Public trees – Bristol Tree Forum

Our proposal for a new Bristol Tree Replacement Standard revisited

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

Revisions are shown in red.

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

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

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

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

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

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

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

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

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

Background

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

Table 1 The Current DM17/BTRS replacement tree table.

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

The Local Government Association says of BNG that it:

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

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

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

Our proposed new BTRS model

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

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

Replacing lost woodland

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

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

Replacing lost trees with hedgerows

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

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

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

Making the BTRS calculation transparent

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

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

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):

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

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

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

Trees

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

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

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

Trees

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

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

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

Appendix 2 – Our proposed changes to BTRS

Trees – Policy Background

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

Trigger for Obligation

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

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

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

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

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

Level of Contribution

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

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

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

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

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

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

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

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

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

Planning obligations will contain the following:

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.

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

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

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

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

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

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

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

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

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

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

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

[12] https://ukhab.org/

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

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

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

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

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

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

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

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

[22]https://ukhab.org/

The vital role of trees in urban development

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.

We have submitted our proposals for how trees lost to development should be replaced as for of the Local Plan Review – Our proposal for a new Bristol Tree Replacement Standard using Biodiversity Metric 4.0

Developing a Tree Strategy for Bristol

Bristol City Council is currently writing a tree strategy for Bristol. This is welcome news, as we have been calling for such a strategy to be developed for more than a decade.

For example, in 2020 we wrote a Manifesto for Protecting Bristol’s Urban Forest.

A tree strategy should be an evolving process rather than a document which may quickly become out of date. This is particularly true in our rapidly changing world – environmentally, climatically and politically. To provide an effective response to the challenges these present, a group of representatives from both civic and professional groups (along the lines of the Bristol Advisory Committee on Climate Change (BACCC), should be established to help coordinate further research and make recommendations to Bristol City Council and other stakeholders as the situation changes.

We also recommend that the development of a tree strategy should take full advantage of exemplars from other local authorities[1]. We should have the ambition to make Bristol’s tree strategy the best.

Here follow 18 key points that we would expect to see included in a strategy.

