Valuing our urban trees

At last, some good news: city trees have been given the same habitat and biodiversity value as their country cousins.

The important contribution that urban trees (native and non-native) make to our cities has finally been recognised by Natural England, with their publication of Biodiversity Metric 3.0 (BM3.0) on 7 July. It states that:

Trees in urban areas can, under the right conditions, provide a large range of habitat opportunities, supporting lichens, bryophytes, invertebrates and birds. Tree planting in urban areas has for over two hundred years also introduced non-native species into towns and cities. In the context of biodiversity, native species are the preferred option. However, non-native tree species can contribute positively to biodiversity richness particularly in relation to providing a seasonal food source for nectar feeders and other invertebrates as well as supporting vertebrates that feed on species that are hosted by non-native trees. Examples are early and late flowering species of Prunus and aphids on varieties of Acer providing food for species higher up the food chain.

Trees in urban areas provide opportunistic sites for biodiversity to colonise and re-colonise, increasing connectivity and contributing to biodiversity critical mass between already established patches or sites. This is especially true where transport corridors are populated with mixed native species.

What’s an urban tree?

The new BM3.0 habitat category, urban tree, includes individual trees, lines of street trees and blocks of trees growing within the urban setting.

BM3.0 Guide – TABLE 7-1: Urban tree definitions

The previous urban tree habitat categories, woodland, orchard and street tree, which appeared in the beta test version of Biodiversity Metric 2.0 (BM2.0) have been discarded.

The urban tree habitat calculation has been set to ‘medium’ distinctiveness and ‘low’ difficulty for both habitat creation and enhancement. Urban trees are categorised into small, medium or large. Their condition may also be assessed as poor, moderate or good.

The problem with BM3.0

The three size bands set out in the table below are useful when creating new habitats or enhancing existing ones (for example, nursery-raised standards ready for planting have a stem diameter of around 30 cm and so are Medium). However, these bands are not useful for assessing the baseline habitat of existing urban trees.

This is the size table used in BM3.0:

BM3.0 Guide – TABLE 7-2: Urban tree size by girth and their area equivalent

(NB: the second column of this table is wrongly labelled. It should read Girth (circumference) at Breast Height, not Diameter.)

This table is based on the methodology adopted for calculating the Root Protection Area (RPA) that must be set aside (protected) during the development of sites where trees are growing or likely to be affected. It is set out in BS:5837 2012 Trees in relation to design, demolition and construction, Recommendations. Its purpose is to make sure that the tree is not damaged during development.

The RPA formula used is simple: RPA radius = 12 x DBH (Stem Diameter is also known as DBH – Diameter at Breast Height). This value is then used to calculate the RPA using the formula DBH = PI * RPAr^2.

Every application to develop land where trees will be affected should produce a BS:5837-compliant survey, called an Arboricultural Impact Assessment (AIA). This will report the stem diameters of all the trees growing on and around the site. The AIA also reports several other tree features including species, height, cardinal point canopy radii, condition, life stage and the BS:5837 category – a measure of the quality of the tree.

However, the BM3.0 table above provides no logical way of establishing whether a given surveyed tree with a stem diameter of, say, 15 cm or 40 cm – halfway between categories – is Small, Medium, or Large.

It would be better if the table gave ranges – say Small up to 10 cm, Medium 10-50 cm and Large 50 cm or more – but this has not been done. Also, doing this would distort the habitat calculation with all Small trees set to their upper range and all Large trees set to their lower range.

Our solution

Why use the table at all? It would be far simpler to calculate a tree’s baseline habitat area just by using the calculated RPA provided in the AIA. It would be better still to use its actual measured canopy area, which will have been reported in the AIA and thus be readily available.

In our view, RPA does not reflect the habitat value of a tree. All it does is use a formulaic approach to solving the problem of finding an acceptable way to protect trees. It bears little relationship to the habitat or biodiversity value of a tree.  It would be far better to calculate a tree’s canopy cover (TCC), the standard method of working out the value of a tree. Every AIA reports the canopy radii of the four cardinal compass points of each tree surveyed. These can be averaged and used to calculate TCC.

The Bristol One City Plan adopted TCC as the measure of tree planting success when it set the target to double TCC by 2046. TCC is a standard measure used by the various i-Tree tools and Forest Research uses it in its UK Ward Canopy Cover Map which used i-Tree Canopy. We used it to calculate the TCC of the city’s wards in our 2018 Bristol Tree Canopy Cover Survey and we are using it to update the new city-wide survey for 2021.

We made these observations when Natural England was consulting on its beta test version, but these seem to have been overlooked. We hope they now take note.

Some further thoughts

The introduction of the three new urban tree poor/moderate/good condition criteria, set out in detail in the BM3.0 Technical Supplement, will require all AIA surveys to include this data. Perhaps BS:5837 should be updated to require this to be recorded in the AIA.

Where tree surveys identify mixed urban tree conditions, the person undertaking the BM3.0 calculation will need to record more than one urban tree baseline habitat to capture this information.

BM2.0, which was only published as a beta test to allow for wider public consultation, is still being used by Bristol’s Local Planning Authority (LPA) for pending applications but needs to be abandoned. Pending applications which require a biodiversity net gain report should be required to recast their calculations using BM3.0 rather than still relying on BM2.0. This is particularly true for the Council’s own, direct applications such as the one pending for the Baltic Wharf Caravan Park.

Our initial analysis shows a significant net gain deficit when BM2.0 is used instead of BM3.0. This is especially true for urban street trees, which are significantly undervalued under BM2.0. Furthermore, the LPA is currently allowing applications which propose a zero net gain outcome, even though the Environment Bill (currently being considered in Parliament) will require a net gain of 10% above the baseline valuation.

Given that the Council has declared climate and ecological emergencies and aims to achieve carbon neutrality by 2030, it is surprising that developers continue to be allowed to present biodiversity net gain proposals that either undervalue biodiversity or offer no net gain whatsoever.

Conclusion

We welcome the publication of BM3.0, but its flaws need to be rectified.

The Council’s declaration of climate and ecological emergencies and its commitment to achieve carbon neutrality by 2030 means that it needs to ensure that the latest, most accurate biodiversity net gain calculations are part of all pending and future planning applications.

As Natural England recognises in its recent blog – Biodiversity Metric 3.0 – a milestone moment for biodiversity net gain:

Publishing Biodiversity Metric 3.0 was a landmark moment for biodiversity net gain, it will become the metric used to calculate and evidence whether a project has achieved the biodiversity net gain requirements set out in the Environment Bill. Biodiversity Net Gain (BNG) is:

an approach to development, and/or land management, that leaves nature in a measurably better state than beforehand‘ …

Biodiversity Metric 3.0 ensures that:

all habitats, from street trees to woodlands, green roofs to grasslands are recorded, scored and valued for their importance for wildlife. At the same time, it provides an evidence-based, transparent, consistent and easy to use way of ensuring that nature is considered within the design of developments and in land management practice, leaving nature in a better place than it was before, benefitting wildlife, people and places.

Bristol’s Tree Canopy

“Bristol ranks as the 5th greyest city in England”

This statement was made in a recent article in Wales Online,  the Express, and elsewhere. The article, with a by-line of Neil Shaw, seems to be based on a press release by OVO Energy who are promoting a petition to create a legally binding target to plant 30,000 ha of new woodland each year to 2050.  The article reported tree cover in a number of countries and cities around the UK based on data supplied by the aerial survey and GIS company BlueSky.  Amongst the results is :

Bristol, known for its green credentials, ranks as the 5th greyest city in England at 8% – and only 1 tree per person. 

