Category: Urban Trees

Small sites, big losses: The BNG exemption is the strongest argument yet for a national tree replacement standard.

Since 6 August this year, the smallest developments in England (those 0.2 hectares or smaller) no longer have to deliver biodiversity net gain (BNG). To understand what this means for trees, we’ve analysed 505 biodiversity gain plans and 320 readable metric calculations gathered from 59 planning authorities.

The answer is uncomfortable: the exemption removes roughly half of all development sites from the BNG regime, and these are precisely the sites on which trees make up the largest share of the habitat being lost.

In December 2023 we argued for a new Bristol Tree Replacement Standard (BTRS) calibrated to the statutory biodiversity metric. That argument was about making the local standard consistent with national policy. It’s now about something much more critical: on half of all sites, a local standard is the only thing left to protect trees being lost to development.

What changed on 6 August 2026

The Biodiversity Gain (Town and Country Planning) (Amendments and Transitional Provisions) (England) Regulations 2026 (SI 2026/790) took effect for planning applications made on or after 6 August 2026. The three changes made matter here:

  1. A site of 0.2 hectares or less, measured by the red-line boundary of the application, is exempt from mandatory BNG unless on-site priority habitat is negatively affected.
  2. Development that’s wholly temporary, with permission granted for five years or less, is also exempt on the same proviso.
  3. For minor development above the area threshold, the mitigation hierarchy has been relaxed: off-site gains are now seen as just as desirable as created or enhanced habitat on the site itself, so a developer may go straight off site without first showing that on-site provision is unachievable.

The self-build and custom-build exemption has also been removed, on the reasoning that most such schemes now fall inside the new small-sites exemption anyway. While applications already submitted before 6 August continue under the old rules, Defra in its 15 April 2026 response to the consultation, estimated that ‘the new area based exemption for sites of less than 0.2 hectares is expected to remove more than 50% of the smallest planning applications which are currently delivering mandatory BNG, from the BNG requirement’.

Trees aren’t priority habitat

The priority habitat proviso sounds protective, and for some sites it is, but it does nothing for trees. Depending on where it is, an individual tree is recorded in the statutory metric as urban tree or rural tree, both of medium distinctiveness; neither is a habitat of principal importance under section 41 of the Natural Environment and Rural Communities Act 2006. Only ancient and veteran trees sit outside these two categories, as irreplaceable habitat, and they are a vanishingly small proportion of the trees felled for development.

So a mature street-side lime, a garden oak, or a row of maturing London planes on the edge of a school playing field — none of these will be protected if they’re in the way of a 0.2 hectare development. Below the threshold, they can be removed without any obligation to count them, replace them, or compensate for their loss anywhere. Unless the tree is protected by a Tree Preservation Order, or unless the local plan says otherwise, nothing now requires their loss to be made good.

This is the gap; and it’s much larger than the word ‘small’ implies.

Half of all development sites now fall outside the system

We measured site area for every gain plan in our dataset that came with a usable metric calculation — 305 sites across 59 authorities. Site area here is the baseline area-habitat total, net of any individual trees, taken from the statutory metric or, for small-sites metrics.

Table 1. Development sites by size, 305 sites with a usable metric calculation
Site size Sites Share Baseline area HUs Post-development area HUs On-site change
0.2 ha or smaller 151 49.5% 48.79 48.67 −0.25%
Over 0.2 ha 154 50.5% 506.88 512.82 +1.17%
All sites 305 100% 555.67 561.49 +1.05%

Habitat units are on-site figures from sections 6.3 and 6.4 of the biodiversity gain plan. Hedgerows and watercourses are excluded as linear features.

Just under half of the sites that have been going through the BNG process fall at or below the new exemption threshold — consistent with Defra’s own estimate. Two further things stand out in that table. First, small sites were barely holding their own on site even under the old regime that’s just been withdrawn: a baseline of 48.79 area habitat units became a mere 48.67 after development, a small net loss, with any gain bought off site from elsewhere. Second, the small-site total is dwarfed by the large-site total, which is exactly why the exemption looked politically cheap. In habitat unit terms these sites are a rounding error. In tree terms, though, they aren’t.