Buy in from all the stakeholders involved. Many council departments (as well as Parks, there is Highways, Education and Planning) have a role to play in the management of Bristol’s trees. We need to see evidence that all such departments are fully involved in the development of the strategy. In particular, with the current review of the Local Plan, it is essential that Planning is fully engaged with the strategy, and that the two documents are consistent and properly cross-referenced. The tree strategy needs to be incorporated into the new Local Plan. In addition, other important landowners (such as the universities, utilities providers, housing associations, schools and hospitals) have a role to play in contributing their expertise to the strategy and implementing its goals. As well as the Bristol Tree Forum, many community groups have an interest in tree planting in Bristol and should be involved and consulted.
When council trees are removed, they must be replaced. At present there are more than 800 street tree stumps and empty tree pits around the city – sites where trees once grew. A plan to plant all these missing trees within five years needs to be included. In the future, when any council trees are damaged or felled, they should be replaced within the next planting season.
There needs to be community engagement in tree management decisions both at the level of individual trees and in strategic decisions. In recent years we have seen a rise in community led campaigns to protect trees, such as the Ashley Down Oak, the M32 maples and Baltic Wharf, and this is indicative of a disconnect between the Council and the communities it serves. When the balance of the Environment Act 2021 takes effect later this year, Councils will be obliged to consult when street trees are being considered for removal[2]. This is too narrow and should be extended to include where any public tree is being considered for removal. Therefore, part of the strategy should be promoting community engagement, providing mechanisms for engagement and then taking account of the concerns of the community and tree campaigners alike.
There should be one person responsible for trees within Bristol City Council. At present we have tree planning officers, tree maintenance officers and tree planting officers with no single individual or office accountable overall, often resulting in a lack of appropriate action or people working at cross-purpose. It is also concerning that Highways are able to remove street trees without any consultation.
There needs to be a plan to address the massive inequality in tree cover in Bristol, which often mirrors social and financial deprivation in the City. For instance, additional protections could be given to trees, and tree planting prioritised, especially in deprived areas such as the City Centre, Harbourside and St Pauls.
When developers remove trees, the replacements required should be planted by BCC. Too often developers have shown themselves incompetent or unconcerned when planting trees, so the trees fail or are never planted. In the case of Metrobus, there has been a more than 100% failure rate of trees in some places (trees have been replaced multiple times). We have an excellent tree planting team in Bristol and we should benefit from requiring them to organise and implement the planting required. The cost should be funded by the developer.
Retaining existing trees must be a major part of the strategy. A tree strategy cannot be just about planting new trees, the benefits of which will not be realised for decades, but crucially about retaining and protecting existing trees and the benefits they are already providing. As such, the strategy must address the threats to existing trees. Planning is crucial in this so we would expect major engagement with Development officers to address the current and future problems.
Planning Enforcement must address the illegal removal of or damage to trees. At the moment there are no consequences following the unauthorised damage or destruction of trees. This must change. Other neighbouring local authorities manage to do this but not Bristol. A strategy must include a review of the reasons for the existing lack of effective enforcement and make recommendations as to how this can be rectified.
Developments should be built around existing trees as is already required[3]. Other local authorities do this but not Bristol. This will require a change of culture in the planning department so that pre-application discussions with developers make it clear that this will be required.
The sites for the replacement trees must be agreed before Planning Applications are approved. This is required by planning policy (BCS9 and DM17), but currently developers are being allowed to, instead, pay a “fee” into Section 106, and frequently the replacement trees are never planted. Trees form an important part of our urban habitat. The calculation of tree replacements required to compensate for their loss must be aligned with the Biodiversity Metric as adopted under the Environment Act 2021.
Spend the £ 900K+ reserved for tree planting. Connected with the above point, a strategy needs to include a mechanism for spending the existing £900K+ of unspent tree planting Section 106 money within the next three years.
A strategy to increase Bristol’s tree canopy cover (or at the minimum, maintain existing canopy cover) needs to have a route to implementation. This must include addressing the loss of street tree canopy cover by being bolder in selecting new tree planting sites and planting large-form trees wherever possible. Trees such as rowans and flowering cherries are short-lived and will never provide much canopy or become robust enough to survive our challenging urban environment in the long-term.
Canopy Cover needs to be measured with an agreed methodology with confidence limits (levels of doubt in the estimate) made clear. In the first instance, we need to establish the baseline year and percentage tree cover from which progress will be measured. Only then will it be possible to show whether a trend has been determined. Two measurements using different methodologies should not be used to claim an increase in canopy cover. The metric should take account of trees lost so that the figure reflects the true increase, or loss.
Include trees within road changes. There needs to be proper engagement with Highways at early stages of the design process for road changes to look at retaining the maximum number of existing trees and including innovative planting opportunities for new large-form trees, such as pavement build-outs.
For new developments, trees should be properly considered at the pre-application stage, with appropriate consultation with stakeholder groups. Too often, the mitigation hierarchy requiring the removal of trees to be a last resort is disregarded, so that it is only after the design has been finalised that the existing trees are considered and removed where they conflict with the design scheme.
Biodiversity Net Gain (BNG) calculations need to be checked by the Local Planning Authority and biodiversity loss must not be monetised as BTRS has been. BNG, if properly implemented, makes sure that biodiversity on development sites is properly measured and will provide a net gain (soon to be least 10%) is factored in. However, at present, developers’ calculations are not being checked. When we have provided properly evidenced calculations, these have been dismissed by the LPA as mere differences of opinion. You cannot have differences of opinion on facts. The LPA must require that BNG calculations are presented in a way that can be checked by anyone interested and actually do the checking. In addition, ensuring BNG must require that the development site does not lose its biodiversity. If this is not possible, then its immediate local environment must be used to offset any onsite losses. Onsite losses must not be compensated for in some faraway place completely removed from Bristol.
Planning Applications involving trees must mention this fact in the title. Too often, applications that involve the loss of important trees (or plans to avoid the planting of new trees[4]) do not even mention this fact in the title. This means that it is extremely difficult for community organisations to engage.
Once a planning application has been issued, no removal of trees. A moratorium should be placed on any tree felling pending the outcome of the planning application. This includes applications to demolish buildings which should exclude tree or other habitat removal.

