the LPA for a particular development, the developer must pay a penalty when calculating the number of habitat units to be offset. If the site is in an adjacent LPA, the penalty is 25%. If it is farther away, the penalty is 50%.
Unfortunately, it appears that developers are not using BGS within their LPA areas (if available) for offsetting but are paying the spatial risk premium, though perhaps this is because they have no choice as there are no local BGS sites available.
Our analysis shows that, to date, the average distance between the centre of the LPA* where the habitat was lost and where its loss is offset is 80.1 km, with the greatest distance between loss and replacement being 344.8 km. Only six sites are within 10 km of the site of the habitat loss, while 23 are over 100 km away.
* It is difficult automatically to calculate the exact site of the habitat loss on the basis of the information provided. If at least Post Codes were provided, this would be possible.
What is particularly notable is that many of the development sites we have examined appear to be in locations where there should be ample opportunities for local habitat to be improved, but nothing has been done to realise this. Even the South Downs National Park LPA has allowed the replacement of habitat lost in two applications on the same site under its care near Petersfield to be exported to a site some 67 km away near Lewes, albeit that it is still in the National Park.
Furthermore, all 46 of the BGS sites are located on private land, in rural settings that are not easily accessible, whereas the lost habitats were largely located in built-up areas.
However, given the requirement that offsite mitigation only be delivered on registered sites, its hard to see what choice developers have apart from testing the BGS market and buying the cheapest habitats required, albeit that this may be miles from the site of the original loss.
This is still a small sample, which will grow over time so, perhaps this will change as more biodiversity gain sites become available and a clearer trend emerges. At the moment, however, the trend is not encouraging and looks like it will result in local nature, especially in urban settings, becoming hollowed out, as we feared it would when the biodiversity net gain requirements became obligatory nearly a year ago. See our article on this: ‘It seems inevitable Bristol will see a steady, inexorable biodiversity decline’
WEMCA’s Local Nature Recovery Strategy (LNRS) will fail to provide Bristol with the benefits promised for nature. While the new Biodiversity Net Gain (BNG) rules require most development in the city to increase biodiversity by at least 10%, unfortunately the LNRS will not apply to most potential development sites.
The LNRS is a locally led and evidence-based strategy which aims to target action and nature investment where it’s most needed. We’re told that the strategy will also focus on biodiversity net gain by increasing the strategic significance of specific habitats. However, it is hard to imagine how the LNRS will help to enhance biodiversity net gain in most, if not all, potential development sites in the city.
We might have been better off, at least as far as the application of biodiversity net gain to new development is concerned, by asking the LPA to specify alternative documents (such as those listed at the end of this article) for assigning strategic significance instead.
The issue
When calculating the impact of a proposed development on biodiversity, one factor taken into account is the strategic significance of any habitat found on a focus area for nature recovery site (coloured purple in the map above). If strategically significant habitats are created or enhanced, then their strategic significance is set to High in the Statutory Metric calculator tool and a 15% uplift to the calculation of its value is applied. Subject to which of the six LNRS areas is being considered, these are the strategically significant habitats in the city:
Ditches
Ecologically valuable lines of trees
Ecologically valuable lines of trees – associated with bank or ditch
Grassland – Floodplain wetland mosaic and CFGM
Grassland – Lowland calcareous grassland
Grassland – Lowland meadows
Heathland and shrub – Mixed scrub
Heathland and shrub – Willow scrub
Individual urban or rural trees
Lakes – Ponds (priority habitat)
Priority habitat (on the River Avon and the Riparian buffers)
Species-rich native hedgerow with trees – associated with bank or ditch
Species-rich native hedgerow with trees
Species-rich native hedgerows – associated with bank or ditch
Species-rich native hedgerows
Urban – Open mosaic habitats on previously developed land
Urban – Biodiverse green roofs
Woodland and forest – Lowland beech and yew woodland
Woodland and forest – Lowland mixed deciduous woodland
Woodland and forest – Other woodland; broadleaved
Woodland and forest – Wood-pasture and parkland
However, a detailed examination of the LNRS map reveals that not all parks and green spaces have been designated as focus area for nature recovery sites. It’s only those which are in one or both of the following:
a location where they can make a greater contribution to ecological networks
deprived areas with a lack of access to nature.