This is very different from the estimate produced by our own tools which estimate tree canopy cover (TCC) in 2020 at around 17.5%. Thankfully, as the following analysis discovers, Bristol can hold its head as a green city.

i-Tree Canopy 

Our estimate is based on a desktop survey using a methodology called i-Tree Canopy.   The methodology is pretty simple:  take any boundary, randomly place a number of points within the boundary, examine each point in Google Maps and decide if the point lies within a tree canopy or not; the ratio of canopy points to the total number of points is the TCC, Uncertainty arises from the nature of the random sampling and interpretation of the image, particularly to distinguish a tree from hedges and low ground cover.

Our version of this approach is integrated with the Trees of Bristol website so that it can used to estimate TCC for any area in our database with a known boundary.  In particular, we have used this tool to estimate TCC for all wards in Bristol which are mapped here.  These values have joined the many hundreds of estimates across the UK  to form the GB Ward Canopy Map  organised by Forest Research.  With this pedigree, we have been advocating this approach for use in Bristol as the means to assess progress towards Bristol’s ambitious goal of doubling tree canopy by 2046.  Aggregating the samples across all 32 wards, we estimated that Bristol had 17.9% TCC in 2018 and by 2020 it was  17.5%. (This change from 2018 to 2020 is not statistically significant)

National Tree Map

The estimates in the press article were based on the National Tree Map, a commercial product from Bluesky.  This uses a combination of their own imagery and LIDAR data.  Complex analysis of the LIDAR data, using the difference in return time from ground and canopy reflections enables an estimate of the canopy above 3m high.   

Discussion with Bluesky revealed a probable cause of the discrepancy for Bristol.  Any comparison between estimates needs to be based on the same boundary definition using imagery from the same time period. For the i-Tree Canopy approach we have used the City of Bristol boundary which has an area of about 11,000 hectares (110 sq km) . In contrast, it turns out that  the data provided to OVO energy by Bluesky was based on the Unitary Authority Boundary.  For Bristol this is a rather odd area, taking in a swath of the Bristol Channel down as far as the islands of Flat Holm and Steep Holm.  This is because historically, the boundary of the Port of Bristol is included.

image

The area within this boundary is 23,500 hectares.  Since Bristol can hardly be criticised for failing to plant trees in the Bristol Channel, this dramatically distorts the estimate.  Adjusting for this difference in definition, I arrived at a figure of 17%, within the statistical bounds of the i-Tree canopy estimate.

The National Tree map was also used back in 2014 as reported in the Daily Mail.  The accompanying map similarly shows a very low value for tree canopy in Bristol so I suspect that the same boundary was used there too.

image

Comparison

After discussion with BlueSky, I supplied four boundaries for assessment using the NTM methodology for comparison with the i-Tree approach: the Bristol City Boundary and three wards chosen to have low, medium and high levels of canopy. These are the results:

image

NTM uses a strict height of 3 metres when assessing canopy whereas using i-Tree canopy, the distinction between tree canopy and lower greenery including hedges is assessed visually, so a slight upward bias might be expected and has also been observed in Forest Research data.  On the whole though, this comparison shows very strong agreement between the two methodologies. 

The bad news

The gross error in Bristol’s tree canopy percentage actually made it easy to see that something was amiss.  One must assume that similar issues will have occurred in the case of other cities whose boundaries are subject to debate.  Indeed, the Unitary authority boundary for Portsmouth, which with only 4% cover is reported to the be worst in the UK, includes the expanse of Portsmouth and Langstone Harbours.  According to the Portsmouth Council website, land is about two-thirds of the area of the authority so a better figure would be 6%, still low.

Problems with boundary definitions plague this data.  Bristol City is only the core of the conurbation with large parts of what we think of as Bristol in South Gloucestershire and Bath and North East Somerset.  Comparison with the figure given for Leeds, also 17%, is not possible since the City of Leeds boundary includes all the surrounding towns and countryside.

It is clear that unitary authority boundaries are not directly suitable for urban canopy evaluation.

The need for full data publication

In addition to the 2014 report and the recent publicity by Ovo Energy, another survey by Bluesky was publicised late last year on the BBC but no figure for Bristol is mentioned.  These press articles give only selective figures rather than the full data across England. I searched for published reports containing the full data, which I expected to include the base area, canopy area as well as the computed percentage and rankings.  I found nothing.  This makes it impossible to correct other derived data, such as the ranking of Bristol as the “5th greyest in England”.

I would hope that in future, companies like Bluesky and Ovo Energy will see that making full data openly available in support of extracts and assertions would reduce mis-interpretations, provide a public good and better promote their company.

Journalists too have a responsibility here, not only to critically assess press releases but to request and link to the supporting data. Neither happened in this case.

The good news

This exercise has turned out to be good news for both the National Tree Map methodology and our own work with i-Tree Canopy. The results are very similar and differences are rather consistent and explainable.  Our implementation of i-Tree Canopy is free to use by citizen-scientists with known error bounds and can be quickly applied to any chosen boundary.  With the inclusion of historical imagery from Google Earth, it can also be used to compare canopy over time.  

This exercise has also confirms the doubts we held about the figure from an i-Tree Eco survey carried out in 2018.  This survey used volunteers to ground-survey 200 random plots in Bristol. The survey arrived at a figure of 12% with wide error bounds but much less than the i-Tree Canopy value.  All methods have some uncertainty but we can be pretty confident that Bristol’s Tree Canopy in 2020  is in the region of 17 – 18%.

The National Tree Map is primarily intended as a means to locate and measure the canopy of individual trees in an area.  The canopy estimate is only a by-product and agrees well with the i-Tree canopy approach.  For its primary purpose, NTM appears to provide a very much more economic solution than on the ground surveying.  Indeed it would be very interesting to compare this map for Bristol with the mapping of individual trees in Trees of Bristol.

Forest Research is at the forefront of research into the UK Urban Tree canopy and their 2017 paper on the Canopy Cover of Englands Towns and Cities remains the most authoritative UK -wide survey. We look forward to an update to this excellent work.

Chris Wallace

First published in The Wallace Line on 11 May 2021

Consultation on proposed changes to NPPF and the National Model Design Code

Individual planning decisions, development designs and local and national plans for development all impact local communities. We urge the Ministry of Housing Communities and Local Government to consider our views on the design codes and to continue to engage communities and groups such as ours in local planning decisions.

Here are our detailed responses to the consultation.


The changes proposed in Chapter 2 – Achieving sustainable development

Paragraph 7 – We agree with the introduction of the 17 Global Goals for Sustainable Development. These have been adopted by Bristol as part of its One City Plan so their adoption in the NPPF will be essential for ensuring that the city’s core planning policies are aligned with its wider goals.

Paragraph 8 states:

‘Achieving sustainable development means that the planning system has three overarching objectives, which are interdependent and need to be pursued in mutually supportive ways (so that opportunities can be taken to secure net gains across each of the different objectives).’

We would also like it to be made as clear as possible that these three overarching objectives are indeed ‘interdependent and need to be pursued in mutually supportive’ ways so that no one objective takes precedence over the others, as has been our experience with a number of recent planning decisions made in Bristol.