A copy of this blog is available here.

02 May 2023

[1] See for example the Wycombe Council Canopy Cover Doc https://buckinghamshire-gov-uk.s3.amazonaws.com/documents/Canopy-Cover-SPD_3qAkk4z.pdf

[2] https://www.legislation.gov.uk/ukpga/2021/30/part/6/crossheading/tree-felling-and-planting/enacted

[3] Bristol Core Strategy, policy BCS9 states that, “Individual green assets should be retained wherever possible and integrated into new Developments.”

[4] See the Avon Crescent Application pp136 – 155 https://democracy.bristol.gov.uk/documents/g10675/Public%20reports%20pack%2010th-May-2023%2014.00%20Development%20Control%20B%20Committee.pdf?T=10

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

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

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

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]

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.

The Local Government Association says of BNG that it:

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 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:

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.

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

8.3.15. 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

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

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

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

A copy of this article is available here.

Appendix 1 – Our proposed changes to BTRS

See the Planning Obligations Supplementary Planning Document at page 20.

Trees – Policy Background

Trigger for Obligation

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

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

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

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

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

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

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

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

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

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

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

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

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

Biodiversity Metric 4.0: what’s it all about?

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

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

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

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

The NPPF basis for achieving biodiversity net gain

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

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

Paragraph 179 c), states that plans should:

…pursue opportunities for securing measurable net gains for biodiversity.

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

Pending planning applications

Natural England advises that:

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

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

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

The BNG 4.0 Guide

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

What is Individual trees habitat?

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

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

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

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

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

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

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

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

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

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

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

Trees in private gardens

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

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

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

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

Measuring Individual trees habitat size

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

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

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

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

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

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

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

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

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

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

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

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

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

Assessing baseline Individual trees habitat condition

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

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

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

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

Assessing baseline Individual trees habitat strategic significance

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

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

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

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

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

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

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

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

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

Post-development Individual trees habitat creation.

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

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

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

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

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

Post-development Individual trees habitat area forecasting

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

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

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

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

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

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

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

Here is the model we use:

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

The Trading Rules

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

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

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

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

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

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

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

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

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

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

[4] This is a girth of 4.08 metres.

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

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

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

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

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

Measuring and Modeling the Tree Canopy of Bristol

A new canopy growth model shows the challenge of increasing Bristol’s tree canopy to 24%, an increase of a third.

Launched in January 2019, Bristol’s One City Plan is a vision of the development of Bristol over the years until 2050. This vision covers many aspects of city life. Of particular interest to the Bristol Tree Forum are two goals:

by 2036 Tree canopy cover has increased by 25% since 2018

by 2046 Tree canopy cover has doubled since 2018

The obvious question to ask is :

What was the canopy in 2018, the baseline for these proportional increases?

But to answer that question, we need to ask another:

How can tree canopy be measured across the city?

We would expect certain properties of a method of measurement, such as accuracy, precision, repeatability, economy (since it will have to be applied repeatedly over the years to assess progress) and scalabilty (so the method can be applied to any boundary to analyse selected areas of the city).

Neither question was addressed in One City Plan publications. A group led by BCC’s Richard Ennion which including Forest of Avon Trust (FOAT), Woodland Trust and Bristol Tree Forum (BTF) met in in 2018-2019 to address these and other tree strategy issues. An ecological survey using the i-Tree Eco method was undertaken by FOAT and volunteers. Here 201 randomly located 11m radius plots are surveyed. This resulted in useful data on the proportion of tree species (Ash was a worrying 16% of trees in Bristol) and estimated the tree canopy at 12%. BTF had also carried out a survey using i-Tree Canopy which is a desk-based method using Google Map imagery able to be carried out by citizen scientists. Our figure was around 18% which was more in line with previous estimates. This figure was later quoted in the Cabot Instutute Review of Progress.

Our arguments in favour of the i-Tree Canopy method were several: reputational (it is used by Forest Research in their nationwide survey); precision (error range is smaller than i-Tree Eco and sample size easily increased to improve precision); economy (i-Tree Eco survey cost around £20k whereas i-Tree canopy is essential free ) and scalability (the method can be easily applied to any bounded area).

