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.
On Thursday 26 September 2024 Waterstones at The Galleries launched Our City. This book chronicles community activism in Bristol through multiple campaigns described in eighteen individually authored chapters.
Suzanne Audrey pulled the whole project together, edited the book and contributed the introduction and conclusion. The book is well worth buying. It tells a story of the diversity and resilience of Bristol communities when faced with threats usually emanating from the council which should be looking after them. The Bristol Tree Forum was asked to contribute. During the launch event, contributors were each given three minutes to describe their work. This is what we said:
Saving Bristol’s Urban Trees by Vassili Papastavrou
In Bristol we are seeing the loss of hundreds of important urban trees each year. They cannot be replaced and it will create a city that is unliveable – we need them to keep us cool now.
I’d like to illustrate the problem with two willow trees which grew not far from here. Two years ago, during National Tree Week, they were chainsawed by Bristol City Council. Highly visible and next to Temple Meads, the trees should have had Tree Preservation Orders but the council does not TPO its own trees, claiming that it is a responsible landlord and that its trees are safe.
I was brought up in Cambridge. Along the banks of the river Cam, willows are such a feature of the city that no one in their right mind would consider chopping them down. But in Bristol they were removed without a moment’s thought.
The removal was filmed by Martin Booth and got a lot of coverage but in reality it was just another ordinary day of tree removal in Bristol. Martin tweeted, “This is so sad to watch. One of Bristol’s most beautiful trees, a majestic weeping willow is being chopped down this morning”.
Trees compete for space with other uses of the city. In Bristol mature urban trees are given lowest priority, so when plans come up, trees are removed. As this incident demonstrates, they have zero value to the Local Planning Authority or the council.
The willows never went in front of the Planning Committee where there could have been a debate as this was a “delegated decision”. Effectively Bristol City Council was giving planning permission to itself in a backroom deal. The planning application was snuck through with no mention of the trees in the title, no arboricultural report from the council, no tree officer report from the local planning authority. The council passed judgement on itself.
And after the deed had been done, was there any recognition that a terrible act had been committed? An inquiry to make sure that this kind of thing would never happen again?
Silence. For in Bristol there is no one in a position of power who cares about retaining important urban trees.
Paul Sander-Jackson, who helped to plant the Baltic Wharf trees, returns to Bristol shocked to learn that they will now be felled.
We came back to live in Bristol nearly two years ago. After living in Somerset for 32 years, and in Bristol 12 years before. Much has changed in the city, but much has remained – the best bits, and especially the trees.
Our new home backs on to Oldbury Court – which has an astonishing range of trees and plants, as well as a river and rocks and old quarries. Our garden has a lime tree – probably about 80 to 90 years, just outside its boundary fence.
In Somerset, we became very involved, in the later stages of life there, in planting new trees and woodlands, hedges and copses. The organisation we were involved with – Reimagining the Levels – has been responsible for planting over 100,000 trees in the past few years. But that is addressing a major problem in Somerset – a tree cover of only between 6% and 7%.
Coming back to Bristol, I wondered where my energy should go. But a few months ago, an old friend mentioned that I might like to get involved in Bristol Tree Forum. The rest is (personal) history. My partner and I are now joint Tree Champions for the Frome Vale ward.
It was a great shock when I was made aware of the application then pending on the Baltic Wharf Caravan Club site, particularly because when I used to do landscaping and gardening for a livelihood in the late eighties and early nineties, I had helped plant trees on that site, and carefully undertaken structural pruning and maintenance. And now a planning application was threatening to fell 80 of them.
“The Baltic Wharf trees were at least 35 years old, providing numerous ecological ‘services’ to the area”
As I made further enquiries, it turned out that there were over 590 objections to the application, with six statements in support. Six!! The trees represented over a quarter of the tree cover in the immediate area. They were at least 35 years old, providing numerous ecological “services” to the communities of Bristol.
And then the terrible shock of a unanimous – yes unanimous – planning committee decision to go ahead without amendments to the felling of the trees, with no objection from the responsible tree officer.
This left me realising many things. How naïve I have been about the problems nature, and especially trees, face in this lovely city. How we need to struggle and advocate for every one. Trees offer so many positive benefits for all creatures, including humans. I reflect on the wonderful life in ‘our’ lime tree. Pigeons, starlings, sparrows, crows, squirrels. Life in all its varied forms. Leaves, blossoms, growth.