Small sites are where the trees are

Of the 320 metric calculations we could read, 163 relate to sites of 0.2 hectares or less. Sixty of these — 36.8% — record individual trees in the on-site baseline. Between them they carry 3.12 hectares of tree canopy ‘oversailing’ 15.22 hectares of land.

Table 2. Individual trees in the on-site baseline, by site size
Site size Metrics With trees Site area (ha) Canopy (ha) Canopy as % of site area Tree baseline HUs
0.2 ha or less 163 60 (36.8%) 15.22 3.12 20.5% 26.71
Over 0.2 ha 157 81 (51.6%) 219.64 13.79 6.3% 133.22
All sites 320 141 (44.1%) 234.85 16.91 7.2% 159.94

Site area is net of individual trees, because tree canopy oversails the habitat beneath it and is not additional land. Canopy can therefore exceed 100% of the site area it overlies.

That last comparison is the heart of it. On sites above the threshold, tree canopy is equivalent to about 6% of the site area. On sites at or below 0.2 hectares it’s 20.5% — more than three times the share. Small sites are disproportionately tree rich, probably because they’re disproportionately urban: they include infill plots, back gardens, corners of school and hospital grounds, and the fringes of existing built-up land. These are the places where we usually find the mature tree canopy of a town.

The concentration is sharper still for twenty of the 163 small-site metrics, which record canopy covering more than half the site area. Seven record canopy exceeding the whole of it. One example is a 0.18 hectare site at 22 York Road in Sutton, which carries 0.33 hectares of canopy across 18 recorded tree entries — canopy equal to 182% of the site, and worth 3.42 habitat units. At an average of 0.019 hectares of canopy each, these are mature trees, not saplings — on the replacement standard set out below, losing 18 medium trees would require 90 replacements. On an application made today, that site would be exempt and the trees could be removed without any obligation to replace them under the new BNG regime.

Expressed as trees rather than hectares, the 3.12 hectares of canopy on small sites is equivalent to 762 small trees, using the 0.0041 hectares the June 2026 metric assigns to a small tree. Because much of that canopy belongs to medium, large and very large trees, the actual number of trees is likely to be lower — and their replacement cost, in the metric’s own currency, correspondingly higher.

And they’re where the new trees were coming from

The exemption removes an obligation to plant as well as an obligation to compensate. Of the 163 small-site metrics, 72 record individual trees created as part of the post-development scheme.

Table 3. Individual trees created in post-development proposals
Site size Metrics With tree creation On-site trees Off-site trees Total trees
0.2 ha or smaller 163 72 (44.2%) 597 8 605
Over 0.2 ha 157 97 (61.8%) 3,135 205 3,340
All sites 320 169 (52.8%) 3,731 213 3,944

Tree numbers are the recorded habitat creation area divided by 0.0041 ha per small tree, rounded up. Off-site units bought from a registered gain site do not appear here, because they sit in the seller’s metric.

Over 600 trees across a dataset of a few hundred plans covering barely two years and 59 authorities out of 306. If you scale that across every planning authority in England and across the years ahead, the tree planting foregone is substantial.

BNG was never going to save urban trees anyway

It’s worth being clear that the case for a local tree standard doesn’t rest solely on the exemption. Even where BNG still applies, it often doesn’t keep compensation anywhere near the trees that were lost.

The Biodiversity Gain Sites register as of 19 August 2026 — 2,328 developments across 306 authorities — shows that 87% of developments in urban authorities offset outside their own authority area. Those authorities needed 1,835 area habitat units and hosted 889. In London the pattern is near total: 98% of the 113 registered London borough developments offset elsewhere, requiring 46.8 units and hosting 3.3.

The units are real, and the receiving sites may well be excellent. But a habitat unit delivered in a rural county doesn’t provide shade in Bristol, intercept its rainfall, cool it in a heatwave, or stand outside anybody’s window. Urban tree canopy is a local public good, yet BNG systematically moves its replacement to some distant field that few, if any, will ever visit.

