Trees a crowd - the movement to green Europe’s city streets

Increasing tree cover can help combat climate challenges and improve mental health - but there are hidden challenges too
Pigeons feeding on a tiled pavement between an avenue of trees in the centre of Almeria a city in Andalucia in southern Spain | Photo: Alamy

By Philippa Nuttall

Philippa Nuttall is a Brussels-based freelance editor and journalist focusing on the climate crisis and sustainability issues

19 May 2023

To get from one campus of Brussels’ Free University (ULB) to another, students and lecturers must walk along a busy main road. The path is lined by beautiful old trees that offer shade in the summer and some much-needed greenery. Yet tree roots have warped the pavement, making it difficult for even the most sure-footed pedestrians.  

It’s a reminder that, despite their ability to cool streets, reduce traffic pollution, and improve city dwellers’ mental health, the question of where and how to plant trees in an urban setting is more difficult than it may first appear. 

There are some people who love the concept of trees, says Kata Tüttő, deputy mayor of the Hungarian capital Budapest, but balk at the reality. “Not everyone likes insects or the loss of parking spaces.”  

For her and many leaders of European cities, tree planting is part of a bigger transformation in urban living. But it’s not without pushback. “We are redistributing public spaces and parks, and creating bike lanes, and this is creating conflict,” says Tüttő.

And while ecologists want cities to be greener, they warn that the right trees must be planted in the right places if they are to have a positive impact on people and pollution.  

Efforts to help European cities find their way through this woody maze are increasing, but there is still some way to go before policymakers, scientists and urban dwellers everywhere in the European Union are ready and able to accept trees as a central part of a city landscape. 

Around 30 per cent of urban areas in Europe are covered by trees, according to the European Environment Agency, an agency of the EU headquartered in Copenhagen. The agency places the Italian port city of Savona at the top of the urban tree cover charts, with nearly 84 per cent tree coverage, with London in bottom place, based on satellite data from 2018.  

Sjoerd van Leeuwen
Illustration by Sjoerd van Leeuwen for The Parliament​​​​​

The agency’s figures make clear the advantages trees can bring to urban areas and their inhabitants, not least their potential to help them adapt to the impacts of climate change.  

Temperatures in Europe have increased at more than twice the global average over the past 30 years, making it the fastest-warming continent in the world, according to a study published by the World Meteorological Organisation, a UN body, in November 2022. Temperatures across the continent have risen at an average rate of around 0.5C every decade from 1991 to 2021, the agency’s data shows.  

Studies by the EU Joint Research Centre, meanwhile, indicate this warming tends to be felt more in urban areas, with surface temperatures generally higher in cities than in rural surroundings. “This phenomenon, known as surface urban heat island, increases the risk of heat-related human illnesses and mortality,” the research authors wrote in a 2022 paper.  

The European Environmental Agency says on its website that trees can help cities adapt to these climate change impacts by reducing air temperature through shading and “evapotranspiration” – when water that enters trees via the roots is released as vapour through pores in the leaves, as well as evaporating from the soil around the roots.  

According to the agency’s research, trees also help protect people and infrastructure against other climate change-linked extreme weather events by reducing the force of stormwater and wind speeds. What’s more, wherever they are planted, trees act as carbon sinks – meaning they absorb more carbon from the atmosphere than they release.  

But trees do not just help with mitigating and adapting to climate change. As numerous studies post-Covid have revealed, green spaces in general are vital for our mental and physical health. Research published in the scientific journal Environmental Research in 2022 showed that people living in residential areas with more tree canopy in Beijing had better mental health during the pandemic.  

Various studies have likewise linked the presence of trees to positive effects on respiration, cardiovascular health, sleep and other health-related issues. A 2016 report from The Nature Conservancy, an American NGO, details how, among other services, tree leaves can filter out fine particulate matter, a dangerous form of air pollution generated from burning biomass and fossil fuels.  

“Planting trees is an obvious thing to do,” says Simone Borelli, an urban forestry officer at the UN’s Food and Agriculture Organisation in Rome. But benefits are being missed because of a lack of thinking about why trees are being planted and what planners or policymakers hope to achieve by increasing tree coverage, he suggests.  

“You see these plans saying: ‘Let’s plant three million or 10 million, or even a 100 million trees,’” says Borelli, a forester by training. But if the trees are of poor quality or not planted in the right place, their lifespan is likely to be limited, he warns.  

Borelli says that before holes are dug and roots placed in the ground, people should have a clear objective: “Why are you planting this tree? What type of tree is best?”  

In cities, trees are often planted “in tiny little pits”, he says, and then people are surprised when they do not grow properly. Borelli advocates “adaptive management” whereby the growth of the tree, its upward reach and its downward root spread over the next 20, 30 or more years, is taken into consideration from the outset. 

We need a paradigm shift, whereby trees are seen as an investment that brings investment

A 2022 study from the Belgian university KU Leuven backs the idea that not all urban trees are equal. After mapping trunks and leaf canopies in the Brussels-Capital Region, researchers found that a single old, large tree with a big, leafy crown had a more significantly positive impact on cardiovascular diseases and mood disorders than the presence of many small trees.  

The importance of older trees is too frequently overlooked, agrees Borelli. Looking after existing urban trees is “less glamorous” than announcing impressive-sounding tree planting targets, “but we have a lot of existing trees and need to take care of them”, he says.  

Just as most European cities have diverse populations, urban trees should display a similar diversity in terms of species, size and the way they are planted. “A more diverse urban forest is more resilient,” says Borelli. 