In the event, the lower figure of 12% was adopted but no decision was made about a suitable method. The 12% figure leads to a goal of 16% by 2036 and 24% by 2046. However, since the i-Tree Eco method was limited to 201 plots across the city, it is unable to give estimates at ward level, so reports about variability by ward have, anomolously, used our i-tree Canopy figures of 9% to 22%. Ward-level estimates are visualized on our ward information page.

The i-Tree Canopy method can be undertaken using a tool provided by i-Tree. BTF have developed our own version of this tool to improve the precision, ease of use by citizen scientists and to integrate into our BristolTrees website. We have used this tool to estimate the canopy for 2020 and while there is a slight numerical increase, it is not statistically significant.

More recently we have had access to the estimates produced by the commercial Bluesky tree map which is based on lidar and aerial imagery. The figure for Bristol, (once corrected to exclude large areas of the Severn Estuary in the Bristol Unitary Authority boundary) is slightly less than the i-Tree Canopy estimate, a difference probably accounted for by BlueSky’s ability to exclude canopy below 3m. Recent BCC reports seem to accept that the baseline is 18% and Bluesky mapping recommended as the method of estimation. It is however unclear how the 18% baseline affects the One City Plan goals. If still based on the initial 12% but measured using Bluesky (or i-Tree Canopy), the 2036 goal of 16% has apparently already been achieved!

The use of the commercial Bluesky service raises questions of the cost of this data, its granularity and the extent to which this data will be publicly available as open data. We look forward to answers on these issues.

Canopy prediction

We have created an online canopy prediction model which computes the canopy over a future period, based on defined planting schemes, which may be so many trees per annum over a period, or so much woodland area.

The BCC report on the planting season 2021-22 shows that 1,352 individual trees and 3 hectares (ha) of woodland were planted. The model predicts that this would yield a total of about 8 ha canopy by 2046. (The BCC report predicts 22.7 ha but this is when all trees have reached their full maturity, well beyond 2046). If repeated every year till then, this planting programme would produce about 120 ha. This is the model used.

To get a fuller picture, we can account for the trees which are lost due to disease, damage or because they outgow the site. On average, about 400 BCC trees are lost each year,and this figure is expected to rise as Ash Dieback takes its toll. If this is added into the model, the result is much less promising. This powerfully demonstates the great benefit of saving the existing tree stock.

However it is unfair to account for tree losses in the BCC tree stock without modeling the canopy growth. This model needs to take account of the age and species profile and to take into account tree management practices. Many large street trees are managed through regular pollarding so that their canopy is essentially constant. This is complex task which is still to be done.

Is the goal achievable?

Achieving the goal of even 24% cover from a base of 18% by 2046 is still a challenging task, even though this would increase tree cover by only a third rather than doubling. It would require adding 660 hectares of tree canopy in 28 years.

A paper by Waters and Sinnett (2021) looked at this issue. Their results are not directly compatible because, thanks to the baseline confusion, they explored the need to increase canopy from 12% to 37.5% using the i-Tree Forecast software. Multiple scenarios are explored but no distinction was made between woodland planting, where the eventual canopy area is limited to the planting area, and planting individual street or park trees able to grow to full canopy width.

In order to create 660 ha, our model indicates that you would have to plant 26 ha of woodland per annum or 14,000 individual trees or some mixture of the two. This model assumes an annual mortality rate of 1%. With a mortality of 3% more typical of urban trees, the planting rate rises to 24,000 trees pa. Urban trees have high early mortality which reduces over time and this is not yet modeled.

In the predictions above, canopy size prediction uses Root Protection Area as defined in BS5837. RPA is a generous proxy for canopy area. Other predictve models are supported, including one derived from data on the Bristol tree stock. The model takes no account of trees lost through the period due to felling existing trees because of age, disease or development, nor for the effect of climate change on tree health. As a result, these predictions, daunting though they are, are likely to be under-estimates of the planting required to achieve the goal.