We propose that the paragraph amended to read: ‘Achieving sustainable development means that the planning system has three overarching objectives, which are interdependent and need to be pursued in mutually supportive ways so that no one objective is treated as having precedence over the others (so that opportunities can be taken to secure net gains across each of the different objectives)’

Paragraph 11 a) – We also endorse the proposed change that ‘all plans should promote a sustainable pattern of development that seeks to: meet the development needs of their area; align growth and infrastructure; improve the environment; mitigate climate change (including by making effective use of land in urban areas) and adapt to its effects’. Trees are an important component of this, particularly where green space is limited.


The changes proposed in Chapter 3 – Plan making

Paragraph 22 – We agree that ‘where larger-scale development such as new settlements form part of the strategy for the area, policies should be set within a vision that looks further ahead (at least 30 years), to take into account the likely timescale for delivery’. Too often, trees that were planted where a site was last developed (often only a few years before) are sacrificed to the short-term goals of the new proposal. Setting longer-term goals can help prevent this.


Proposed changes to Chapter 4 – Decision making

Paragraph 53 – Of the two options offered[1], we prefer the second – ‘where they relate to change of use to residential, be limited to situations where this is necessary in order to protect an interest of national significance’. In our view, the phrase ‘wholly unacceptable adverse impacts is open to too wide an interpretation which may not be rooted in wider national goals.

We agree that that Article 4 directions should be restricted to the smallest geographical area possible. 


The changes proposed in Chapter 8 – Promoting healthy and safe communities

We welcome many of the additions and changes proposed, including the recognition that a well-connected network of high-quality, open, green and wooded spaces is important for both our mental and physical health.

Paragraph 97 – We believe that access to a network of high-quality open spaces and opportunities for sport and physical activity ‘should always deliver wider benefits for nature and efforts to address climate change.


The changes proposed in Chapter 12 – Achieving well-designed places

Paragraph 128 – We agree that all guides and codes should be based on effective community engagement and reflect local aspirations for the development of their area.

Meaningful community engagement at all stages of the planning process is essential if the changes proposed are to succeed. Too often, communities are not asked to engage with planning proposals until they are published and the formal approval process has started. By this time most of the key decisions have been agreed between the developer and the planner and it is too late for any meaningful consultation with the wider community.

Paragraph 130 – We welcome the introduction of this new paragraph:

‘Trees make an important contribution to the character and quality of urban environments, and can also help mitigate and adapt to climate change. Planning policies and decisions should ensure that new streets are tree-lined, that opportunities are taken to incorporate trees elsewhere in developments (such as community orchards), that appropriate measures are in place to secure the long-term maintenance of newly-planted trees, and that existing trees are retained wherever possible. Applicants and local planning authorities should work with local highways officers and tree officers to ensure that the right trees are planted in the right places, and solutions are found that are compatible with highways standards and the needs of different users.’

We must learn to value our urban trees and woods growing in Bristol (and in other cities), so we were pleased to see this addition with the ambition to ensure that all new streets are treelined, but city-wide planning involving existing streets and road networks must also make space for new tree planting in the design process as well as ensuring that existing trees are retained.

Generally, planning requirements must be tightened to ensure that existing trees are retained. Only in exceptional cases where there are clear, justifiable and compelling reasons to do so should trees be removed. In all cases the cascading principles of the Mitigation Hierarchy must be applied and, where there is no option but to remove a tree, the loss of habitat and biodiversity that the tree provided must be compensated for by an adequate tree replacement calculation such as that used in the Biodiversity Metric calculation.

We agree that ‘development that is not well designed should be refused (paragraph 133). Designs that fail to make provision for preserving existing trees and providing new trees are not, in our view, well-designed and so should be refused.


The changes proposed in Chapter 13 – Protecting Green belt Land

New Paragraph 149 – We propose the deletion of this text, which is too general and open to interpretation. Certain other forms of development are also ‘not inappropriate in the Green Belt provided it preserves its openness and does not conflict with the purposes of including land within it’.

In Bristol there are just over 596 hectares of Green Belt left within the metropolitan boundary, mostly confined to the few remaining green margins of the city. The last draft of the Local Plan proposed the removal of some 50 hectares for development. Already parts of the Green Belt are disappearing without any hint that this ‘preserves its openness and does not conflict with the purposes of including land within it’. Little by little, development by development, Green Belt land is being lost.


The changes proposed in Chapter 14 – Meeting the challenge of climate change, flooding and coastal change

Paragraph 160 c) – Tree preservation and the planting of new trees are key elements of ‘using opportunities provided by new development and improvements in green and other infrastructure to reduce the causes and impacts of flooding, (making as much use as possible of natural flood management techniques as part of an integrated approach to flood risk management)’ We would like to see text added that states this.


The changes proposed in Chapter 15 – Conserving and enhancing the natural environment

Paragraph 179 d) – This states that ‘development whose primary objective is to conserve or enhance biodiversity should be supported; while opportunities to improve biodiversity in and around other developments should be pursued as an integral part of their design, especially where this can secure measurable net gains for biodiversity and enhance public access to nature’.

It is essential that core planning policies mandate a standard metric for measuring baseline and created and enhanced habitat biodiversity proposals. Developers must be obliged to provide a Net Gain calculation when submitting their proposals. The latest version of the Biodiversity Metric Is designed for this purpose and should be mandated for all new planning proposals. All planning permissions should require the delivery of Biodiversity Net Gain plans of at least 10%.


We would be grateful for your views on the National Model Design Code, in terms of a) the content of the guidance b) the application and use of the guidance c) the approach to community engagement

The design codes must deliver three key things to ensure that new developments always provide access to high-quality, local green space and to trees, with all the benefits these provide for communities.


  • Protect and integrate existing trees  

New developments must incorporate and protect existing trees from the outset. There must be a presumption that the design will accommodate the existing trees growing on and around the site – especially those growing around the edges of sites. Designs should consider the long-term health of trees in and adjacent to new developments and aim to promote this. This will include providing adequate buffers for ancient, veteran and self-seeded trees and woods.

  • Increase canopy cover  

New developments must have a target of providing a combined minimum of 30% canopy cover on and off site. This should be made up of a mix of tree-lined streets, community woodlands, Tiny Forests, parks and gardens. Where tree provision will be made off site, the cost of providing, planting and caring for the trees on a long-term basis should be funded by the developer and incorporated into tree-specific S106 agreements (T&CPA 1990). Where possible, trees should be native and sourced and grown in the UK. Trees that will become large and are long-lived should be selected where possible.

  • Ensure trees thrive for the long term  
<p value="<amp-fit-text layout="fixed-height" min-font-size="6" max-font-size="72" height="80">Local authorities must be properly resourced so that they can implement design codes and other areas of planning policy. Resource needs to be available for decisions to be enforced and to ensure long-term management of trees by tree officers.Local authorities must be properly resourced so that they can implement design codes and other areas of planning policy. Resource needs to be available for decisions to be enforced and to ensure long-term management of trees by tree officers.
  • Community engagement

As we have already noted, meaningful community engagement is essential if communities are going to consider that they ‘own’ planning decisions rather than having them imposed on them.

We have published a paper on the issue as it relates to consultation on the management of trees which we commend to you: ‘Community engagement in urban tree management decisions: the Bristol case study’.

3 March 2021

You can download a copy of our submission here.

