How ambitious and creative tree planting projects could help keep Bristol cool during heatwaves

Green infrastructure could help make Bristol more resilient to climate change, improving the lives of its residents. What can the city learn from Milan?

Photo: Boeri Studio

Future of Cities 9th October, 2023

Neill Talbot

High rise buildings like the 1960s blocks in Lawrence Hill and Easton are prone to overheating due to poor ventilation, often cramped conditions and the lack of shade from other surrounding buildings or trees.

For Marta, who lives in Barton Hill high rise Longlands House with her mother and her newborn child, her flat can become unbearable during hot weather, even with the curtains drawn during the day.

“We struggled to sleep, even with the fan,” she tells the Cable, recalling the effect on her home from the hottest September on record this year. “There’s not much ventilation so even with our windows open it’s hard to do anything about it.”

Keith, who lives with his family in Twinnell House, another council-owned tower block in nearby Easton, says that during last year’s intense heatwave it was hard for him to breathe in his 16-floor flat.

“Since the heatwave we can’t breathe, no air up here at all, can’t open windows due to the kids, can’t sleep,” he said at the time of the block which is surrounded by very little green space and major roads. 

As our summers get hotter, the severity of these kinds of experiences are only going to get worse unless we learn to better adapt the city for the impact of climate change, as well as trying to prevent it.

Bristol was the first city to declare a climate emergency in 2018, and as it’s in the fire line for regular temperatures of 40C and over, a drive to make the city heat resilient is becoming rapidly more urgent.

People living in cities experience much warmer temperatures than rural areas due to what’s called the ‘heat island effect’, where materials like concrete and asphalt absorb more heat. But how can planners and developers help mitigate this?

Here we investigate how nature-based infrastructure could help Bristol become a climate resilient city in the future, improving the lives of its residents.

Heatwaves hit us unequally

Hot weather is known to have significant effects on our health, for example by placing a strain on the heart and cardiovascular system. Those with pre-existing medical conditions, older people and very young children are particularly at risk.

Those on a lower income are more adversely affected by a lack of natural cooling due to things like green space or tree cover near their homes, according to Friends of the Earth research. Meanwhile, people of colour make up 65% of the population of neighbourhoods with the least cooling.

Mapping of Bristol on behalf of Friends of the Earth showed, for instance, that the trees in the north-west leafy district of Clifton starkly contrasts with the largely treeless streets of Southville.  

And at present, according to the council’s Keep Bristol Cool data tool, Lawrence Hill residents are most at risk from extreme heat due to the high number of flats, deprivation, lack of green space and the fact the area is low-lying and in the east of the city. Easton, St George, central Bristol, and Hillfields have also been identified as the areas next most at risk.

Central and eastern parts of Bristol are more likely to see higher temperatures than north-western or southern parts of the city due to the prevalence of hard surfaces like concrete, and being further away from the cooling effect of coastal sea breezes. Prevailing westerly winds can transport heat from the urban island effect in the centre to the east of the city. Areas at a higher elevation, in the hillier north-west and southern parts of Bristol, are generally cooler than central or eastern parts.

During a cabinet meeting on 3 October, councillors said average summer temperatures in Bristol could increase by seven degrees by the 2080s. 

The local authority is now preparing a range of adaptations to make Bristol more resilient to hotter summers. The plans to keep Bristol cool include planting many more trees, making sure care homes, council housing and offices can stay cool during heatwaves ,and creating more shaded walkways and river paths. But there is little available space on pavements for planting more trees, partly due to utilities, so they could be planted in space currently used for cars instead.

During last year’s hottest day on record, research also found that Bristol areas with fewest green spaces and trees could be 4.8^oC hotter than those that have more tree cover and other plants.

While it’s clearly a solution is to plant more trees, because the areas in most need for planting them there tends to be little space to factor in green space this isn’t so straightforward.

Bristol City Council’s One City Plan includes a target to increase the city’s tree canopy by 25% by 2035, doubling it by 2046. That’s from a baseline of 12% cover in 2018 to 24% in 2046. 

To achieve this target, about 1.7 million trees will need to be planted as saplingings, or 110,000 trees planted as specimens, with a girth of at least 25cm. However, by the end of last year only 80,000 predominantly young trees had been planted, so the rate of planting needs to significantly increase. 

The council said it plans to plant a further 16,000 this year.

The One City plan also aims to address the disparity between different wards in Bristol, which range from 9% to 27% tree cover. The council has in place several active tree planting programmes, albeit reliant on external funding.

However, despite the council’s projects to increase tree cover in the city, there are loopholes in the council’s planning policies that allow trees to be lost during development projects.

If trees are lost, replacement trees must be planted within a one mile radius to compensate, according to the Bristol Tree Forum. 

But developers are given an alternative to planting trees, and that is to pay a fee to the council. This fee is supposedly for the council to plant trees, but as there are almost always few (if any) sites to actually plant trees, this money remains unspent. 