However, the major constraint is the lack of suitable street space and land to achieve this level of planting and competition for land use from other One City Plan goals, such as increased housing, food security and greater ecological diversity. Trees alone provide some ecological benefits although this is species-dependant. In general, British native trees provide better ecological support than introduced trees. 37% of the existing council-owned tree stock are natives but only 18% of the trees ear-marked for planting are natives. Woodland areas have a much higher proportion of natives. BCC is undertaking research into the availablity of both street and parkland planting to explore the opportunities for street and park planting.

So our assessment is that even the goal of 24% is unachievable. This does not of course mean that we should not do our utmost to increase tree canopy. The benefits of trees in an urban environment are well-documented.

The private realm

This analysis has focused on the role of the council in expanding tree canopy on council land. However the majority of land and hence tree canopy in Bristol is in private and commercial hands. The need for private and commercial landowners to use their land to help move the city forward is clear. BTF is particularly concerned over the loss of mature trees due to housing and other development. Mature trees are an irreplaceable (in the short and medium term) loss of canopy and sequestered Carbon. Likewise for private homes, the trend seems to be in the wrong direction, with paving of front gardens, astroturfing of back gardens and existing trees often deemed more of a nuisance that a benefit.

The need to bring the public on-side with this goal is urgent.

Our 2022 AGM is being held on…

Monday, 7 November 2022 starting at 6:00 pm – City Hall, College Green, Bristol

Our Agenda

Introduction from Chair, Mark Ashdown
Finding our remarkable UK & Irish urban trees – Paul Wood
Panel Discussion between Paul Wood and Andy Bryce
Our Tree Champions
Bristol’s emerging local plan
Election of officers and financial report
AOB

We are looking forward to seeing you there.

Paul Wood

Paul is the author of three books: London’s Street Trees: A Field Guide to the Urban Forest; London is a Forest; London Tree Walks: Arboreal Ambles Around the Green Metropolis and is the editor of the Great Trees of London map. He has a lifelong passion for nature, especially trees, and was formerly a trustee of the London Wildlife Trust. He is currently working on a book about 1,000 remarkable urban trees throughout the UK and Ireland, to be published by Penguin in 2023.

Paul regularly leads walks and gives talks about trees in urban areas. As well as London, Paul has led walking tours around the streets of Bath, Sheffield, Dublin and even Philadelphia.

Andy Bryce

Andy is the Trees and Woodlands Manager at Bristol City Council. He manages Bristol’s team of tree officers and is responsible for managing our existing tree stock. Andy joined BCC in November 2021 having previously worked as an arborist at The National Arboretum, Westonbirt, latterly as its Collections Manager. Andy has worked in arboriculture for over 20 years, six of which were as an arborist in Bristol. Andy’s current research interest is tree pests and diseases.

Trees for Streets – will we see more trees being planted in more Bristol streets? Hopefully.

You will all have seen young trees planted in vacant tree pits in the streets of Bristol. These trees are replacement trees. There was once a tree growing there before – maybe some time ago.

These replacement trees are paid for by sponsorship, or by funds paid by Developers when they have felled trees on a building site and there is no room to replace the felled trees on the building site. In the latter case more than one tree has been “lost” – the one on the building site and the one that was previously in the tree pit.

In order to increase Bristol’s tree canopy – vital in this time of a climate emergency – we must see trees being planted in new places as well as getting all the “old” sites being filled more quickly.

Trees for Streets

To try to get this initiative going, Bristol has joined Trees for Streets.

Quotes from the Flyer for Trees for Streets

‘Bristol City Council has joined the Trees for Streets national street tree sponsorship scheme, which aims to plant thousands of additional trees in streets and parks across the city, by supplementing the council’s tree budgets through public and corporate sponsorship.’

and

‘Trees for Streets is the National Street Tree Sponsorship Scheme from the urban tree charity Trees for Cities, funded by the government’s Green Recovery Challenge Fund and City Bridge Trust. The project uses technology to empower people and makes it easy for residents and organisations to get involved in greening their communities.’

and

‘Our mission is to fund the planting of more than 250,000 additional street trees nationwide over the next ten years by hosting online tree sponsorship schemes on behalf of local councils and delivering local promotion and engagement activity to bring these schemes to life.’

