Measuring and Modeling the Tree Canopy of Bristol

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

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

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

by 2046  Tree canopy cover has doubled since 2018

The obvious question to ask is :

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

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

How can tree canopy be measured across the city?

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

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

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

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

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

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

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

Canopy prediction

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

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

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

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

Is the goal achievable?

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

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

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

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

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

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

The private realm

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

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

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

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

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

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


Trees for Streets

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

Quotes from the Flyer for Trees for Streets

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

and

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

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

Comment

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

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

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

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

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

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

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

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

How it will work:

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

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

Urgent help needed to water new trees

Lots of newly-planted trees on The Downs and elsewhere are suffering from lack of water in this unusually dry weather.  Many trees are dying.

The soil around the roots of each tree was so dry that (despite the recent rain) it would now take a lot of water to become hydrated.  Your help is urgently needed to water any of these trees that you see.  Even if they look nearly dead, with a lot of water they may come back to life.  These trees were paid for by members of the public and local organisations.

A number of people have raised concerns and Bristol City Council has said that it will now water each tree twice a week.  Some are in good shape – one sponsor has been watering her own tree.

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Just about hanging on…

Last year, the same thing happened and after six months of raising concerns with Bristol City Council, they said that last year’s problems would not happen again and watering would be sorted out for this year.  Much of the cost of planting a new tree is to cover sufficient watering for the first couple of years.

Clearly, it is a terrible waste of trees, time and effort and upsetting for the sponsors for the trees to die.  The Bristol Tree Forum  will keep raising this problem so that future sponsors can be sure that their trees thrive and that dead trees from this year and last year’s plantings are replaced.  A proper guarantee needs to be obtained for the future.

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Too late – these trees on Redland Green are dead

If you know anyone who sponsored one of these trees alert them and encourage them to water their own tree.  If you see a new tree that is dead or dying please email us a location and photo.

Vassili Papastavrou

Trees under threat at the Eastgate Centre – Comments so far…

Many thanks to all of you who have lodged comments on this application (nine so far). Here is one great example:

  1. This proposal flies in the face of the objective of the city council to double tree canopy cover in a generation.
  2. There is clear evidence that climate change is in part being driven by the city heat island effect. Bristol is already two degrees warmer than the surrounding area. A mitigation of this is to ensure that all car parking areas are shaded by trees- and not simply by a perimeter screen, but the use of suitable trees 20 metres apart to cover the entire area. This particular complex already has huge areas of unshaded car parking, and the proposal would only increase this.
  1. The Frome Valley is a key feature of the city’s biodiversity. It is one of a series of wildlife corridors that form a key feature of the attractiveness of the city to humans. This corridor is increasingly being eroded by development. As the River Frome has a huge water catchment area, which is increasingly being developed, creating much greater and faster run off, it floods rapidly and frequently. The fact that flood water is now diverted at the site of the Eastgate shopping centre into the northern stormwater interceptor will not prevent future floods upstream.
  1. Visually this remnant woodland of the Frome Valley is crucial it counteracting the utter ugliness and dreariness of the developed site. This of course originates from the original use of the site as a football and greyhound racing stadium. Bristol deserves better.
  1. What is desperately needed throughout this site is more trees being planted on the land owned by the various firms in the area, and not the destruction of the trees that by happy chance have survived.
  1. The wonderful veteran oak in particular, probably 300 years old, should become the centrepiece of a revival of this dreary area.

This is our earlier blog. Time is running out to lodge your objections. If you want to do so, please lodge your objections here in the Planning application comments section.

These are the Important dates:

Eastgate Trees3

Bristol’s trees in crisis!

With Bristol City Council’s budget cuts, two decisions have been made regarding the management of Bristol’s treescape that make no economic sense, and threaten the reputation of the City as a Green and pleasant place to live and locate a business.

With Bristol City Council’s budget cuts, two decisions have been made regarding the management of Bristol’s treescape that make no economic sense, and threaten the reputation of the City as a Green and pleasant place to live and locate a business.