And perhaps most of all, I recognise that in Bristol, in 2024, there is perhaps an equal if not greater task to planting new trees. To protect the ones we have, and love. And to share that love with as many people, in our local communities and beyond, that we can.
“Maybe we can teach even more people how important it is to retain trees and how we should never allow a good tree in a good place to be arbitrarily felled”
My partner and I began our life in Bristol in St Werburghs, starting the City Farm there in 1980. As it got going, with great help from some wonderful City Council officers in the planning and parks departments, we planted trees with children from six local primary schools. Those children, I hope, will always carry the memory of that planting. And I hope they go and visit the trees – now over 40 years old and flourishing in the wondrous environment of the City Farm.
It is about a will, an attitude, a set of values. Above all, it is about respect for our fellow plants and creatures, who share this planet and this city with us.
Interested in becoming a Tree Champion like Paul? Click below to learn more.
We urge the Bristol City Council Cabinet to reject the current draft. More nuanced rules, that meet identified real needs, need now to be developed following a proper consultation with Bristol’s allotmenteers. The PGSS must be also integrated into the new Local Plan so that all the sites that have been identified are protected either as Local Green Space or as Reserved Open Green Space and given proper planning weight when decisions about their future use and development are made.
We are particularly concerned that the draft rules relating to trees on allotments are too narrowly drawn and will result in the loss of allotment tenants’ ability to make the best use of the particular conditions of their plot. We urge the relaxation of these proposed limits on the number and types of trees which can be planted.
Food growing as part of the draft Parks and Green Spaces Strategy
We are concerned that the Strategy for Food Growing and Allotments has been included as Chapter 6 of the draft Parks and Green Spaces Strategy (PGSS). This is a departure from the 2008 PGSS, which makes it clear that such spaces are not included, since they are not public spaces: ‘The Strategy does not consider green spaces that are not freely accessible to the public, including allotments, city farms, school grounds, or Sites of Nature Conservation Interest in private ownership.’
The proposed PGSS recognises that: ‘There are other green spaces managed by the council that are not included because they do not offer full public access for recreation …’ [our underlining]. Given that allotments and other land used for growing food (including grazing land) are subject to private tenancies, with rental payments and mutual rights and obligations agreed between tenant and Council, it is hard to understand why they have been included in the strategy proposal.
We need to recognise the importance of this specific type of land use and ensure that it is not made an ancillary element in the PGSS. It deserves greater attention, which is why we suggest that it should have been integrated into and protected by the current proposals for the new Local Plan (as, indeed, Parks and Green Spaces should also be) rather than treated as an add-on to the proposed PGSS.
In particular, the current status of and strategy for market gardens, smallholdings and grazing land (and other land not used for food growing), which have an area greater than all current allotments combined, get only passing consideration. Little or no thought is given to the opportunities these sites present for nature recovery, biodiversity enhancement and other environmental gains.
Instead, we are told that these sites:
… have limited benefit beyond the individual users … and many smallholdings tend to use less than 10% of their site for food growing … we now need to review our current landholding agreements so that we can start to explore opportunities to repurpose land to diversify our food growing spaces and ensure land is being used effectively to meet Bristol’s food growing targets. … Once suitable land has been identified we will review how these opportunities are promoted to ensure that the system of securing land is fair and equitable. We are particularly interested in opportunities that will not only ensure land is being sustainably and efficiently managed but that also delivers social value and benefits the residents of Bristol.
In the face of the demand for allotments and the need to increase local food production, which has led at least in part to the proposed changes in the tenancy agreement, these areas must surely warrant greater protection than is given them in the proposed PGSS. These areas also contain many trees (often veterans) and other ecologically sensitive habitats, and many are in SNCIs. It is vital that these matters are properly considered when planning any change of use of such land. This ought to be addressed properly in the proposed new Local Plan, not in this draft strategy.
Trees on allotments
Trees on allotments fall into two categories: those within allotment plots and those on allotment sites but outside the plots.
Trees outside allotment plots
From our analysis of data on the Council’s Open Data site, we estimate that allotment sites total some 105 hectares when community orchards are included. However, only about 87 hectares of these sites are productive. This means that around 17% of allotments, some 18 hectares, are unproductive. We welcome the clarification in the new tenancy agreement that trees in these unproductive areas remain in the care of the Council and may not be damaged by tenants.