Why we need a tree replacement standard

A tree replacement standard is a local plan policy, not a national biodiversity calculation. It attaches to the tree, not to the site area; it applies whether or not a metric has to be completed; and it keeps the replacement in or close to the place where the loss occurred. It’s the mechanism best suited to fill the gap the exemption has opened.

This is not a novel invention. Bristol has operated the BTRS since 2014 under Core Strategy policy BCS9, and a replacement table is carried forward into proposed local plan policy BG4. What we argued in 2023 was that the ratios should be recalibrated so that the standard delivers what the statutory metric would have delivered, plus the 10% uplift. Our proposal was not adopted then, though our argument stands. What’s changed is who needs it: not just Bristol, but every planning authority that wants any control at all over tree loss on the majority of its planning applications.

We would encourage authorities without such a policy to adopt one, and those with one to check that it’s calibrated to the metric rather than to a historic rule of thumb whose rationale was and remains hard to understand. As the standard is a local plan policy, it can be applied to exempt sites without cutting across the national BNG regime: BNG sets a floor for biodiversity, and a replacement standard sets a separate, local requirement for canopy.

Recalculating the standard against the June 2026 metric

Our 2023 proposal derived replacement numbers from the tree area equivalents then in force. Those figures have since changed. Table 15 of the Statutory Biodiversity Metric User Guide (June 2026), at page 62, now assigns the areas set out below. Applying the same method — the area of the lost tree divided by the area of one small tree, since a newly planted tree is recorded as small, plus the 10% net gain, rounded up because a fraction of a tree is not a tree — gives revised numbers.

Table 4. Replacement trees required, recalculated against the June 2026 metric
Tree size (DBH) Area equivalent (ha) Multiple of a small tree Plus 10% net gain Replacement trees 2023 proposal
Small (>7.5–30 cm) 0.0041 1.00 1.10 2 1
Medium (>30–60 cm) 0.0163 3.98 4.37 5 3
Large (>60–90 cm) 0.0366 8.93 9.82 10 10
Very large (>90 cm) 0.0765 18.66 20.52 21 20

Areas are from Table 15 of the Statutory Biodiversity Metric User Guide (June 2026), p62. Replacement numbers are the tree’s area divided by the area of one small tree, plus the 10% net gain, rounded up.

Two categories of tree size move. A medium tree is now worth almost four small trees rather than three, so its replacement requirement rises from three to five. A very large tree rises from 20 to 21. Large trees are unchanged at ten.

The small-tree row needs a decision rather than an arithmetic answer. One small tree replaced by 1.1 small trees rounds up to two, which is a real doubling for the commonest category in the surveyed population. The alternative is to hold the small category at one-for-one and take the 10% uplift from the scheme as a whole. We think the honest position is to state the arithmetic and let the policy make the choice explicitly, rather than round the uplift away in silence.

What we’re asking for

  • That local planning authorities adopt, or retain and strengthen, a tree replacement standard in their local plan, applying to all development including sites exempt from mandatory BNG.
  • That the replacement ratios be calibrated to the current tree area equivalents in the statutory metric, so that a local standard delivers no less than the national regime would have done.
  • That replacement be required within the same authority area and as close to the loss site as is practicable, so that canopy is replaced where people live.
  • That authorities continue to require a BS 5837:2012 tree survey with applications on exempt sites, so that the loss is properly recorded even where no metric is required.
  • That Defra keep the 0.2 hectare exemption under review, and specifically consider whether the priority habitat proviso should be extended to sites carrying significant tree canopy.

The exemption was introduced to reduce costs and delays for small applications. Whatever one makes of that aim, it shouldn’t be paid for with the mature canopy of our towns and cities. A tree replacement standard is a cheap, well-understood, locally enforceable way of making sure it’s not.