Many European cities are beginning to move in the right direction, he concedes, acknowledging that “in general, there is a better understanding of urban nature for people’s wellbeing”. Yet, in too many instances, “green is still seen as a luxury, a nice-to-have”, he says.

“We need a paradigm shift, whereby trees and green spaces are seen as an investment that brings investment.”  

Borelli cites Lee Kuan Yew, the former prime minister of Singapore, who famously said that if the city-state became “a nice place to live, then people will come and invest”, a vision that was seen as instrumental in Singapore’s transformation from a polluted economic backwater 60 or so years ago into the green powerhouse it is today.  

“It is a matter of thinking differently,” says Borelli. “Do the Googles and Amazons want to have their headquarters in a grey industrial city or in a nice, environmentally-friendly green city?” 

Urban trees can also help make city dwellers more aware of the global biodiversity crisis and the links between the advantages of trees locally and the wider benefits of healthy nature for people and the planet, believes Kelsey Perlman from environmental NGO Fern, which works on influencing EU policies around forests.  

“We need to integrate [urban trees] into a larger conversation about European forests,” she says. “Everything is seen as quite separate.”  

This approach includes focusing on connectivity, says Perlman, “where we make sure trees in urban areas link out to peri-urban areas and to the few biodiversity hotspots we have left”.  

Research shows that protected areas in isolation aren’t enough to allow wildlife and their habitats to thrive. This has led to increasing calls for “ecological connectivity”, where green areas join together to allow natural processes to flow without being interrupted by human infrastructure such as roads.  

Until recently, it has been difficult for mayors and other city leaders to make a clear argument for spending money on trees rather than other services or infrastructure, but methodologies making the economic and environmental case for preserving old trees and for planting saplings are becoming the reality.  

A 2016 report entitled ‘Valuing London’s Urban Forest’ from Treeconomics, a UK-based social enterprise tackling projects “that highlight the value of trees”, puts a monetary value on the services trees provide to the UK capital in terms of air pollution, carbon storage or stormwater management for example.  

Likewise, a study by the US Forest Service and the University of California, Davis, found that for every $1 (€0.9) spent in California’s cities on tree planting and maintenance, there were $5.82 (€5.31) in benefits. 

Another study of 10 US cities found that urban trees remove enough particulate matter to reduce annual health impacts significantly, with reductions in health impacts ranging from $1.1m (€1m) in Syracuse, New York to $60.1m (€54.8m) in New York City. 

The European Commission’s Nature Restoration Law should encourage EU cities to take notice of such benefits. The ruling, which still needs to go through the EU legislative process, proposes an end to any net loss of green urban spaces by 2030; a five per cent increase of such spaces by 2050; and a minimum 10 per cent of tree canopy cover in every European city, town and suburb.  

The Committee of the Regions (CoR), the political assembly that brings together Europe’s locally and regionally elected representatives, is already trying to encourage its members to take steps to reach this target with its Trees for Life campaign, which aims to plant 3 billion extra trees across Europe’s countryside, towns and cities by 2030. 

Tüttő is leading on this initiative as First Vice-President of the CoR’s Commission for Environment, Climate change and Energy. “Planting trees in cities is hard,” says the deputy mayor of Budapest, explaining that the asphalt and urban pollution make the process analogous to keeping animals in a zoo. “They can live but they don’t thrive.” She adds: “Trees want to connect [to each other and to nature], but there are pipelines; there is polluted water.” 

Nonetheless, Tüttő is clear that Budapest and cities everywhere in Europe are committed to ensuring existing trees are looked after and that the right new trees are planted in places that give them the best possible chance of thriving, despite sometimes difficult conditions. 

Tüttő says Budapest is working on its own methodology to count the number of trees in the city and their economic value based on four indicators: the amount of carbon dioxide a tree can capture; the shade it can give depending on the size of its leaves and crown; the dust and pollution it filters out of the air; and the amount of water it releases through evaporation.  

The tree’s age and its situation in the city will also be taken into consideration, she says. The idea is that, ultimately, it will be possible to say how much each tree is worth in euros.  

Where exactly trees are placed can make a big difference to the lives of residents. While various studies suggest trees can increase the value of houses, in particular in wealthier areas,     finds that cleaning up green spaces in lower-income areas, including planting trees, can make people feel safer and reduce levels of violence and crime.  

Trees and green space increase social cohesion by making it easier for people to mix outdoors, says Borelli. “If you know your neighbour, you are less likely to steal from them.”  

Likewise, the heat islands that develop in cities during heatwaves tend to be worse in poor areas with fewer parks and trees, he adds. “Let’s invest in trees where we can make the most difference,” says Borelli. “In a refugee camp, when you plant a tent, you should plant a tree; the same for a housing estate.”  

While it is currently impossible to check whether every new tree planted as part of the Trees for Life initiative, or other plans, survives and thrives, Tüttő suggests this might be about to change. Budapest is using artificial intelligence and satellite data to map its tree coverage.  

“With good data, we can identify the species, the colour and the size [of a tree],” says Tüttő. “AI can work out how healthy a tree is and if it has grown. The future will bring lots of opportunities.”  

Budapest has also created a special nursery, or “tree school”, she adds, so the city can grow trees that are used to the local climate and soil rather than importing them from countries such as the Netherlands, thereby giving the trees a better chance of survival.  

Central to ensuring trees play a greater role in making our cities happier, healthier and safer places and in making urban green spaces part of wider efforts to protect nature, is the need to plan “green and grey” infrastructure together, says Borelli.  

“Green infrastructure too often comes as an afterthought and is not really integrated in the urban planning process,” he says. “We need some critical thinking.” 

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