Trees within allotment plots
Proposed allotment Rule 4.0 Trees deals with trees growing within allotment plots. We distinguish between newly planted trees and existing trees below.
New trees
Regulation 4.4 limits the height of trees to 2.5 metres (a common constraint in other councils). The limit in Regulation 4.3 of one tree per allotment sized between band B and band C is far too restrictive, particularly since the majority of plots fall within these bands.
Rule 3.1 requires that the tenant uses at least 75% of the plot for ‘a cultivated crop of vegetables, fruit bushes [not fruit trees], ornamental flowers, and herbs.’
Rule 3.3 states that fruit trees will occupy the non-cultivated area of the plot (which may be no more than 25% of the plot) unless they are underplanted with a productive crop, in which case they will be counted as being part of the cultivated area. No trees are permitted on band A or Micro Plots, nor is any consideration given to whether it is possible to underplant ‘a productive crop’. This implies that the food value of the tree does not warrant the space they occupy. According to the RHS, having fruit trees on an allotment adds both structure and permanence, while fruit tree crops have a higher value than many other allotment crops and are easier to manage.
The transitional arrangements require that all trees, except the permitted dwarf rootstock fruit trees, be removed from plots within three years, though mature trees may be permitted.
These rules devalue the role of trees on allotments, not only for their contribution to the ecology of the area but also their food production value. These rules also effectively exclude such fruit-tree-growing techniques as cordons, espaliers or step-over planting.
Regulation 4.3 also constrains the type of fruit trees that may be grown. Since only rootstocks M27 and M26 are allowed (though not the intermediate M9 rootstock), it seems that that only apple trees may be grown, even though dwarfing rootstocks are available for a number of other fruit trees such as Pear, Quince (Quince C), Cherry (Gisela 5), Plum (including Gages and Damsons), Peach and Apricot (VVA-1, Pixy).
The exclusion of hazel and other non-fruit trees (most trees bear fruit) removes the ability of tenants to grow valuable nut crops or to provide locally grown stakes through coppicing – a good way to reduce our carbon footprint. This restriction also precludes the use of allotment space for growing trees from seed with a view to planting them out elsewhere, at a time when there is a great need for locally grown trees for local tree planting.
Established trees
Fruit trees
Many fruit and other trees are already growing on many allotment sites. They provide not only good-value food, but they also provide biodiversity, a rich ecological resource for insects, birds and mammals and are important legacy features of the site. The allotment and its users have adapted to the presence of these trees and their removal would not only be counterproductive to the value of the site and cause distress to tenants but would also risk damaging the soil and the allotment more broadly.
Non-fruit trees
Although such trees may not have obvious value for food production, other considerations apply. We agree that allotments should not be taken over by trees, resulting in the reversion of productive land to woodland and the attendant loss of statuary allotment protection for such land. However, it would be a mistake to create rules which put biodiversity, nature and ecology in conflict with food production; trees are crucial for biodiversity, which in turn is essential for the productivity of the places where we grow food.
Site-appropriate regulations
Allotment plot conditions both within allotment sites and across Bristol vary widely. The gradient and aspect of each plot, its soil type, soil depth and historical development all pose different challenges to tenants, requiring a localised approach to the best way to utilise a site. Imposing a one-size-fits-all set of regulations, based on some notional idealised allotment plot, will not provide sufficient scope to allow for these variations. Tenants’ associations also need to be able to vary the regulations to fit their local conditions. Indeed, arguments in favour of the distribution of local decision making, which we endorse, have recently been made by the Bristol Mayor.
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.
‘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 UrbanIndividual 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 UrbanIndividual 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.
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:
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:
The number and identity (using Id used in the BS5837:2012 survey) of each tree to be removed.
The number and species of the trees to be planted on the development site.
The number and species of the trees to be planted on public land.
Which offsite trees are in to be planted open ground and which in hard standing.
The agreed location and species of each offsite replacement tree which and should be within one mile radius of the lost tree.
Trees planted under BTRS should not replace lost public trees, such as street trees removed in the normal course of tree management.
Like for like replacement. Compensation for the loss of large-form trees should result in large-form trees being planted.