About the data

Figures are drawn from the Bristol Tree Forum biodiversity gain plan dataset (v11): 505 gain plans logged from 59 planning authorities, of which 320 carry a readable statutory or small-sites metric calculation and 305 a usable site area. Plans are dated 2024 (58), 2025 (254) and 2026 (165). Register figures are from the Biodiversity Gain Sites register extract of 19 August 2026, covering 2,328 developments and 2,686 allocations across 306 authorities.

Two caveats:

  1. Site area in this analysis is the metric’s baseline habitat area net of individual trees, which is a close proxy for the red-line boundary the exemption uses but not identical to it; a handful of sites near the threshold may fall on the other side of it in practice.
  2. The sample is of applications that produced a gain plan, so it describes the population the exemption removes rather than all development across the country.

Take a look at our recent follow-up which looks at the immediate impact of this new exemption for Bristol :

Looking at Bristol’s development sites

The Local Nature Recovery Strategy fails to deliver for Bristol

WEMCA’s Local Nature Recovery Strategy (LNRS) will fail to provide Bristol with the benefits promised for nature. While the new Biodiversity Net Gain (BNG) rules require most development in the city to increase biodiversity by at least 10%, unfortunately the LNRS will not apply to most potential development sites.

The West of England Mayoral Combined Authority (WECA as was) Local Nature Recovery Strategy was published to much fanfare last November. Defra’s blog, Kickstarting local nature recovery: a new strategy for the West of England, hailed it as the first in the country.

The LNRS is a locally led and evidence-based strategy which aims to target action and nature investment where it’s most needed. We’re told that the strategy will also focus on biodiversity net gain by increasing the strategic significance of specific habitats. However, it is hard to imagine how the LNRS will help to enhance biodiversity net gain in most, if not all, potential development sites in the city.

We might have been better off, at least as far as the application of biodiversity net gain to new development is concerned, by asking the LPA to specify alternative documents (such as those listed at the end of this article) for assigning strategic significance instead.

The issue

When calculating the impact of a proposed development on biodiversity, one factor taken into account is the strategic significance of any habitat found on a focus area for nature recovery site (coloured purple in the map above). If strategically significant habitats are created or enhanced, then their strategic significance is set to High in the Statutory Metric calculator tool and a 15% uplift to the calculation of its value is applied. Subject to which of the six LNRS areas is being considered, these are the strategically significant habitats in the city:

  • Ditches
  • Ecologically valuable lines of trees
  • Ecologically valuable lines of trees – associated with bank or ditch
  • Grassland – Floodplain wetland mosaic and CFGM
  • Grassland – Lowland calcareous grassland
  • Grassland – Lowland meadows
  • Heathland and shrub – Mixed scrub
  • Heathland and shrub – Willow scrub
  • Individual urban or rural trees
  • Lakes – Ponds (priority habitat)
  • Priority habitat (on the River Avon and the Riparian buffers)
  • Species-rich native hedgerow with trees – associated with bank or ditch
  • Species-rich native hedgerow with trees
  • Species-rich native hedgerows – associated with bank or ditch
  • Species-rich native hedgerows
  • Urban – Open mosaic habitats on previously developed land
  • Urban – Biodiverse green roofs
  • Woodland and forest – Lowland beech and yew woodland
  • Woodland and forest – Lowland mixed deciduous woodland
  • Woodland and forest – Other woodland; broadleaved
  • Woodland and forest – Wood-pasture and parkland

However, a detailed examination of the LNRS map reveals that not all parks and green spaces have been designated as focus area for nature recovery sites. It’s only those which are in one or both of the following:

  • a location where they can make a greater contribution to ecological networks
  • deprived areas with a lack of access to nature.

These designations were based on Bristol’s previous work on ecological networks within the city and where wildlife-friendly interventions are most likely to be feasible. This means that the existence, creation or enhancement of these special habitats outside these areas will not attract the 15% strategic significance uplift.

The BNG requirements

The now compulsory Statutory Metric Guide, used for calculating Biodiversity Net Gain (BNG), advises (at page 27) that: ‘Strategic significance is the local significance of the habitat based on its location and habitat type. You should assess each individual habitat parcel, both at baseline and at post-intervention, for on-site and off-site.’