These designations were based on Bristol’s previous work on ecological networks within the city and where wildlife-friendly interventions are most likely to be feasible. This means that the existence, creation or enhancement of these special habitats outside these areas will not attract the 15% strategic significance uplift.
The BNG requirements
The now compulsory Statutory Metric Guide, used for calculating Biodiversity Net Gain (BNG), advises (at page 27) that: ‘Strategic significance is the local significance of the habitat based on its location and habitat type. You should assess each individual habitat parcel, both at baseline and at post-intervention, for on-site and off-site.’
If the LPA has adopted an LNRS then only the High or Low strategic significance multipliers can be used (High – formally identified in local strategy = 1.15. Low – area compensation not in local strategy = 1). If it has not adopted an LNRS, then the Medium strategic significance multiplier may also be used (Location ecologically desirable but not in local strategy = 1.10).
Where an LPA has adopted an LNRS, all those sites which have not been identified as a focus area for nature recovery site will be designated as having Low strategic significance and so attract no uplift, even if they’ve been identified as important habitats in the Local Plan or in another strategic document adopted by the Council. These documents (used where an LPA has not adopted an LNRS) can include:
Draft Local Nature Recovery Strategies
Local Plans and Neighbourhood Plans
Local Planning Authority Local Ecological Networks
Parks and Green Spaces Strategies
Tree and Woodland Strategies
Area of Outstanding Natural Beauty Management Plans
Biodiversity Action Plans
Species conservation and protected sites strategies
Green Infrastructure Strategies
River Basin Management Plans
Catchment Plans and Catchment Planning Systems
Shoreline management plans
Estuary Strategies
Baseline habitats cannot be uplifted
Despite the BNG strategic significance guidance, Defra has stated that LNRS designations only apply to the creation or enhancement of post-development biodiversity mitigation habitats. They don’t apply if these habitats – called the baseline habitats – are found on the site before development begins.
This means that the 15% strategic significance uplift can only be applied where offsite biodiversity mitigation is being delivered in a focus area for nature recovery site. If these habitats are being delivered elsewhere, the uplift may not be applied.
However, even if the baseline habitats were included, it is unlikely to make any difference This is because the focus area for nature recovery sites identified in Bristol are, for the most part, located in public parks or green spaces, on river banks, in riparian buffers or on railway margins, none of which are likely ever to be developed or, in many cases, used to offset habitat lost to development elsewhere.
So far, no announcement has been made as to whether any of Bristol’s focus area for nature recovery sites will be made available for offsite habitat mitigation and the proposed new Local Plan does not commit to using these sites for this purpose.
This, combined with the challenge of finding LNRS suitable for offsite habitat mitigation, registering them as biodiversity gain sites and then managing them, effectively, in perpetuity, suggests that few feasible LNRS sites will be found, especially as many sites are also in demand for public access for recreation.
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.
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
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:
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.’
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:
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?
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.
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.
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 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:
[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.
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
A
The tree is a native species (or at least 70% within the block are native species).
B
The 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).
C
The tree is mature (or more than 50% within the block are mature).
D
There 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.
E
Natural ecological niches for vertebrates and invertebrates are present, such as presence of deadwood, cavities, ivy or loose bark.
F
More 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 criteria
Good (3)
Passes 3 or 4 criteria
Moderate (2)
Passes 2 or fewer criteria
Poor (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.
The Bristol Tree Replacement Standard (BTRS), which was adopted nearly a decade ago in 2013, 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.
The presumption should always be that trees should 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) is the Mitigation Hierarchy[2] which states, firstly, avoid; then, if that is not possible, minimise; then, if that is not possible, restore; and, as a last resort, compensate (the purpose or BTRS). BCS9 adopts this approach and states that:
Individual green assets should be retained wherever possible and integrated into new developments.
However, with the emergence 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 trees to the city in the years since the last version 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 new Environment Act 2021 (EA 2021) is coming into force late next year.
Our proposal provides a mechanism for complying with the new legal requirement for 10% Biodiversity Net Gain (BNG) which will be mandatory when EA 2021 takes effect.