Comment

Bristol has long had a Tree Sponsorship scheme, run by TreeBristol (part of Bristol City Council).

In the 2021/2022 planting season £456,000 was spent by Bristol Council in planting of trees. A portion of this money is retained by BCC for maintaining the trees planted 55% of this money came from mitigation funds paid by Developers who had felled trees somewhere in the city in order to build on the land released. (So, the money was not being spent on NEW trees, just on replacements).

10% of that money came from sponsorship, with 6.5% coming from private sponsorship (individuals and groups) and 3.5% coming from business sponsorship. Even then a lot of that money was spent on replacing trees which had been lost i.e., not on providing trees in new sites. It is a difficult “sum” to achieve. Money from Developers is for the replacement of trees lost to development. The Bristol Tree Replacement Standard achieves an amount for replacement trees based on the size of the trees lost. Eventually the trees may grow to a size which more than compensates for the environmental value of trees lost. But it remains true that each replacement tree goes in to a tree site that has lost a tree formerly growing there – so the Council is spared the expense of replacing lost trees that it owned.

Representatives of the Bristol Tree Forum have attended two meetings now where this new scheme has been explained and described.

The Trees for Streets scheme is not going to fund the trees, nor plant the trees, so we would have worded the sentence “Our mission is to fund the planting of more than 250,000 additional street trees…….” slightly differently with instead “Our mission is to facilitate and organise the funding of the planting of more than 250,000 additional street trees…”

The Trees for Streets scheme is similar to Bristol’s former scheme in that it will provide a web based choosing and ordering and paying for system, whereby residents and organisations and businesses can find available tree sites for planting trees in Streets and Parks.

There are differences between the Trees for Streets Scheme and Bristol’s former scheme, and they are:

Bristol’s former sponsorship scheme was largely one of replacement for trees lost. A sponsor (an individual, a group or a business) would select, from the Council’s mapping, a site where formerly there had been a tree, and would pay for its planting. New site planting came from One Tree per Child (whips) or from national grants where Bristol would win a bid for a grant and spend the money.
The new scheme hopes to facilitate, through sponsorship, the planting of a new tree in a new site. These sites have to be found, and checked for Services (underground utility provision), and then put forward in the Council mapping for planting with a tree.
Residents, and other types of sponsor, will be able to suggest new sites for trees by answering the question “Where would you like to see a tree planted?” with their own suggestions.
The sponsor would need to pay for the tree, but Trees for Streets might be able to assist with organising the funding, using their funding know how.
Initially this kind of new planting of Street Trees will only be possible in streets that currently have green verges, or in new sites in Parks.
(Trees in “hard ground” – pavements, plazas, city squares, etc. will need to be planted in engineered tree pits – and that is expensive. If a sponsor (which can be an individual, a group or a business) is prepared to meet that cost, then efforts will be made to agree suitable sites and then check them for Services and other criteria, such as the width of the pavement.)
Trees for Streets has national funding and this gives it an improved platform with web support and advertising which could see many more trees sponsored. Maybe businesses reached by the advertising will see a role in supporting tree planting in the more “tree poor” areas of Bristol?
Bristol is to offer residents the option to water their sponsored tree when it is outside their property – at a reduced cost (£160/tree v £295/tree). It gives people an option at a lower cost – and it avoids trucks driving about with lots of water in a bowser. It has worked elsewhere, and Bristol is going to try it.
DEFRA has provided funds for the setting up of Trees for Streets, and maybe future DEFRA grants will be channelled through this new national scheme. Bristol has, by making individual bids, obtained grants for tree planting from DEFRA in the past, and will still want to continue to make these bids for new funding for the actual purchase and planting of trees for new sites.

How it will work:

Go to the Trees for Streets website at https://treesforstreets.org.bristol.
Choose the location of your tree from the map or suggest a spot in a grass verge in your street or neighbourhood. The questions on the website take you through the choices.
Answer a few questions about the location and you.
If all works out your tree will be planted during the next available planting season.

Bristol Tree Forum’s Tree Champions are to be offered training from Bristol’s Tree Officers so that they can help residents, organisations and businesses with determining the suitability of sites that are suggested.