Here are copies of the draft National Planning Policy Framework and National Model Design Code.

The consultation closes on 27 March 2021 and can be accessed here – National Planning Policy Framework and National Model Design Code: Consultation proposals.


[1]  ‘a) where they relate to change of use to residential, be limited to situations where this is essential to avoid wholly unacceptable adverse impactsorb) where they relate to change of use to residential, be limited to situations where this is necessary in order to protect an interest of national significance’.

The Campaign to Save the M32 Maples

The M32 Norway maples are, or were, a group of mature and majestic street trees on Lower Ashley Road, St Pauls. They were due to be felled as part of a development on the adjacent site, an action the Save the M32 Maples campaign group contend is illegal as the protected trees are on public land and not the property of the landowner.

The Development

In 2016 outline planning permission was granted to Mr John Garlick to build a four storey development, comprising ground floor offices and student accommodation above on the site of the former probation office, 31-45 Lower Ashley Road. Reports stated that the development required removal of five mature Norway maple trees. There was no mention in the application or the arboricultural report that the trees were protected by Tree Preservation Orders, a crucial consideration in any consultation, and the supposition was that the trees were sited on land owned by Mr Garlick. Full planning permission for this development was granted in May 2019.

In May 2019, a second application was submitted for construction of a 4-storey block of flats to provide 28 units including some affordable housing. Over 400 objections to this development were submitted, mostly citing the loss of trees, air pollution, noise and flooding issues.

The Bristol Tree Forum was one of the objectors on the basis that legally binding planning policies aimed at protecting green infrastructure (BCS9, DM15), protecting against air pollution (BCS23, DM33), addressing climate change (BCS13), reducing flood risk (BCS16), limiting noise pollution (BCS23, DM35) and protecting public health (DM14),  were contravened in the proposal (https://bristoltreeforum.org/2020/07/09/bristol-city-development-where-did-all-the-green-go/).

Felling by the cowboy method.

Illegal tree felling

At 6 am on June 19th 2019, a crew of workmen arrived with chainsaws and proceeded to attack the maple trees on site. There were no safety precautions applied either for the workmen or the public. None of the required permits were in place and no pavement or road closures were implemented. Local residents stepped in and the police intervened to stop the illegal felling. However, two of the trees were severely damaged. This contravention of planning conditions was reported to the Council’s Planning Enforcement Officer, but no action was taken.

Protesters chain themselves to a tree.

At 5.30 am on December 31st 2019, the felling crew returned. Again, no permit was in place and no safety measures were applied. Again, the public intervened, with some chaining themselves to the trees. The area tree officer attended, and members of the Bristol Tree Forum arrived in support. As previously, the police put a stop to the illegal activity, and Mayor Rees attended to witness the damage. Two of the 5 original trees had been felled. The contravention of planning conditions was again reported to the council Enforcement officer, and, as previously, no action was taken, on the basis that other unrelated enforcement issues were being investigated.

At 5.50 am on November 2nd 2020, an anonymous chainsaw crew arrived, and whilst still dark, during a storm and fully 2 hours before permitted construction hours, proceeded to fell two of the trees. As on previous occasions, there were no safety measures applied, either for the workers or the public. Local residents again intervened, and the workmen attempted to flee the site, which residents prevented, despite being threatened. Again, the police arrived to bring the illegal activity to an end, and the site was left in a dangerous state, with only one of the original five trees still standing. The contravention of planning conditions was again reported to the Council’s Planning Enforcement Officer, but no action was taken on the basis that this was an “isolated incident”. This was despite the fact that three previous illegal fellings, stopped by the police, had been reported to the enforcement office.

A severed tree suspended off the ground by entangled branches

The legal case disputing the ownership of the land on which the five maples are located

Early in 2020, a local residents group, the Save the M32 Maples campaign, was formed to protect the trees. The group has been active on a number of fronts.

  • Challenging the legality of the developer to remove the five maple trees, as it has been demonstrated the trees are on public land not actually located on Mr Garlick’s property.
  • Building and occupying tree houses in the remaining trees.
  • Mounting a vigil to guard the trees.
  • Opposing planning applications for development on the site.
  • Community outreach activities and public protesting against the removal of the trees.
  • Local and national media campaigning.

The legal dispute is whether the five Norway maple trees were, and are, sited on public land or land sold to private landowners in 2005. The Save the M32 Maples group have presented substantial legal documentation showing that that the strip of land sold by Bristol City Council (BCC) to private landowners in 2005 did not include a strip of land on which 5 mature maple trees were located. These include the deed Register, Sale Contract (1), Heads of Terms, plan of Adopted Highway asset 384 (2), Deed of Covenant Release, BCC Corporate Estate Document, and BCCs Pinpoint map (3), all of which are in agreement. If the group’s contention is correct, it is illegal for the landowner to fell the trees as part of this development.

However, BCC Development Officers refused to accept legality, accuracy and validity of these documents, instead presenting a Registry Transfer document overdrawn with a thick black line (4), claiming the thickness of this line undermined the accuracy of all legal documents. On the basis, therefore, that all drawn boundaries are unreliable, BCC regard that text in some documents referring to “trees” is proof that the 5 maples are part of the land sold in 2005.

Accurate measurements from all the above legal documents showed the land transferred was 180m2, in agreement with the 180m2 cited in the Heads of Terms. That this 180m2 excludes the five trees is not disputed by either party.

In response, BCC undertook a survey in 2020. The area in question increased to 210m2 with no reason given, transferring some of Asset 384 to the landowner and depleting Adopted Highway accordingly.

The 180m2 land sold does not include the five maples…. but the 210m2 does.

Thus, the campaign group contend that BCC has illegally given away 30m2 of public land without legal sale/transfer, and that it is on this land that the five maples were located. This has led to a long running dispute and police investigation, and incited illegal felling of publicly owned and protected trees.

The Save the M32 Maples group have instigated a formal complaint about the BCC’s alleged misconduct in dealing with this case. This complaint was not upheld by BCC, and therefore the group began proceedings to mount a Judicial Review, with the help of Paul Powlesland, barrister and environmental rights activist. BCC resisted the move to have the case heard in a court of law, and the group ultimately had to abandon the case because BCC threatened imposing punitive costs. Alleged misconduct by Council Officers has been investigated by the police, who recognised “irregularities”, but were unwilling to pursue the case until the council’s complaints procedure had been exhausted. There is currently an appeal being prepared for the Local Government Ombudsman.

Ground contamination

There is a strong possibility that the 31-45 Lower Ashley Road site is contaminated. The adjacent property is the site of a former petrol station which suffered a major contamination event in the 1990s in which a long-standing leak from fuel storage tanks was detected. A confidential report, commissioned in 2003 by the former owners, revealed that a cocktail of toxic chemicals, including lead, chromium, benzene, toluene, xylene, naphthalene and MTBE, a lead replacement, soaked into the ground over at least five years. Many of the chemicals are associated with neurological disorders and can cause miscarriages and severe learning disabilities in children.

Despite a statutory requirement, the latest planning applications for the 31-45 Lower Ashley Road site did not include a ground contamination report, and referred only to a “desk report” prepared for the 2016 application. This report makes no mention of the documented contamination event, despite this report being copied to BCC and the Environment Agency, and no samples were analysed for pollutants. The Councils Public Protection group has insisted that a full investigative survey is undertaken, but this has still not been carried out.