Background
Under current policy – BCS9 and DM17 – trees lost to development must be replaced using this table:
Table 1 The Current BTRS replacement tree table
However, when the balance of the Environment Act 2021 (EA 2021) takes effect late in 2023, the current version of BTRS will not, in most cases, be sufficient to achieve the 10% biodiversity net gain (BNG) that will be required for nearly all developments. Section 90A will be added to the Town and Country Planning Act 1990 and will set out the level of biodiversity net gain required ( Schedule 14 of the 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.[3]
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.[4]
This aligns perfectly with Bristol’s recent declarations of climate and ecological emergencies and with the aspirations of the Ecological Emergency Action Plan,[5] which recognises that a BNG of 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 3.1 and that the BTRS table (Table 1) be replaced with Table 2 below:
Table 2 The proposed new BTRS tree replacement table
The Replacement Trees Required number is based on the habitat area of each of the three BNG 3.1 tree categories (Table 7-2 below) divided by the area habitat of one 30-year old BNG 3.1 Small tree (Table 3 below) plus 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:
Applying the Biodiversity Metric to Urban trees
The most recent Biodiversity Metric (BNG 3.1) published by Natural England, defines trees in urban spaces as Urban tree habitats. The guidance states that:
the term ‘Urban tree’ applies to all trees in urban situations. Urban trees may be situated within public land, private land, institutional land and land used for transport functions.
Table 7-1 divides Urban tree habitats into three categories:
Paragraph 8.5 of the 3.1 BNG Guidance makes it clear that lines of trees in an urban environment should not be treated as a linear habitat:
Urban trees are considered separately to lines of trees in the wider environment, since they generally occur in an urban environment surrounded by developed land.
Calculating Urban tree habitat
Urban tree baseline habitat area is measured in hectares and is based on the Root Protection Area[7] (RPA) of each tree impacted by a proposed development. RPA is used instead of tree canopy because it is considered to be the best proxy for tree biomass.
In most cases, RPA is obtained from an Arboricultural Impact Assessment (AIA), which complies with British Standard 5837 2012 – Trees in relation to design, demolition and construction (BS:5837).
Where no AIA is available, Table 7-2 is used:
Note that the tree’s size will still need to be ascertained, and that any tree with a stem diameter (DBH) 75mm or more and of whatever quality (even a dead tree, which offers its own habitat benefits) is included . Under BTRS, trees with a DBH smaller than 150 mm are excluded, as are BS:5837 category “U” trees.
The guidance also makes it clear that, given the important ecosystem services value provided by trees, where possible like-for-like compensation is the preferred approach, so that lost Urban trees are replaced by Urban trees rather than by other types of urban habitat.[8]
Replacing lost trees
To calculate the number of trees required to replace Urban tree habitat being lost, table 7-2 above is used on this basis:
Size classes for newly planted trees should be classified by projected size at 30 years from planting.
We have used the median DBH sizes for new stock trees as set out in BS 3936-1: Nursery Stock Specification for trees and shrubs as the basis for calculating the eventual size of a newly planted trees after 30 years and assumed that a tree adds 2.54 cm (1”) to its girth annually.
This results in a predicted stock tree size after 30 years’ growth. This is then assigned to one of the three Urban tree categories set out in table 7-2: Small, Medium or Large. In all cases save for Semi-mature tree stock, the eventual size of stock trees after 30 years falls within the BNG 3.1 size category Small, which has a habitat area of 0.0041 hectares. This value is then used to calculate how many new trees will be required to replace trees lost to the development, plus a 10% biodiversity net gain. This gives a compensation size per replacement tree of 0.0045 ha (0.0041 hectares + 10%).
Table 3 below shows the basis our our calculation:
Table 3 Annual stock tree growth predictions
The Trading Rules
It may be that a notional positive biodiversity net gain can be achieved by replacing fewer trees than this analysis indicates. However, this is not enough. The calculation should also comply with the Trading Rules that apply to Urban tree habitats.
Paragraph 7.6 of the 3.1 BNG Guidance states:
The mitigation hierarchy and trading rules apply to Urban trees. Given Urban trees are a ‘Medium’ distinctiveness habitat, trading rules stipulate that the same broad habitat type (or a higher distinctiveness habitat) is required. However, given the important ecosystem services value provided by trees, where possible ‘like for like’ compensation is the preferred approach (i.e. where possible any loss of Urban trees should be replaced by Urban trees – rather than other urban habitats).