John Tarlton of Bristol Tree Forum told the Cable: “This [fee] is supposed to pay for replacement trees, but as there are few if any sites to plant trees, this money remains unspent, and currently stands at around £900,000 for the whole of Bristol with little likelihood that the money will ever be used for tree replacement.”

Tarlton says there are other local authorities Bristol could learn from to inform a review of its ‘tree strategy’, including Wycombe Council which is planting at least one tree per resident on large areas of council-owned land. A total of 43,000 trees are due to be planted in order to achieve carbon neutral status by 2050.

“Most councils have a tree strategy, and Bristol City Council is currently updating theirs,” says Tarlton. “Wycombe Council have a very proactive approach to increasing their canopy cover which Bristol could learn from, including a supplementary planning document dedicated to maximising tree cover.”

But as Bristol aims to ramp up its tree planting, are there even more ambitious projects ongoing further afield that Bristol could also learn from?

Tree towers

The Milan Forest is an ambitious project to plant 3 million trees by 2030.  To date 427,475 trees have been planted.  If successful, the increase in tree cover would have a major impact on the lives of the 3.25 million in the city and surrounding area.

Experts say that the projected 30% increase in trees would lower the city’s temperatures by 2^o C, offering relief from the city’s muggy, sometimes tropical weather.  

It could also absorb an additional 5 million tons of carbon dioxide a year, The plans to increase the number of trees include planting trees in more than 2,000 school courtyards and on 10 million m₂ of flat rooftops. 

Although a much larger city, Milan does appear to be taking action more quickly than Bristol. Their target is to increase the city’s canopy coverage by nearly 30% in the space of a decade from 16% to 21%, while Bristol’s target is a 25% increase in around 15 years – from 12% to 16%. 

But Milan’s target of 3 million trees by 2030 was also meticulously calculated as part of a two year research phase before the launch of the project. Researchers have mapped the current canopy cover and potential for tree planting across the city region, which is publicly available. 

In February 2022, Bristol City Council committed to developing a tree planting strategy, but this has not yet been published. The report brought to the council’s cabinet notes that ‘no comprehensive study of the potential to plant trees in Bristol has been undertaken’ and ‘it is not clear that a 24% tree canopy target is achievable’.

More than this, Vassili Papastavrou of Bristol Tree Forum told the Cable much more effort is needed to retain existing street trees in the city.

“When large street trees are removed, they must be replaced with large-form trees within the next planting season and not left for a decade, or worse, not at all as planting sites are lost if not utilised within a few years,” Papastavrou said.

“It’s also important to consider which tree species are best to adapt to warmer conditions and will provide the best habitats for wildlife… Rowans and flowering cherries are not long-lived and will never provide the shade that we desperately need.  

“For street trees we need instead to plant large-form trees such as planes, maples and limes that are robust, grow fast, are long lived trees and will provide shade. For trees that are planted in green spaces and parks, then I think the focus should be on biodiversity.” 

Labour Cllr Don Alexander, cabinet member for transport, says a challenge for tree planting is competition for space on the city’s streets. 

He told a cabinet meeting on 3 October: “A lot of these street trees are going to go in the carriageway. And that’s when it becomes political, because that’s when they are actually competing with car parking and other things that people want to do in the carriageway. 

“But just to warn you, this is going to become political. We have to be quite clear about the importance of greening our streets, quite clear that there will be a cost to it in terms of changes in our lifestyle.”

In densely built areas where space is an issue, architects and city planners must be creative to make best use of what’s available.

In Milan, an example of this is architect Stephano Boeri’s ‘Vertical Forest’ residential towers near the city’s Garibaldi train station, which were completed in 2014. The pair of high rises have balconies brimming with shrubs and trees that absorb carbon dioxide and PM10 particles – a pollutant with links to respiratory ailments.

On Boeri’s green towers, for each resident there are two trees, eight shrubs and 40 bushes, reducing the external amount of typical urban building materials such as concrete and asphalt, which absorb the sun’s radiation. This means the buildings contribute to reduced temperatures in the vicinity of the building – as well as inside – and represent a mitigation to the ‘heat island effect’. 

The green facades also result in reduced energy consumption as the tree cover provides increased shading and insulation against outside temperatures – keeping the building warmer in the winter and cooler in summer.

Last year was the hottest year on record in the UK. Temperatures reached 40C for the first time, leading to the highest number of heat-related deaths on record.

Kye Dudd, Bristol City Council cabinet member for climate, said: “This year so far has seen the hottest June on record and also we had the September heatwave, where for the first time in that part of the year there were seven consecutive days where the temperature reached at least 30 degrees,” Dudd said. “Quite clearly our climate is changing, and we need to protect people in the city.”

What’s also clear is Bristol needs a clearer plan for how to do this, and to be open to ambitious approaches like those in Milan, which could help secure its future as a climate-resilient city.

The Future of Cities project is funded by the European Journalism Centre’s Solutions Journalism Accelerator, supported by the Bill & Melinda Gates Foundation.