Because of the likely contamination of the site, and the threat this poses to the public, the Bristol Tree Forum have advocated that this site is allowed to recover naturally through the actions of the trees on site removing toxins from the soil.

The campaign continues

The last tree standing…

Though there is only one of the original five maple trees remaining, the Save the M32 Maples group continues to campaign, in part to save the final maple tree, but also as a matter of principal that the council and developers should not fell mature trees unless absolutely unavoidable. In this case, as the trees are, at worst, on the margins of the development, efforts could and should have been made to incorporate the trees in the construction. The council and developers regard that planting replacement trees, in accordance with the Bristol Tree Replacement Standard (BTRS), is adequate mitigation. However, recent work by the Bristol Tree Forum (https://bristoltrees.space/trees/tree-benefits/interactive-1.xq) demonstrates that, even with the BTRS applied, the carbon (also representative of the other benefits of trees) will not be recovered for 40 to 60 years.

The one remaining tree has been reinforced by layers of protection around the trunk, and a daily vigil has been mounted to guard the tree, should the illegal felling crew return. The media campaign continues, and local support for the group continues to grow. The group has also begun coordinating with other tree protest groups nationally, to share experiences and effective campaign strategies.

The campaign continues!

Sadly, the issue that this campaign highlights is all too common in Bristol, that mature trees are not sufficiently valued, either by developers or the council’s own development office. A major rethink is needed if we are going to protect the trees of Bristol and the benefits they provide. (https://bristoltreeforum.org/2020/01/18/shocking-treatment-of-lower-ashley-road-trees-shows-urgent-need-for-bristol-planning-rethink/).

John Tarlton.

Bristol Tree Forum tree planting campaign – free Oak saplings available for planting

STOP PRESS

We delighted to report that nearly 1,600 tree orders have been received. We have bought another 600 trees to cover the extra orders and expect delivery soon.

Many thanks to all of you who have placed an order. We shall soon let you know when and where you can collect your trees.

Due to COVID-19 restrictions and delays in government funding, there has been postponements and cancellations of many major tree planting projects. As a result, large numbers of tree saplings are due for destruction in tree nurseries. This includes 750,000 two year old English oak tree saplings at the Maelor Forest Nursery in Wrexham.

Rather than see these trees destroyed, Bristol Tree Forum has purchased 1,000 of the oak saplings for free distribution to anyone able to plant them, whether this is one tree or a hundred.

We will get delivery early in November. The trees can be collected from a site in Redland, Bristol and a few collection dates will be organised hopefully to suit all. They should be planted as soon as possible afterwards.

The saplings are between 10cm and 90cm high. They come bare-rooted (i.e. out of the soil) and need to be planted as soon as possible after collection, although the viability of the trees over winter can be extended a little by storing the trees with the roots covered in damp soil.

This form is to find out who would like to have saplings for planting and how many, and for you to provide basic contact details (email and/or phone number) for us to organise collection of the trees. Contact details will not be used for any other purpose.

Why plant a tree?

A single mature oak tree is the equivalent of 18 tonnes of CO2 or 16 passenger return transatlantic flights.

Despite advances in carbon capture technology, the most efficient and cost-effective way to sequester carbon from the atmosphere is to plant trees.

Recent scientific reports calculate that planting trees wherever we can, without occupying land used for other purposes, would absorb up to two thirds of the carbon emitted in the last century.

Oak trees can support over 2300 different species, including birds, mammals, invertebrates, mosses, lichen and fungi.

Trees improve air quality by absorbing both gaseous (e.g. NO2) and particulate pollution.

Trees reduce traffic noise and flooding, reduce excessive heat in cities and improve physical and mental wellbeing.

Trees valued at over £4.6m are under threat at Bonnington Walk, Lockleaze

Whatever the merits of this application of achieving its primary goal to provide much needed housing may be, it should not be permitted to proceed unless and until it has properly addressed how it will replace and build upon the Green Infrastructure (including trees) that will inevitably be lost if this application proceeds as presently formulated.

Summary of our submission

We object to this application for the following reasons.

Bristol City Council has:

Declared climate and environmental emergencies.

Committed to becoming carbon neutral by 2030.

Committed to doubling tree canopy cover by 2046.

As currently formulated, these plans to build new houses can only set back the work needed to resolve these emergencies and achieve these commitments.

  1. The need to build housing to meet sustainable economic or social development objectives should not be allowed to take precedence over ensuring that the development is also both environmentally sustainable and meets Net Gain objectives.
  2. Whatever the merits of this application of achieving its primary goal to provide much needed housing may be, it should not be permitted to proceed unless and until it has properly addressed how it will replace and build upon the Green Infrastructure (including trees) that will inevitably be lost if this application proceeds as presently formulated.
  3. The existing trees have a significant asset value which should not lightly be ignored. Using CAVAT, we have valued them at £4,674,918.
  4. Under the Mitigation Hierarchy, trees should not be removed unless there is no realistic alternative. One alternative would be to build around the trees rather than remove them.
  5. BCS9 of the Core Strategy also states that “Individual green assets should be retained wherever possible and integrated into new development”. Clear felling nearly all the trees to the east of the cycle/footpath should not, as it so often is, be the default option.
  6. Trees should not be removed merely because they are diseased or self-sown, or because they are small or not perfect specimens of their species.
  7. The removal of existing trees inevitably means that the eco-services they provided will not be replaced for decades, if at all.
  8. The adverse knock-on environmental impact on biodiversity of removing existing trees far outweighs any short-term benefits achieved by replacing them.

Our submission

The planning background

The National Planning Policy Framework

The National Planning Policy Framework (NPPF) seeks to ensure that new development is sustainable. It stresses the importance of Green Infrastructure as one of three overarching, interdependent objectives – economic, social, and environmental. This means that the presumption in favour of sustainable environmental development is just as important as any in respect of economic or social development objectives.

Trees are an integral part of this because of the importance of trees in relation to the management of air, soil and water quality along with other associated ecosystem services, climate change adaptions and beneficial health effects. The NPPF also seeks to achieve the protection and enhancement of landscapes and achieve Net Gain in biodiversity.

The Natural England Joint Publication JP029 – Biodiversity Metric 2.0 (BDM2) provides a way of measuring and accounting for biodiversity losses and gains resulting from development or land management change. It defines Net Gain as an:

“approach to development that aims to leave the natural environment in a measurably better state than beforehand. This means protecting existing habitats and ensuring that lost or degraded environmental features are compensated for by restoring or creating environmental features that are of greater value to wildlife and people. It does not change the fact that losses should be avoided where possible, a key part of adhering to a core environmental planning principle called the mitigation hierarchy.”

The Mitigation Hierarchy

Avoid – Where possible habitat damage should be avoided.

Minimise – Where possible habitat damage and loss should be minimised.

Remediate – Where possible any damage or lost habitat should be restored.

Compensate – As a last resort, damaged or lost habitat should be compensated for.

This is a cascading decision process – only if the preceding choice is unavailable is the next considered.

Local Planning Authorities (LPA) in the UK have a statutory duty to consider both the protection and planting of trees when considering planning applications. The potential impact of development on all trees is therefore a material consideration. In particular, BCS9 of the Core Strategy states that “Individual green assets should be retained wherever possible and integrated into new development”.

We have summarised Bristol’s planning policies as they relate to trees here – Planning obligations in relation to trees in Bristol.