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:
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.
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]
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.
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.
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):
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:
The number and identity (using Id used in the BS5837:2012 survey) of each tree to be removed.
The number and species of the trees to be planted on the development site.
The number and species of the trees to be planted on public land.
Which offsite trees are in to be planted open ground and which in hard standing.
The agreed location and species of each offsite replacement tree which and should be within one mile radius of the lost tree.
Trees planted under BTRS should not replace lost public trees, such as street trees removed in the normal course of tree management.
Like for like replacement. Compensation for the loss of large-form trees should result in large-form trees being planted.
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.
[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.
There’s a climate emergency and we need to act. With higher temperatures and more severe weather events than just a decade ago, we must take action at the local as well as the global level.
Bristol City Council declared a Climate Emergency in 2018, reflecting the need to reduce the city’s contribution to the causes of climate change, and to adapt and be resilient to further expected climate impacts. For the declaration to be meaningful, it has to result in practical changes, for example the protection of existing trees on development sites. With important urban trees being routinely felled, there is no evidence that this is the case. If Bristol continues in this way, the city will become unliveable in the climate crisis.
The Council is now drafting a tree strategy for the city, which we hope will become a key element of the forthcoming revised Local Plan. We hope that the strategy will protect existing trees and prioritise the planting of replacement and new trees across the city. We have asked for 18 principles to be included in the strategy.
If our urban environment is going to be liveable in the long term, we need to create new developments that can cope with the changes in the local climate expected in the future. The benefits of trees in the fight against climate change are now well understood: trees lock up carbon, reducing pollution and flooding. They are also the best way of reducing the urban heat island effect, decreasing the temperatures of heatwaves by up to 10°C . It’s therefore vital that green infrastructure forms part of any proposed development. This is particularly crucial in the city centre.
On every occasion that trees are felled, we’re told it will be all right, as they will be replaced. Often these replacement trees are never planted because there is nowhere to plant them, or if planted, they die and are not replaced. At any rate, we need tree canopy and shade now, not in 50 years’ time when any new trees that might survive will replace the canopy lost. This is why we must protect existing trees, and if trees must be lost, local tree replacements must be planted and not just promised.
A warmer climate increases the risk of overheating and heat-related illness, even death. In the heat wave of 2003, around 70,000 people died across Europe due to the extreme heat, with older people and children particularly vulnerable. However, we can reduce much of the risk without the need for active cooling, by incorporating effective measures into development proposals from the earliest design stage. New buildings and external spaces must be designed to provide year-round comfort and support well-being. On-site tree planting for shade will contribute to this by minimising the amount of heat entering buildings. All new developments will be expected to demonstrate through ‘sustainability statements’ how they would incorporate such measures into their design from the outset.
How green (and blue) infrastructure reduces climate impacts
Developers must take into account that changes in the local climate are likely to: increase flood risk and water stress; change the shrink-swell characteristics of clay soils affecting foundations and pipework; affect slope stability; and affect the durability of building materials. Incorporating green and blue infrastructure, such as trees and water features, in developments will help to reduce all these effects. Green and blue infrastructure should be multifunctional, that is, provide ecology and biodiversity benefits as well as climate adaptation in developments. Where appropriate, this should include the use of living roofs with a sufficient substrate depth to maximise cooling benefits. However, the cooling effect of green roofs is a fraction of that afforded by trees.
Long-term thinking
As we build more homes, businesses and communities, it’s essential that we retain and integrate important existing trees within any new development. We must also consider carefully the size, species and placement of new trees provided as part of any planned landscape treatment, for example in terms of:
ensuring that any new streets are tree-lined
focusing once again on large-form trees that will be long-lived and provide substantial shade, rather than small, short-lived trees such as Rowan or Amelanchier
reducing or mitigating run-off and flood risk on the site
increasing on-site canopy cover and providing shade and shelter
ensuring that newly planted trees will be maintained in the long term and replaced if necessary.
Where tree loss or damage is unavoidable, and not merely expedient, within a development site, new replacement trees of an appropriate species must be provided either on or off site and their long-term management and maintenance secured.
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.
UseallavailableplanningandforestrylegislationandpowerstosafeguardPlymouth’strees.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.
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.
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.
Bristol Tree Forum 