If the LPA has adopted an LNRS then only the High or Low strategic significance multipliers can be used (High – formally identified in local strategy = 1.15. Low – area compensation not in local strategy = 1). If it has not adopted an LNRS, then the Medium strategic significance multiplier may also be used (Location ecologically desirable but not in local strategy = 1.10).

Where an LPA has adopted an LNRS, all those sites which have not been identified as a focus area for nature recovery site will be designated as having Low strategic significance and so attract no uplift, even if they’ve been identified as important habitats in the Local Plan or in another strategic document adopted by the Council. These documents (used where an LPA has not adopted an LNRS) can include:

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

Baseline habitats cannot be uplifted

Despite the BNG strategic significance guidance, Defra has stated that LNRS designations only apply to the creation or enhancement of post-development biodiversity mitigation habitats. They don’t apply if these habitats – called the baseline habitats – are found on the site before development begins.

This means that the 15% strategic significance uplift can only be applied where offsite biodiversity mitigation is being delivered in a focus area for nature recovery site. If these habitats are being delivered elsewhere, the uplift may not be applied.

However, even if the baseline habitats were included, it is unlikely to make any difference This is because the focus area for nature recovery sites identified in Bristol are, for the most part, located in public parks or green spaces, on river banks, in riparian buffers or on railway margins, none of which are likely ever to be developed or, in many cases, used to offset habitat lost to development elsewhere.

So far, no announcement has been made as to whether any of Bristol’s focus area for nature recovery sites will be made available for offsite habitat mitigation and the proposed new Local Plan does not commit to using these sites for this purpose.

This, combined with the challenge of finding LNRS suitable for offsite habitat mitigation, registering them as biodiversity gain sites and then managing them, effectively, in perpetuity, suggests that few feasible LNRS sites will be found, especially as many sites are also in demand for public access for recreation.

We set out the process used to assess the strategic significance of habitats on our blog, Assessing habitat parcels: strategic significance explained.

Our City book launch

On Thursday 26 September 2024 Waterstones at The Galleries launched Our City. This book chronicles community activism in Bristol through multiple campaigns described in eighteen individually authored chapters.

Suzanne Audrey pulled the whole project together, edited the book and contributed the introduction and conclusion. The book is well worth buying. It tells a story of the diversity and resilience of Bristol communities when faced with threats usually emanating from the council which should be looking after them. The Bristol Tree Forum was asked to contribute. During the launch event, contributors were each given three minutes to describe their work. This is what we said:

Saving Bristol’s Urban Trees by Vassili Papastavrou

In Bristol we are seeing the loss of hundreds of important urban trees each year. They cannot be replaced and it will create a city that is unliveable – we need them to keep us cool now.

I’d like to illustrate the problem with two willow trees which grew not far from here. Two years ago, during National Tree Week, they were chainsawed by Bristol City Council. Highly visible and next to Temple Meads, the trees should have had Tree Preservation Orders but the council does not TPO its own trees, claiming that it is a responsible landlord and that its trees are safe.

I was brought up in Cambridge. Along the banks of the river Cam, willows are such a feature of the city that no one in their right mind would consider chopping them down. But in Bristol they were removed without a moment’s thought.

The removal was filmed by Martin Booth and got a lot of coverage but in reality it was just another ordinary day of tree removal in Bristol. Martin tweeted, “This is so sad to watch. One of Bristol’s most beautiful trees, a majestic weeping willow is being chopped down this morning”.

Trees compete for space with other uses of the city. In Bristol mature urban trees are given lowest priority, so when plans come up, trees are removed. As this incident demonstrates, they have zero value to the Local Planning Authority or the council.

The willows never went in front of the Planning Committee where there could have been a debate as this was a “delegated decision”. Effectively Bristol City Council was giving planning permission to itself in a backroom deal. The planning application was snuck through with no mention of the trees in the title, no arboricultural report from the council, no tree officer report from the local planning authority. The council passed judgement on itself.

And after the deed had been done, was there any recognition that a terrible act had been committed? An inquiry to make sure that this kind of thing would never happen again?