Rule 3 of the User Guide states: ‘”Trading down’ must be avoided. Losses of habitat are to be compensated for on a ‘like for like’ or ‘like for better’ basis. New or restored habitats should aim to achieve a higher distinctiveness and/or condition than those lost…’
The likely impact of this policy change
We have analysed tree data for 1,038 surveyed trees taken from a sample of AIAs submitted in support of previous planning applications. Most of the trees in this sample, 61%, fall within the BNG 3.1 Small range, 38% within the Medium range, with the balance, 1%, 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):
Table 4 Proposed BTRS impact analysis
The spreadsheet setting out the basis of our calculations can be downloaded here – RPA Table 7-2 Comparison.
This article was amended on 7 November 2022 to include references to Lines of Trees in the urban environment, the application of the Trading Rules to Urban tree habitats and fix a broken link.
Appendix 1
Our proposed changes to BTRS, set out in the Planning Obligations Supplementary Planning Document, 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.
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.
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[9]:
Tree in open ground (no tree pit required) £765.21
Tree in hard standing (tree pit required) £3,318.88
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 7-2 of the Biodiversity Metric (BNG), published from time to time by Natural England. This may be updated as newer versions of BNG are published.
The following table will be used when calculating the level of contribution required by this obligation:
[6] DBH = Diameter at Breast Height. RPAr = Root Protection Area radius. Area = the calculated BNG habitat area.
[7] RPA area = π × r2 where r is 12 x the tree’s DBH for a single stemmed tree. For multi-stemmed trees, the DBH of the largest stem in the cluster should be used to determine r.
[9] These values should be updated to the current rates applicable at the time of adoption. The current indexed rates as of April 2022 are £1,041.6 & £4,517.89 respectively.
[10] DBH = Diameter at Breast Height. RPAr = Root Protection Area radius. Area = the calculated BNG habitat area.
Bristol’s trees are constantly under threat from development, especially when the commercial value of the land they grow on is so great and the public amenity value they offer is not thought worthy of consideration.
Here is (yet) another example.
An outline planning application – Number 17/01580/P – has been made to at the Eastgate Centre on Eastgate Road for the demolition of the existing drive-thru restaurant. It will be replaced by new retail units with a health and fitness club above and a replacement drive-thru restaurant.
Part of this application will require the destruction and removal of a delightful stand of trees that grow on a triangle of land between the roundabout on Eastgate Road and the existing retail park. This is so that larger retail units can be built and goods vehicles can more easily gain access to the rear of the site. This is a plan of the trees affected.
This is the proposal for what will be planted in their place – a souless echo of what is already there:
The Council’s own arboricultural officer has objected to the proposal. He advises:
“I have conducted a site visit and reviewed the supporting arboricultural documentation. The trees on site are located on the edge of the proposed development area and provide a significant screen to the already extensive retail development. The group of trees fall within 2 distinct age ranges, a mature group of ash, oak and poplar and a young understorey of secondary infill planting.
The mature trees are protected by TPO 282. The ash and oak are a historic remnant of a landscaped garden (Circa 1900) from the former gas works that occupied the site, the ash appear to of been managed as old pollards which have now grown out. They are historic trees with potential veteran tree characteristics that warrant the TPO status and must be retained. Due to poor management or lack of management the trees have a number of less than satisfactory defects associated with them that have in part been identified within the supporting tree survey from February 2015. The understorey planting appears to date back to the original development of the retail park, this understorey now has a more complex relationship with the larger TPO trees. They reduce the target area of people and property by the restriction of movement within this area and they also provide shade to the lower portions of the main stems which when considering the potential veteran tree characteristics offer significant ecological benefits. The management of this area for the benefit of the mature TPO’d trees would need careful consideration.
The supporting arboricultural survey is out of date and only provides basic survey detail that does not consider age and historic relevance of the TPO’d trees. The survey in not a full BS5387 report as required with DM17: Development Involving Existing Green Infrastructure.
The proposal seeks to redevelop the current Burger King site to increase the number of commercial units with associated HGV delivery bays to the rear.
This proposal would remove the vast majority of the historic TPO’s trees and associated understorey, This would be detrimental to the TPO status of the trees. The final design would be in constant conflict with the trees identified for retention leading to further applications to remove the tree following occupation of the individual units.