Summary of the proposal in relation to trees

This site covers just over six hectares. The Lockleaze Allotments (a 0.8 hectare Statutory Allotment[1]) is located to the south east of the widest part of the site. It appears to be disused. Most of the substantial trees growing on the site are growing in or around this allotment or to the north of it. We have calculated that, taken together, they cover at least 1.3 hectares of the site – a tree canopy cover (TCC) of around 20% which is well above the estimated TCC for Bristol as a whole which is just under 12%.

All our calculations, summarised below, can be examined in this linked spreadsheet.

The Arboricultural Impact Assessment Report (the AIS) dated June 2020 (based on a survey done on the 19th and 20th of September 2019) identified a combined total of 58 individual trees and 40 tree group features. The number of trees in each group is not given, so it is not possible to say how many trees in total are growing on the site.

Of all the trees growing on site 24 individual and at least 251 group trees are identified for removal. The trees growing in Groups G69 and G74 are all to be removed, but the number of trees in each group is not identified so we have not been able to include or count these in our calculations.

The only reason for given for felling these two groups is because they show evidence of Ash Dieback (Hymenoscyphus fraxineus). As the AIS recognises, the mere presence of Ash Dieback is not a sufficient reason for the removal of a tree. We oppose the removal of these tree unless it can be shown that they there is a better reason for their removal.

The CAVAT calculation

Using CAVAT we have calculated that those identified trees which have a measured stem Diameter (DBH) are worth £4,674,918.  As the AIS fails to give the upper life expectancy ranges[2] of the majority of trees, we have assumed that all those trees given a 10+ or 20+ years life expectancy will survive between 40 and 80 years. This attracts a 5% discount on the base valuation. We have applied a CTI factor for Bristol of 150[3]. All the other factors are set to their default values.

The BTRS calculation

These two tree groups and five individual trees are categorised as Category ‘U’ trees under BS5837:2012 Trees in relation to design demolition and construction, and so have not been taken into account for the purpose of the Bristol Tree Replacement Standard (BTRS) calculation. A further 10 trees are also excluded from the BTRS calculation because their stem diameters are under 15 cm. We advocate that all trees identified for removal should be replaced no matter what their size.

Notwithstanding this and based on the current guidance, we have calculated the BTRS value at 455 trees as per the AIS calculation.

Net Gain calculation

No Net Gain calculation has been undertaken using BDM2 in support of this application.

We have undertaken our own BDM2 calculation in respect of just the trees surveyed in support of this application. A full calculation needs to be undertaken in respect of the whole of the site. This will inform any future decision about achieving Net Gain if this development is to be allowed to proceed.

Using BDM2, we have calculated that the combined tree canopy cover[4] of just the known, measured trees is 1.21 hectares. We have set the A-1 Site Habitat Baseline Habitat Type to Urban – Street Tree in the calculation. This assumes, amongst other things, that any replacement trees will reach maturity in 27 years and so uses a multiplier of 0.3822 to reflect this.

This gives Base Habitat Units of 5.864 and a Base Replacement value of 3.17 hectares. If we add an arbitrary Net Gain value of 10%[5], then the Base Habitat Units increases to 6.451 and the Base Replacement value to 3.49 hectares. Assuming that a 27-year-old tree has a canopy of .00403 hectares, then 866 replacement trees are needed to replace what has been removed and to achieve Net Gain.

Loss of the ecosystem services of trees

We invite you to consider the decades-long damage that felling just one tree (let alone over 277 trees) will cause by inputting the DBH of any tree identified for removal into our Tree CO2 Calculator.

As you will see, when an equivalent tree is replaced on a one-for-one basis, the lost CO2e is never recovered. Even when the largest tree (with a DBH of 100 cm) is replaced with eight trees in accordance with BTRS, it will still take some 40 years to recover the 10.4 tonnes of lost CO2e. And this is just one of the eco-services that trees provide us!

Impact on wildlife from tree loss

We endorse the following passages from the Bonnington Walk Breeding Bird Survey Report which observes at 5.2 Habitat Loss:

The Proposed Development will include the loss of scrub, trees and buildings which provide habitat for breeding birds. The extent of habitat loss is likely to include all the scrub and trees in the centre of the Site with some edge habitat along the boundaries retained…The loss of this habitat will have an impact on any birds using it for foraging or breeding at the time. The Site is located within an urban landscape with limited natural habitats. Alternative habitats are not readily available adjacent to the Site, though alternative habitat is available in the wider landscape including Stoke Park Estate and connected habitats further east. Habitat loss on Site will have an impact at a Local level by reducing breeding bird habitat in the local area…

and at 6.2.1 Habitat Loss:

Where possible, habitat loss should be avoided, and natural habitats retained. Scrub and trees are of most value to breeding birds at this Site. When natural habitats are retained these should be protected during construction to prevent damage including root compaction and knocking off or damaging over hanging limbs.

This is just one example of the likely adverse impact on wildlife resulting from these tree removal plans. There is evidence of a diverse range of both flora and fauna that likewise will also be adversely affected by the loss of these trees.

The Bristol Tree ForumJuly 2020

You can find more detail about the application here:

20/02523/FB – Land on south side of Bonnington Walk, Bristol


[1] Owned by BCC under its asset number 8397.

[2] CAVAT uses six age ranges to set the discount factor.

[3] Bristol has a population of 459,300 and a land area (as opposed to the Administrative area which covers large parts of the River Avon and coastal margins) of 10,970 hectares. Using this gives a population per hectare of 41.9 (459,300/10,970) and so a CTI Index value of 150.

[4] Under BDM2 each tree’s Root Protection Area (RPA) is calculated at 12 times its stem diameter. RPA is roughly equivalent to a tree’s canopy.

[5] The choice is arbitrary chosen only for the sake of illustration. We are not advocating a Net Gain of 10%, though the concept of Net Gain implies an improvement on the base values.

Wales and West Utilities helps to protect Bristol’s precious trees

Wales and West Utilities has been congratulated by the Bristol Tree Forum, and thanked mightily for their understanding and practical approach to a possible future environmental catastrophe at one of their installations in Bristol.

Stoke Lodge Playing Field is a 26-acre site in Stoke Bishop in Bristol. In the north west corner of the field is a gas “kiosk” which houses gas pressure regulation equipment. It was built in 2009, replacing a smaller installation nearby which was not on the same land. It is the responsibility of Wales and West Utilities.

Many locally and nationally notable trees grow on the parkland, a number of them getting quite old now, and in need of some love and our protection.

The access to the compound is over the roots of some of these important trees, most of which are the subject of Tree Preservation Orders. Protected trees surround the compound, and their roots, which are very superficial (as is the way with tree roots) and in places even exposed, are at risk of damage from vehicles driving over them and parking on them.

The W&W gas kiosk

Tree Roots

Tree roots extend radially in every direction to a distance equal to at least the height of the tree (assuming no physical barriers) and grow predominantly near the soil surface.

Typically, 90% of all roots, and virtually all the large structural supporting roots, are in the upper 60cm of the soil.

Soil disturbance within the rooting area should be avoided, whenever and wherever possible as this can significantly adversely affect tree health and tree stability. 

Associated with roots are much finer, thread-like, mycorrhizae. Mycorrhizae are symbiotic fungi which grow on or in roots, an association which is mutually beneficial to both the tree and the fungus. They are extremely efficient at nutrient absorption, especially phosphorus, and many trees cannot survive without them.