Silence. For in Bristol there is no one in a position of power who cares about retaining important urban trees.

We can only hope that that will change.

Why we must protect the trees we love

Paul Sander-Jackson, who helped to plant the Baltic Wharf trees, returns to Bristol shocked to learn that they will now be felled.

We came back to live in Bristol nearly two years ago. After living in Somerset for 32 years, and in Bristol 12 years before. Much has changed in the city, but much has remained – the best bits, and especially the trees.

Our new home backs on to Oldbury Court – which has an astonishing range of trees and plants, as well as a river and rocks and old quarries. Our garden has a lime tree – probably about 80 to 90 years, just outside its boundary fence.

In Somerset, we became very involved, in the later stages of life there, in planting new trees and woodlands, hedges and copses. The organisation we were involved with – Reimagining the Levels – has been responsible for planting over 100,000 trees in the past few years. But that is addressing a major problem in Somerset – a tree cover of only between 6% and 7%.

Coming back to Bristol, I wondered where my energy should go. But a few months ago, an old friend mentioned that I might like to get involved in Bristol Tree Forum. The rest is (personal) history. My partner and I are now joint Tree Champions for the Frome Vale ward.

It was a great shock when I was made aware of the application then pending on the Baltic Wharf Caravan Club site, particularly because when I used to do landscaping and gardening for a livelihood in the late eighties and early nineties, I had helped plant trees on that site, and carefully undertaken structural pruning and maintenance. And now a planning application was threatening to fell 80 of them.

“The Baltic Wharf trees were at least 35 years old, providing numerous ecological ‘services’ to the area”

As I made further enquiries, it turned out that there were over 590 objections to the application, with six statements in support. Six!! The trees represented over a quarter of the tree cover in the immediate area. They were at least 35 years old, providing numerous ecological “services” to the communities of Bristol.

And then the terrible shock of a unanimous – yes unanimous – planning committee decision to go ahead without amendments to the felling of the trees, with no objection from the responsible tree officer.

This left me realising many things. How naïve I have been about the problems nature, and especially trees, face in this lovely city. How we need to struggle and advocate for every one. Trees offer so many positive benefits for all creatures, including humans. I reflect on the wonderful life in ‘our’ lime tree.  Pigeons, starlings, sparrows, crows, squirrels. Life in all its varied forms. Leaves, blossoms, growth.

And perhaps most of all, I recognise that in Bristol, in 2024, there is perhaps an equal if not greater task to planting new trees. To protect the ones we have, and love. And to share that love with as many people, in our local communities and beyond, that we can.

“Maybe we can teach even more people how important it is to retain trees and how we should never allow a good tree in a good place to be arbitrarily felled”

My partner and I began our life in Bristol in St Werburghs, starting the City Farm there in 1980. As it got going, with great help from some wonderful City Council officers in the planning and parks departments, we planted trees with children from six local primary schools. Those children, I hope, will always carry the memory of that planting. And I hope they go and visit the trees – now over 40 years old and flourishing in the wondrous environment of the City Farm.

It is about a will, an attitude, a set of values. Above all, it is about respect for our fellow plants and creatures, who share this planet and this city with us.

Interested in becoming a Tree Champion like Paul? Click below to learn more.

Find out about tree champions

Our response to the proposed changes to the allotment agreement

We urge the Bristol City Council Cabinet to reject the current draft. More nuanced rules, that meet identified real needs, need now to be developed following a proper consultation with Bristol’s allotmenteers. The PGSS must be also integrated into the new Local Plan so that all the sites that have been identified are protected either as Local Green Space or as Reserved Open Green Space and given proper planning weight when decisions about their future use and development are made.

We are particularly concerned that the draft rules relating to trees on allotments are too narrowly drawn and will result in the loss of allotment tenants’ ability to make the best use of the particular conditions of their plot. We urge the relaxation of these proposed limits on the number and types of trees which can be planted.