The Mature Oak T10 is a key amenity feature located in an elevated position over the roundabout at the gateway of the Eastgate centre; this is a TPO’d tree and no evidence has been provide to justify its removal.
I object to the proposed and would recommend refusal of the application on the grounds of a detrimental impact to the only green infrastructure on site and historic environment. Insufficient detail has been presented in accordance with BCS9, DM15 & DM17. There has been no consideration of the TPO status of the trees or their current or future management.
The arboricultural documentation is poor, out of date and insufficient to support an application, the tree planting plan produced to mitigate the loss of such significant trees has not considered the “Planning obligation SPD, Tree.” (Bristol tree replacement standard (BTRS). I hope you find these comments of use.
Matt Bennett Arboricultural Officer (Planning) City Design Group – Place Directorate City Hall.”
We agree with Matthew! We shall be lodging our objections.
With Bristol City Council’s budget cuts, two decisions have been made regarding the management of Bristol’s treescape that make no economic sense, and threaten the reputation of the City as a Green and pleasant place to live and locate a business.
With Bristol City Council’s budget cuts, two decisions have been made regarding the management of Bristol’s treescape that make no economic sense, and threaten the reputation of the City as a Green and pleasant place to live and locate a business.
Decision 1: Slashing of street tree management budget
The budget for managing street trees has been cut by nearly 78% from £240,000 to £53,000.
Emergency cover outside normal working hours is no longer being provided through the tree management contract, despite having no cost benefit.
Tree management will be limited to felling to address safety risks, despite greater initial costs and the long term consequent loss of tree sites – felling costs the same as 16 years of maintenance. As a result, Bristol’s street tree population will rapidly fall into decline as they are steadily removed, never to be replaced.
Decision 2: No planting of street trees, either replacement or new, even when cost neutral
Bristol City Council has operated a number of innovative schemes allowing residents or community groups to sponsor replacement or new street trees. Despite fully funding the planting, and maintenance for two years, such planting will no longer be permitted.
At the moment, when a tree is replaced in an existing tree pit it costs £295. This covers regular watering until the tree is established and two years maintenance. If the trees dies whilst establishing itself, it is replaced at no extra cost.
If the Council can be persuaded to change its mind about not planting new trees, then this cost could to increase to the £765 that developers are required to pay – the overall costs of planting a tree and maintaining it during its lifetime. Planting a tree at a brand new site could add around £2,000-£2,500 if a special tree pit needs to be installed.
Currently there is huge support from the community for replacing lost trees. Around £200,000 of developers’ money is set aside for this purpose, Metrobus is committed to planting 200-300 trees as part of their planning condition, and Bristol University has donated funds to plant 60-100 public street trees.
It makes little sense, in times of budget constraints, to renounce external funding sources that fully cover planting and maintenance costs.
These decisions are a false economy for Bristol City Council
With no pollarding of street trees, increased tree growth will lead to more subsidence claims against the Council, and create more highway damage, pavement trip hazards, and infrastructure damage.
New tree growth from previous pollarding points will become unstable, increasing the probability of personal injury and property damage claims against the Council.
Just a couple of additional subsidence or injury claims could negate the Council’s entire “cost saving”.
This short-term decision makes no sense – if a tree can be maintained for some 16 years – the one-off cost of felling it, then surely it makes better economic sense to spread this inevitable cost and maintain the tree rather than fell it as a short-term ‘solution’ – a ‘solution’ which loads all the costs up front and will lead to greatly increased and unavoidable expenditure in not very many years time?
These decisions will threaten Bristol’s reputation as a Green City
Bristol currently has a great reputation as a Green City and a fantastic place to live.
Reputational damage will result from any decision that damages the Green environment of the City, particularly in the period following its place as the 2015 European Green Capital.
These decisions were taken with no consultation with stakeholders
There has been no consultation regarding these decisions with other Departments within Bristol City Council, who will have to deal with the foreseeable consequences, with insurers, who will face additional damage claims, Avon and Somerset Police, who will have to address public order consequences of mass felling, or Bristol Tree Forum, with its wide-reaching understanding of tree issues.
What you can do
Contact your Councillor and email the Mayor and demand that these decisions be overturned.
Contact us here at Bristol Tree Forum to register your support and offer to help defend Bristol’s public tree spaces.
Spread the word and forward this blog to others interested in saving trees.
Bristol Tree Forum 