Valuing our urban trees – part III

When is tree not a tree?

Figure 1 Leyland cypress trees on the boundary of the former Police Dog & Horse Training Centre, Bristol.

The Biodiversity Metric 3.0 (BNG 3.0) User Guide defines Urban Tree habitats as follows:

Individual Trees	Young trees over 75mm in diameter measured at 1.5m from ground level and individual semi-mature and mature trees of significant stature and size that dominant their surroundings whose canopies are not touching but that are in close proximity to other trees.
Perimeter Blocks	Groups or stands of trees within and around boundaries of land, former field boundary trees incorporated into developments, individual trees whose canopies overlap continuously.
Linear Blocks	Lines of trees along streets, highways, railways and canals whose canopies overlap continuously.

These habitats are measured by area (hectares). Using this measurement and other parameters (Distinctiveness, Condition and Strategic Significance), their baseline biodiversity value is calculated in area biodiversity habitat units (ABHUs).

BNG 3.0 also includes separate calculations for two types of linear habitat, one of which is ‘Hedgerows and Lines of Trees’. These linear habitats are measured in kilometres. Using this measurement and the same parameters used for ABHUs, their baseline biodiversity value is calculated in hedgerow biodiversity units (HBUs).

Hedgerow habitats are a feature almost unique to the British Isles, but ‘Lines of Trees’ have been included as a linear habitat as they ‘display some of the same functional qualities as hedgerows’.

Box 8-2 of the BNG 3.0 User Guide (Figure 2) uses this key to help identify Hedgerow or Line of Trees habitat types:

The BNG 3.0 User Guide states that ‘Urban trees are considered separately to lines of trees in the wider environment, since they generally occur in an urban environment surrounded by developed land’. However, it is possible for disagreements to arise where the site is not clearly part of ‘an urban environment’, even though the trees fall within the Urban Tree habitat definition as either Perimeter or Linear Blocks.

A recent example demonstrates the issue. It involved 34 Leyland cypress trees growing along the boundary of the former Police Dog & Horse Training Centre on Clanage Road, Bristol, on the edge of the city. These trees were planted to form a screen between Clanage Road and the training centre (Figures 1 & 3).

This issue was argued before the Planning Inspector when the Secretary of State called the matter in (APP/Z0116/V/21/3270776) following a grant of planning permission for a change of use to a touring caravan site.

It was agreed at the inquiry that these trees had been planted between 1.5 to 2 metres apart, had developed average stem diameters of 33 cm and had grown to about 10 metres high and eight metres wide. The whole row is about 72 metres (0.072 km) long.

Figure 3 The site on the edge of the city (red boundary line)

Using the flow chart at Box 8-2 above, the developer’s ecologist argued that these trees were a Hedge Ornamental Non-native habitat. So, using the BNG 3.0 calculator, they would be assessed as a linear habitat 0.072 kilometres long. This habitat is given a Very Low Distinctiveness (score 1) and has a Poor Condition (score 1) [1]. Because of its location, it was given a Strategic Significance of Within area formally identified in local strategy (score 1.15). As such, the baseline habitat value is calculated as 0.072 x 1 x 1 x 1.15 = 0.08 HBUs.

We argued that these trees formed an Urban Tree habitat and that, using the BNG 3.0 calculator, it should be treated as 34 Medium-sized trees with a combined area of 0.1384 hectares with a Medium Distinctiveness (score 4) and is in Poor Condition (score 1) – even though it was agreed that the trees were in good condition and could be categorised as B2 using BS 5837:2012. Because of its location, it was given a Strategic Significance of Within area formally identified in local strategy (score 1.15). On this basis, the baseline habitat value is calculated as 0.1384 x 4 x 1 x 1.15 = 0.64 ABHUs (nearly 8 times the HBU value).

Whilst Rule 4 of the BNG 3.0 User Guide (page 37) states that ‘… the three types of biodiversity units generated by this metric (for area, hedgerow and river habitats) are unique and cannot be summed’, it is clear that adopting either of these two approaches will result in very different outcomes when assessing biodiversity net gain.