Food growing as part of the draft Parks and Green Spaces Strategy

We are concerned that the Strategy for Food Growing and Allotments has been included as Chapter 6 of the draft Parks and Green Spaces Strategy (PGSS). This is a departure from the 2008 PGSS, which makes it clear that such spaces are not included, since they are not public spaces: ‘The Strategy does not consider green spaces that are not freely accessible to the public, including allotments, city farms, school grounds, or Sites of Nature Conservation Interest in private ownership.’

The proposed PGSS recognises that: ‘There are other green spaces managed by the council that are not included because they do not offer full public access for recreation …’ [our underlining]. Given that allotments and other land used for growing food (including grazing land) are subject to private tenancies, with rental payments and mutual rights and obligations agreed between tenant and Council, it is hard to understand why they have been included in the strategy proposal.

We need to recognise the importance of this specific type of land use and ensure that it is not made an ancillary element in the PGSS. It deserves greater attention, which is why we suggest that it should have been integrated into and protected by the current proposals for the new Local Plan (as, indeed, Parks and Green Spaces should also be) rather than treated as an add-on to the proposed PGSS.

In particular, the current status of and strategy for market gardens, smallholdings and grazing land (and other land not used for food growing), which have an area greater than all current allotments combined, get only passing consideration. Little or no thought is given to the opportunities these sites present for nature recovery, biodiversity enhancement and other environmental gains.

Instead, we are told that these sites:

… have limited benefit beyond the individual users … and many smallholdings tend to use less than 10% of their site for food growing … we now need to review our current landholding agreements so that we can start to explore opportunities to repurpose land to diversify our food growing spaces and ensure land is being used effectively to meet Bristol’s food growing targets. … Once suitable land has been identified we will review how these opportunities are promoted to ensure that the system of securing land is fair and equitable. We are particularly interested in opportunities that will not only ensure land is being sustainably and efficiently managed but that also delivers social value and benefits the residents of Bristol.

In the face of the demand for allotments and the need to increase local food production, which has led at least in part to the proposed changes in the tenancy agreement, these areas must surely warrant greater protection than is given them in the proposed PGSS. These areas also contain many trees (often veterans) and other ecologically sensitive habitats, and many are in SNCIs. It is vital that these matters are properly considered when planning any change of use of such land. This ought to be addressed properly in the proposed new Local Plan, not in this draft strategy.

Trees on allotments

Trees on allotments fall into two categories: those within allotment plots and those on allotment sites but outside the plots.

  • Trees outside allotment plots

From our analysis of data on the Council’s Open Data site, we estimate that allotment sites total some 105 hectares when community orchards are included. However, only about 87 hectares of these sites are productive. This means that around 17% of allotments, some 18 hectares, are unproductive. We welcome the clarification in the new tenancy agreement that trees in these unproductive areas remain in the care of the Council and may not be damaged by tenants.

  • Trees within allotment plots

Proposed allotment Rule 4.0 Trees deals with trees growing within allotment plots. We distinguish between newly planted trees and existing trees below.

  • New trees

Regulation 4.4 limits the height of trees to 2.5 metres (a common constraint in other councils). The limit in Regulation 4.3 of one tree per allotment sized between band B and band C is far too restrictive, particularly since the majority of plots fall within these bands.

Rule 3.1 requires that the tenant uses at least 75% of the plot for ‘a cultivated crop of vegetables, fruit bushes [not fruit trees], ornamental flowers, and herbs.’

Rule 3.3 states that fruit trees will occupy the non-cultivated area of the plot (which may be no more than 25% of the plot) unless they are underplanted with a productive crop, in which case they will be counted as being part of the cultivated area. No trees are permitted on band A or Micro Plots, nor is any consideration given to whether it is possible to underplant ‘a productive crop’. This implies that the food value of the tree does not warrant the space they occupy. According to the RHS, having fruit trees on an allotment adds both structure and permanence, while fruit tree crops have a higher value than many other allotment crops and are easier to manage.

The transitional arrangements require that all trees, except the permitted dwarf rootstock fruit trees, be removed from plots within three years, though mature trees may be permitted.