Decision 1: Slashing of street tree management budget

  • The budget for managing street trees has been cut by nearly 78% from £240,000 to £53,000.
  • As a result there will be no pollarding of street trees or removal of epicormic growth around the tree.
  • Emergency cover outside normal working hours is no longer being provided through the tree management contract, despite having no cost benefit.
  • Tree management will be limited to felling to address safety risks, despite greater initial costs and the long term consequent loss of tree sites – felling costs the same as 16 years of maintenance.  As a result, Bristol’s street tree population will rapidly fall into decline as they are steadily removed, never to be replaced.

Decision 2: No planting of street trees, either replacement or new, even when cost neutral

  • Bristol City Council has operated a number of innovative schemes allowing residents or community groups to sponsor replacement or new street trees. Despite fully funding the planting, and maintenance for two years, such planting will no longer be permitted.
  • At the moment, when a tree is replaced in an existing tree pit it costs £295. This covers regular watering until the tree is established and two years maintenance. If the trees dies whilst establishing itself, it is replaced at no extra cost.
  • If the Council can be persuaded to change its mind about not planting new trees, then this cost could to increase to the £765 that developers are required to pay – the overall costs of planting a tree and maintaining it during its lifetime. Planting a tree at a brand new site could add around £2,000-£2,500 if a special tree pit needs to be installed.
  • Currently there is huge support from the community for replacing lost trees. Around £200,000 of developers’ money is set aside for this purpose, Metrobus is committed to planting 200-300 trees as part of their planning condition, and Bristol University has donated funds to plant 60-100 public street trees.
  • It makes little sense, in times of budget constraints, to renounce external funding sources that fully cover planting and maintenance costs.

These decisions are a false economy for Bristol City Council

  • With no pollarding of street trees, increased tree growth will lead to more subsidence claims against the Council, and create more highway damage, pavement trip hazards, and infrastructure damage.
  • New tree growth from previous pollarding points will become unstable, increasing the probability of personal injury and property damage claims against the Council.
  • Just a couple of additional subsidence or injury claims could negate the Council’s entire “cost saving”.
  • This short-term decision makes no sense – if a tree can be maintained for some 16 years – the one-off cost of felling it, then surely it makes better economic sense to spread this inevitable cost and maintain the tree rather than fell it as a short-term ‘solution’ – a ‘solution’ which loads all the costs up front and will lead to greatly increased and unavoidable expenditure in not very many years time?

These decisions will threaten Bristol’s reputation as a Green City

These decisions were taken with no consultation with stakeholders

There has been no consultation regarding these decisions with other Departments within Bristol City Council, who will have to deal with the foreseeable consequences, with insurers, who will face additional damage claims, Avon and Somerset Police, who will have to address public order consequences of mass felling, or Bristol Tree Forum, with its wide-reaching understanding of tree issues.

What you can do

  • Contact your Councillor and email the Mayor and demand that these decisions be overturned.
  • Contact us here at Bristol Tree Forum to register your support and offer to help defend Bristol’s public tree spaces.
  • Spread the word and forward this blog to others interested in saving trees.

Green space and our health

The evidence for physical and mental health benefits from contact with nature, such as reducing rates of non-communicable diseases is clear.

A range of bodies, including Government agencies, have promoted the potential physical and mental health benefits of having access to green spaces.

The evidence for physical and mental health benefits from contact with nature, such as reducing rates of non-communicable diseases is clear. So are the challenges for preserving and extending urban green spaces.

Green space is natural or semi-natural areas partially or completely covered by vegetation that occur in or near urban areas and provide habitat for wildlife and can be used for recreation. They are many and varied – from tree-covered streets & avenues to squares, play areas, schools, cemeteries, parks, woodlands, nature reserves and allotments.
Sadly, only half of us live close to green space. Green space is expected to decrease as urban infrastructure expands.

Key benefits include:

  • Physical and mental illnesses associated with sedentary urban lifestyles are an increasing economic and social cost.
  • Areas with more accessible green space are associated with better mental and physical health.
  • The risk of mortality caused by cardiovascular disease is lower in residential areas that have higher levels of ‘greenness’.
  • There is evidence that exposure to nature could be used as part of the treatment for some conditions.
  • There are challenges to providing green spaces, such as how to make parks easily accessible and how to fund both their creation and maintenance.
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