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.

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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.

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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:

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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

London Planes on Narrow Quay

In the early ‘70s, Great Britain was in the throes of the calamitous spread of a new virulent strain of Dutch Elm Disease which would eventually kill nearly all the 25 million mature elm trees and change the face of the English countryside forever.

In the face of this devastation, the government launched  National Tree Planting Year  in 1973, with the slogan ‘Plant a tree in 73′ . The scheme was supported by the Forestry Commission and the Crown Estate who donated thousands of trees which were planted by local authorities, schools, businesses and voluntary organisations.  The Tree Council was established in 1975 to build on the momentum generated by this campaign.

In Bristol, the Civic Society worked closely with the city council and over the following years, 2000 urban trees were planted. One of the architects of this bold urban plan was the council planner Frank Kelf (February 5, 1925 – August 28, 2013) who was instrumental in persuading a cash-strapped council to invest in this major undertaking.

The centre of Bristol post war was a rather neglected space, particularly the dock area.  Narrow Quay runs along the left bank of St Augustines Reach in the heart of the city.

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Bristol Archives ref 40826/DOC/40:  City Docks: The ‘Rosedene’ at Narrow Quay : 1960

In the 1950s, Bristol’s role as a port was in decline and slowly the cranes and warehouses fell into disuse and many were demolished, leaving a neglected and ill-used post-industrial landscape. The photo shows the quay in 1975.

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Demarco Digital Archive : Opening of the Arnolfini Art Centre : 1975

An exhibition of “Ideas for Bristol” was run at the Bristol Museum and one idea for planting trees on Narrow Quay was shown from BCC’s Urban Design team.  Peter Floyd was then chairman of the Civic Society as well as having been a city civic planner.  Peter successfully gained the support of the businesses fronting the quay who provided the funds to buy ‘extra heavy standard’ trees able to deter vandalism. This photo by John Trelawny-Ross, city conservation officer, shows these substantial trees in Sept 1978 .

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Bristol Archives 4512/Of/12/21 : Bordeaux Quay(sic) John Trelawny Ross 1978

Here is the avenue in September 2019 forty years later, with Peter Floyd, recently honoured for this and other contibutions to Bristol.

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The trees have grown remarkably well.  With perhaps only one which may be a replacement, all the original trees remain and appear in good health. The chart shows the growth of the trees over about a 7 year period:

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There is quite a range of girths, perhaps reflecting trauma in early life or differences in the ground in which they are planted.  The average girth of 221 cm would (using our age calculator) suggest an age of 59 years. In fact they were planted about 43 years ago although perhaps already 10 years old when planted.

The avenue is mapped here on BristolTrees

Eastgate woodland

Over the summer, the owners of the Eastgate Retail Park, Consolidated Properties Group, submitted plans for the redevelopment of the east end of the retail park. (170/01580/P)  Currently the area comprises a drive-thru Burger King, a car park in front and an area of woodland behind.

The proposal is to replace the Burger King building with 5 new retail units and move the drive-thru restaurant into the car park. Rear service entrance to the units will be required, necessitating a service road which effectively removes the woodland. Marked for re-development were a fine 120 year-old Oak, protected by a Tree Preservation Order (TPO), overlooking the roundabout and and an area of woodland also protected by a TPO which includes 12 specific trees with an understory of younger trees and bushes.

eastgate_oakOver summer there was great public outcry about the loss of the prominent oak  (Bristol Tree Forum) and plans were resubmitted which mark the oak, two poplars and an ash for retention, but with no change to the location of the buildings. The woodland area is still destroyed, leaving only three isolated trees from the original canopy and understory.

revised-construction

The aboricultural report produced by Matthew Bennett of the Bristol City Planning Department is very critical of the plans, pointing out that the proposal takes no account of the Bristol core strategy nor of the tree replacement scheme and remarks that “The group of trees are an important green infrastructure asset which has a historical reference and provided a significant visual amenity to an already heavily developed site.”

eastgate-spinney

Consolidated Properties Group  owns over 40 retail parks and retail units throughout the UK, having bought Eastgate in 2011 . The company is one the the richest family-owned businesses in the UK and is chaired by the founder, Peter Stuart Dawson. The company acquired Eastgate in 2011.

Looking at the aerial photographs of retail parks on their website, the absence of green spaces is very striking . Retail parks take up large areas of ground, comprising only buildings and tarmac. Very little land is given to green spaces or exposed ground. Trees where they are planted are largely functional, used for screening purposes.

Isn’t it high time that a significant part of retail parks should be reserved for trees and woodland.  This is after all more in line with the meaning of “park”. If not, then at least we should resist the urge to remove what little woodland does still exist.

Public consultation on the revised proposal is now closed but comments can still be addressed to councillors and officers.

Chris Wallace

17th October 2017

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