In our view it is vital not to undervalue baseline habitats by the selective use of the habitat definitions given in BNG 3.0.

The planning inquiry decision (refusal) has now been published – APP/Z0116/V/21/3270776.

A copy of this blog is available here.

Valuing our urban trees – part I

Valuing our urban trees – part II

[1] The Very Low Distinctiveness and Poor Condition parameters are the only options available for this habitat type under BNG 3.0.

Valuing our urban trees – part II

Assessing the condition of urban tree habitats using Biodiversity Metric 3.0

Our recent blog – Valuing our urban trees I, pointed out the failings of the methodology for calculating the size of urban tree habitats as set out in Biodiversity Metric 3.0 (BNG 3.0). We would now like to show how this is compounded by the inappropriate assessment criteria used to determine the condition of Urban Tree habitats, as also set out in BNG 3.0 (see Annex 1).

We use the following example – taken from a recently approved planning application [1] which will result in the removal of 13 urban trees – to demonstrate why this is approach is inappropriate.

Figure 1 The example tree – Google Street View 2020

This street tree is a London Plane (Platanus × acerifolia) with a stem diameter (called DBH) of 118 cm. It is a non-native species planted in hard standing on Bridge St, Bristol BS1 2AN in about 1967. Using BS 5837:2012 – Trees in relation to design, demolition and construction – Recommendations (a BSI Standards Publication), it has been categorised as A,1,2 (see Annex 2). The developer’s Arboriculturalist described it as having a ‘Large, broad crown with excellent form and vigour.’

The tree’s BS 5837:2012-calculated Root Protection Area (RPA) radius[3] is 14.6 metres, so it has an RPA of 630 square metres. The tree has an average crown radius of 9.88 metres and a calculated canopy area of 306 square metres.

Using BNG 3.0 TABLE 7-2: Urban tree size by girth and their area equivalent (see Annex 1), the calculated RPA of the tree is set at Large, so its habitat size is limited to just 113 square metres – a discount of 82% of its calculated RPA and 37% of its canopy area.

Notwithstanding categorisation of the tree as A,1,2, the BNG 3.0 Condition Assessment Criteria categorises the condition of this tree as Poor because it meets only two of the six criteria, as shown below:

Using BNG 3.0, the calculation of the baseline habitat (called Habitat Units) of this tree is as follows:

Had the BS 5837:2012 condition of the tree been allowed for and its condition set to ‘Good’, then the habitat units of this tree would be three times the habitat unit value of 0.0452, i.e., 0.1356 as shown below.

Not only has the true size of the urban tree habitat been significantly undervalued (because its actual RPA has not been used), but its assessed condition using the BNG 3.0 criteria is also clearly inappropriate given that this tree has been assessed at the highest category under BS 5837:2012:

Category A – Trees of high quality with an estimated remaining life expectancy of at least 40 years …that are particularly good examples of their species, especially if rare or unusual; or those that are essential components of groups or formal or semi-formal arboricultural features (e.g., the dominant and/or principal trees within an avenue).

The proposed solution

BNG 3.0 is seriously flawed when it comes to evaluating Urban Tree habitats. We have already commented on this when it comes to calculating habitat size.

In our view, the solution to the issue of assessing the correct condition of urban tree habitats is already available in BS 5837:2012. The standard may require some amendment to align it with BNG 3.0, but it is a well-established and practical approach used by the arboricultural community. This British Standard gives recommendations and guidance on the relationship between trees and design, demolition and construction processes and is used whether or not planning permission is required.

A copy of this blog can be downloaded here.

Our third blog dealing with habitat selection is available here – Valuing our urban trees – part III.

Annex 1

The Biodiversity Metric 3.0 – auditing and accounting for biodiversity

USER GUIDE (page 68)

TECHNICAL SUPPLEMENT (pages 193-194)

Annex 2

BS5837:2012 – 4.5 Tree categorization method – tree category definitions

[1] The Developer used BNG 2.0 in its submissions and applied a different Condition assessment to the one used here.

Category: Public trees

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

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Measuring and Modeling the Tree Canopy of Bristol

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Valuing our urban trees – part II

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Canopy prediction

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