These rules devalue the role of trees on allotments, not only for their contribution to the ecology of the area but also their food production value. These rules also effectively exclude such fruit-tree-growing techniques as cordons, espaliers or step-over planting.

Regulation 4.3 also constrains the type of fruit trees that may be grown.  Since only rootstocks M27 and M26 are allowed (though not the intermediate M9 rootstock), it seems that that only apple trees may be grown, even though dwarfing rootstocks are available for a number of other fruit trees such as Pear, Quince (Quince C), Cherry (Gisela 5), Plum (including Gages and Damsons), Peach and Apricot (VVA-1, Pixy).

The exclusion of hazel and other non-fruit trees (most trees bear fruit) removes the ability of tenants to grow valuable nut crops or to provide locally grown stakes through coppicing – a good way to reduce our carbon footprint. This restriction also precludes the use of allotment space for growing trees from seed with a view to planting them out elsewhere, at a time when there is a great need for locally grown trees for local tree planting.

Established trees

  • Fruit trees

Many fruit and other trees are already growing on many allotment sites. They provide not only good-value food, but they also provide biodiversity, a rich ecological resource for insects, birds and mammals and are important legacy features of the site. The allotment and its users have adapted to the presence of these trees and their removal would not only be counterproductive to the value of the site and cause distress to tenants but would also risk damaging the soil and the allotment more broadly.

  • Non-fruit trees

Although such trees may not have obvious value for food production, other considerations apply. We agree that allotments should not be taken over by trees, resulting in the reversion of productive land to woodland and the attendant loss of statuary allotment protection for such land. However, it would be a mistake to create rules which put biodiversity, nature and ecology in conflict with food production; trees are crucial for biodiversity, which in turn is essential for the productivity of the places where we grow food.

Site-appropriate regulations

Allotment plot conditions both within allotment sites and across Bristol vary widely. The gradient and aspect of each plot, its soil type, soil depth and historical development all pose different challenges to tenants, requiring a localised approach to the best way to utilise a site. Imposing a one-size-fits-all set of regulations, based on some notional idealised allotment plot, will not provide sufficient scope to allow for these variations. Tenants’ associations also need to be able to vary the regulations to fit their local conditions. Indeed, arguments in favour of the distribution of local decision making, which we endorse, have recently been made by the Bristol Mayor.

Here is a copy of this response

Calculating tree habitat area

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

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

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

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

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

  • Woodland and forest
  • Individual trees

Defining woodland and forest habitat

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

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

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

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

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

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

Defining individual trees habitat

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

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

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

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

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

Other clarifications

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

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

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

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

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

Recording individual trees at baseline 

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

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

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

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

Recording baseline trees within private gardens

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

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

Recording baseline trees within hedgerows and lines of trees 

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

Recording baseline trees within orchards and wood-pasture and parkland 

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

Calculating the number of post-development trees required

Post-development size classes 

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

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

You should not: 

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

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

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

You should not: 

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

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

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

How these rules affect the tree habitat area calculation

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

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

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

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

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

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

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

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

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

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

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

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

Other Blogs in the series

Calculating habitat units

The trading rules explained

Why we need a new Bristol Tree Replacement Standard

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

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

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

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

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

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

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

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

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

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

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

Background

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

Table 1 The proposed BG4 tree replacement table.

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

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

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

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

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

Our proposed new BG4 (BTRS) model

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

Table 2 Our proposed BG4 tree replacement requirement

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

The reasoning for our proposal is set out below:

1. Applying the Biodiversity Metric to Urban trees

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

When to record individual trees

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

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

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

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

  • woodlands
  • orchards
  • wood-pasture and parkland

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

2. Calculating Individual trees habitat

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

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

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

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

Recording trees within private gardens

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

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

Recording trees within hedgerows

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

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

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

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

3. Forecasting the post-development area of Individual trees

The SBNG User Guide provides this guidance:

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

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

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

Exceptions

You cannot count:

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

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

4. The likely impact of this policy change

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

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

Table 4 Proposed BG4 impact analysis.

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