Diagram of a typical tree root system:

Cars, lorries and vans are heavy. They leak oil and hydraulic fluids from braking systems and power steering pipework and pumps. They also leak windscreen washer fluids. These chemicals are toxic for trees.  In the root area of a tree soil compaction caused by vehicles and the deposit of toxic or impermeable materials should be avoided. The nearer to the trunk these things take place the greater is the damage done and the greater the loss of roots.

Local residents are very protective of the trees – this whole Parkland is hugely important for them, and they have taken its care to their hearts.

Vans and lorries from Wales and West attend the site, both for routine maintenance visits and for any “emergencies”.

Recently one of the residents noted a Wales and West van parked on the exposed roots of one of the trees, so they contacted the Company to point out the dangers for the tree’s future that could be caused by this.

The response from a manager at the Company was immediate and most gratifying. Within hours a site meeting had been arranged with the W&W Manager, the Resident and the BTF BS9 Tree Champion in attendance – all suitably socially-distanced!

The Manager listened to everything we said. He told us that Wales and West had not previously been aware of the importance of these trees, nor aware of the peculiarities of the siting of the compound in relation to the trees.

He went on to say that he would do everything he could to inform future Wales and West employees visiting the site of the sensitive nature of the ground they would have to drive over, and that they would keep traffic passing over the root areas to a minimum, allowing only one vehicle to park on site at a time, parking any others required nearby on the highway. The one vehicle needed would not park under the canopies of the trees. It is possible for one vehicle to park clear of tree canopy areas.

He arranged for good quality signs to be affixed to their entrance gate and to the fence enclosing the kiosk, so that Wales and West operatives would be aware of the need to avoid damage to tree roots at this particular site.

This is the sign W&W attached.

The Bristol Tree Forum has been working hard in recent years, with local residents and their representatives, to encourage Bristol City Council, as owners of the land and as Landlord, to ensure that Tree Preservation Order regulations are complied with by their tenant using the Playing Field, and if necessary enforced. We have had some limited success. This made the attitude and actions of Wales and West Utilities all the more overwhelming.

So, we would like to thank W&W’s manager again for his actions on behalf of the trees, and to compliment Wales and West Utilities for supporting an ethos which encourages community engagement and action like this.

Postscript

The sign (see above) riveted to the main entrance gate onto the site and to the gas kiosk has been removed. It looks like the rivets have been drilled out, rather than the sign being removed by snapping it off, so it must have taken some effort and maybe even some planning to do this. The sign appears to have been taken away.

Who could possible think that doing this ‘vandalism’ could be for anyone’s benefit? It cannot have been Bristol City Council and it is hard to imagine who else would do such a thing. We are investigating.

Post Postscript

Wales & West have now told us that they removed the sign, saying “We put them in the wrong place. Now moved to the right place as agreed with the leaseholders of the land.” What leaseholders? As far as we know this bit of land is not leased. It belongs to Bristol City Council.

Finally

This is the notice that was on the gate:

This is the sign W&W attached.

It is all about protecting precious trees and safeguarding our environment.

There is no single Leaseholder with control of this gate. It is owned by Bristol City Council and the use of the gate and the access to the Field it grants is shared between Cotham School and Wales and West Utilities (W&W), two Leaseholders who use separate parts of the land beyond the gate. We do not know who asked W&W to remove the notice from the gate, but we are bound to ask what reasonable person, with any regard at all for trees, the environment and climate change, would ask W&W to remove it from a shared gate that they may not control, and why would they? Own up please?!

Bristol City Development – Where did all the Green go?

The Climate and Ecological Emergency

In 2018, with much fanfare, Bristol City Council (BCC) declared a Climate Emergency, the first UK city to do so, preceding the UK government by over a year. This has been followed up by the declaration of an Ecological Emergency, and a raft of sustainability aspirations detailed in the Bristol One City plan including doubling the tree canopy by 2046, doubling wildlife abundance by 2050, and City-wide carbon neutrality by 2030.

So why is it that so much of our informal green spaces are still being lost, and so many of our trees continue to be felled?

Is the BCC Development Office blocking Climate and Environmental Action?

A clue to this came out of a recent planning application to build a 4-storey block of flats in St Paul’s, in a street with one of the highest illegal levels of pollution in Bristol, above recommended noise levels, in a known high flood risk area and on land thought to be contaminated.  It was shown that the planned development would increase pollution and noise levels. Furthermore, in an area with one of the lowest tree density in Bristol, five mature maple trees were to be felled, removing the last mitigation for noise, pollution and flooding in the street. The trees are on the very edge of the development site and could therefore have been retained, readily complying with BCS9 which states “Individual green assets should be retained wherever possible and integrated into new development”.

Bristol’s Planning policies are contained in two main documents:

These are supplemented by the Planning Obligations Supplementary Planning Document. All were variously adopted and implemented by the Council between 2011 and 2014.

Despite contravening core strategy planning policies on green infrastructure (BCS9, DM15), pollution (BCS23, DM33), climate change (BCS13), flood risk (BCS16), noise (BCS23, DM35) and health (DM14), the Development Office did everything in its power to promote and advocate this development.

The reasons for this became clearer when officers were asked during the Planning process specifically why they supported a development which breached so many core policies aimed at protecting the health of citizens, the environment and the City’s crucial green infrastructure.

The Head of Development Management responded, “With regard to this application, the policy aims of the Core Strategy could be seen as the delivery of housing (BCS5), including affordable housing (BCS17)”. Further, “Loss of green infrastructure will only be acceptable where it is…… necessary, on balance, to achieve the policy aims of the Core Strategy”.

The statement effectively says that, whilst the need for new and affordable houses remains, BCS5 and BCS17 can override other policies including those mentioned above. Thus, green infrastructure that could have been retained is ignored, pollution and noise levels above legal limits are permitted, and the worsening health of residents would be tolerated. This position seems to be contrary to that previously held, with development under BCS5 and BCS17 needing to be also in compliance with the other core policies. As there will always be a need for new homes and affordable homes, the concern is that all other policies can be set aside indefinitely.

We would suggest that BCC Development Office interpretation is in contravention of the National Planning Policy Framework (NPPF) which states that: “the purpose of the planning system is to contribute to the achievement of sustainable development (remember that phrase), including “an environmental objective” – to contribute to protecting and enhancing our natural environment, including helping to improve biodiversity, mitigating and adapting to climate change and moving to a low carbon economy”.

So how has the BCC Development Office responded to BCC’s Climate and Ecological declarations?

The Development Office was also asked how implementation of planning policies had been influenced by the Climate and Ecological Emergencies. Their response was:

“Whilst Climate and Ecological Emergencies have been declared by the Council, the Bristol Local Plan has not been fully reviewed in the light of these and the policies referred to remain unchanged. Changes to Local Plan policies would have to balance the objectives of the respective declarations with the requirement to deliver sustainable development for the city”.   

By “balance”, it seems they may effectively mean “ignore”. Clearly their definition of sustainable development is somewhat different to that defined in the NPPF, with no intrinsic “environmental objective”, and, as one Councillor on the Committee remarked, the development will “lead to poorer people having shorter lifespans”. Unpacking their response still further, the implication is that there are currently no core policies in place to implement the Climate and Ecological emergencies. As described above, this is not true. Were BCS9, DM15, BCS23, DM33, BCS13, BCS16, DM35 and DM14 to be applied as intended in the NPPF, there would be sufficient policy support at least for the principles of the two emergency declarations.

Is this being led by bureaucratic or political decision making?

It is not clear why the Development Office has taken this position, but there are two possibilities that should be of concern:

  • The Development Office is acting contrary to the aspiration of the City’s political leaders.
  • Senior Council politicians who have made much political capital from the highly praised environmental declarations, have at the same time permitted, or perhaps even encouraged, Council Officers to disregard existing planning policies that would otherwise enable implementation of these declarations.

Thus, selective policy compliance allows development of second-rate housing in a race for quantity over quality.

It seems that Bristol City Council are choosing to emphasise some core strategic policies aimed at hastening house building, whilst demoting other core strategic policies aimed at protecting public health, green infrastructure, air quality and the environment. This is a recipe for slum development, and we deserve to know whether these decisions are being taken at a political or bureaucratic level.

Professor John Tarlton.

A letter to our Councillors

Dear Bristol City Councillors,

We recognise the fundamental importance of the natural environment, the value that nature has in an urban setting and the global threat posed by the ongoing climate catastrophe. We also recognise that trees are a crucial component in all these concerns.

We are supportive of Bristol City Council’s declaration of a Climate Emergency and an Ecological Emergency and the goals detailed in the One City Climate Strategy, including the commitment to carbon neutrality by 2030 and doubling the abundance of wildlife by 2050. We are also supportive of their commitment to doubling the tree canopy by 2046.

However, we have a real concern that the commendable words are not being matched by effective actions.

A principle aim of the BTF is to promote the planting and preservation of trees in Bristol for the well-being of its citizens, the sustainability of urban habitation, the enhancement of nature in the cityscape and as our contribution to combating climate change (see A Manifesto for protecting Bristol’s existing Urban Forest).

A recurrent concern we have is the continued loss of trees as a result of environmentally insensitive developments that are not sympathetic to the City’s declared commitments outlined above. On the other hand, the BTF supports developments that favour a sustainable environment over high density occupancy, and those that prioritise retention of existing trees.

Bristol’s policy on replacing trees lost to development – adhering to the Bristol Tree Replacement Standard (BTRS) – is widely well regarded. As such, decision makers believe that tree loss is mitigated by subsequent tree replacement. However, recent studies undertaken by the BTF have shown that this is not the case over the timescales committed to by Bristol City Council and the Green Party.

Typically, tree planting undertaken under the BTRS takes between 30 and 50 years to recover the biomass (and therefore the CO2e) lost by felling, well beyond the 10-year commitment on carbon neutrality, and even beyond dates set for doubling the tree canopy or doubling wildlife abundance.

The BTF study has been developed into a versatile online tool for calculating the extent and timescale of the carbon deficit, with a wide range of inputs. This can be accessed via the link Tree Carbon Calculator, and we encourage you to try this yourself. See also the BTF blog Tree replacement and carbon neutrality.

In the example shown here, a mature tree felled in 2020 is replaced by four trees (as per BTRS) of the same species. The carbon released (2 tonnes CO2e) is not recovered until 2064, a full 34 years beyond the date Bristol aims to be carbon neutral.

This model can also be used to determine how many replacement trees are needed to recover lost carbon within a particular timescale. In the example shown, to be carbon neutral by 2030, a reasonable expectation as this is the declared aim of BCC, the felled tree would need to be replaced by 37 plantings of the same species. Scaled up to, for instance, 500 trees, new plantings would need to number 18,500 to mitigate the lost carbon.

This new information represents a fundamental change in the evidence base for tree replacements, and emphasises the need to retain existing mature trees, and not to consider replacement by new plantings as adequate mitigation.

We request that you consider this new information with urgency and make a commitment to oppose developments where mature trees are removed and tree replacements do not deliver carbon neutrality by 2030.

Tree replacement and carbon neutrality

The UK aims to be carbon neutral by 2050. Bristol is more ambitious and aims to reach that goal in 2030. Both are massive challenges in which trees have been enrolled to play their part in mitigating the carbon dioxide (CO2) created by human activity.

Background

There are plans for extensive tree-planting.  The government pledged to plant 30 million trees a year, nationally. This a huge challenge partly because seedlings and land has to be found for these trees. However even when planted, these trees will take a long time to grow and extract CO2 from the air.  We in Bristol Tree Forum are concerned that not enough attention is given to the role of existing mature trees.  

Trees grow and add to their mass each year. Most of this mass is in the form of cellulose and lignin and about 50% of those organic compounds is carbon, obtained through photosynthesis using the energy of sunlight and CO2 from the atmosphere.  The rate at which mass is accumulated increases with age so whilst a 10 year old tree might put on a few kilograms a year, a 50 year old tree might add 50 kg.  So the older the tree the better for CO2 fixation. However mature trees are constantly under threat – from development for housing and industry, from home owners overshaded by large trees, from councils assessing maintenance costs and risks.

Here in Bristol, the Bristol Tree Replacement Standard (BTRS) is part of local planning regulations and specifies how many replacement trees are needed to be paid for by the developer and planted to mitigate the loss of mature trees. The BTRS is a very welcome and forward-thinking strategy, but is it enough to support the Carbon Neutrality goals? Should BTRS  apply also to council-owned and indeed privately owned trees for which no funded replacements are available?

The Bottom Line

In an attempt to understand how this standard works in practice, we have developed an on-line calculator to explore different scenarios.

Tree CO2 Calculator

The general conclusion from this analysis is stark:  it will take 25 to 40 years before the replacement trees are able to compensate for the loss of the mature tree.

The graph shows the scenario of the replacement of a mature tree such as a Maple with a diameter of 60 cm by the 6 trees as determined by BTRS which are faster growing but shorter lived such as Rowens.

Assuming that the original tree is felled, chipped and used as fuel in a biomass boiler (the practice in Bristol), the carbon stored in the mature tree is returned to the atmosphere within months of felling.  The replacement trees start to grow, but absorb much less carbon than the original mature tree would have done, so they take many years to catch up. In the case shown in the graph, it takes 35 years (ie, to 2055) before the new trees mitigate the loss of the original tree.

Modelling

A model of this scenario needs to take into account:

  • the rate at which different species of tree grow at different ages in different conditions.
  • the estimated mortality of the tree over time.
  • the calculation of a tree’s biomass from its girth for different species.
  • the relationship between the tree’s biomass and the amount of carbon stored.

There is a lot of uncertainly in these relationships, partly because of the paucity of data on urban, as opposed to forest, trees. Urban trees are under threat not only from natural processes and disease, but also from the vagaries of vehicles and humans. Planting sites are often less than optimal and urban trees have no support from the ‘wood wide web’.

The interactive calculator allows the user to vary the parameters of the model using the sliders. This allows the sensitivity of the overall outcome to variation in values to be tested. Different policy choices can also be explored and can be used in a predictive sense to determine the number of replacements needed to achieve a given carbon neutral date.

Summary

Documentation on the website explains the thinking behind the model in more detail, and the sources of data used. The model is still under development, in particular to make it easier to select conditions for different species and situations, and to improve the quality of the model itself. The research literature is extensive but often of limited applicability to urban conditions.

We would be grateful to receive additional or better sources of this information, and indeed any comments on the model itself at co2@bristoltrees.space.

Chris Wallace, Bristol Tree Forum