I recently moved back to my hometown, York, a small city in the north of England. It is historically rich and culturally vibrant, but lacking in one department: trees. According to the council, its canopy cover was in 2022, which is on the low side. The European average is around 15 per cent and London has around .
York has plans to do something about this, in recognition of the fact that urban trees have myriad benefits. These include cooling, carbon sequestration, pollution control and flood mitigation; they stimulate economic activity by making cities more attractive and are good for mental well-being. In a nutshell, trees make cities more liveable and prosperous.
The council is planning to increase York’s canopy cover to 13 per cent by 2050. That means planting around 4000 trees over the next few years, a process that has begun with the of two “micro woods” not far from the city centre. York is hardly a global city, but it is part of a global trend. “Today, we are in a very positive place when it comes to talking about urban trees,” says at Gothenburg Botanical Gardens in Sweden. “Cities are almost competing with each other over their plans for planting trees.”
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But, as I discovered when I attended the recent , planting thousands of trees doesn’t guarantee success. It is critically important what you plant and where, and also to look after what you already have.
One of the main benefits of urban trees is that they keep cities and inhabitants cool by providing shade and transpiration – the process by which they absorb water from the ground and vent it into the atmosphere. This cooling effect will only become more important as the climate warms. at the Technical University of Munich in Germany told the symposium that increasing canopy cover in a big city like Manchester in the UK – say from 20 to 30 per cent – will keep urban temperatures roughly where they are today, even as the world gets hotter overall.
But at the same time, climate change is a threat to urban trees. Between 2016 and 2022, Pauleit and his colleagues monitored 1.34 million trees in five German cities. Each year, between 1 and 2 per cent of them died – which is to be expected, as trees get old and expire. But there was a major spike in mortality in the of 2019. At the overall rate of attrition they found, half of urban trees will have died by 2050.
The loss of mature trees is a major problem because they supply way more cooling than younger trees
The loss of mature trees is a major problem. According to Pauleit, an 80-year-old lime tree supplies an order of magnitude more cooling and sequestration of carbon than a 20-year-old one. “It’s really important to protect old trees,” he said.
There are other threats. Two invasive species of longhorn beetle, Asian and citrus, recently arrived in North America and Europe. Both lay their eggs under tree bark and their larvae can eat a tree to death. A recent by Sjöman found that a serious outbreak of either of these pests could kill a quarter of the trees in 10 Nordic cities; in a simultaneous infestation of both species, urban tree mortality could exceed 90 per cent. “The question is not if we have an outbreak of these beetles, but when,” he told the symposium.
Both of these problems are exacerbated by the limited diversity of urban trees. In the London borough of Camden, for example, just five species account for three-quarters of trees, according to Sjöman. There is a similar lack of diversity in Beijing, Pittsburgh in Pennsylvania, and Barcelona, and nearly half of trees in Finland’s capital Helsinki are a single species, the common lime.
Such trees are tried and tested urban choices, but as the climate warms, they may no longer be fit for purpose. So cities need to plant with the future in mind. But as to what to grow instead, there is a lack of information and an unwillingness to branch out. “It’s not surprising,” said Sjöman. “You don’t want to be the famous landscape architect… designing the square where everything died.”
That is why Sjöman and his team have been experimenting in what he calls his “torture chamber” at the University of Gothenburg. There, they subject non-traditional species to extreme conditions, such as drought and flooding, to see which tolerate them the best. He has already identified three candidates for future urban afforestation – the chestnut-leaved oak from Azerbaijan, silver lime from Romania and Lebanese wild apple. “They love heat and drought. These are trees we could, should and have to see more of in urban environments.”
Back in York, I got on my bike and went to one of the micro woods. There are about 50 saplings, mostly (as far as I could tell) from the standard repertoire. They will take decades to mature. I am rooting for them, but I fear the world they are growing up in won’t be kind to them.
Graham’s week
What I’m reading
I’d like to have a look at The Essential Tree Selection Guide: For climate resilience, carbon storage, species diversity and other ecosystem benefits by Henrik Sjöman and Arit Anderson.
What I’m watching
Alma’s Not Normal on the BBC.
What I’m working on
A reporting trip to Germany.
Graham Lawton is a staff writer at New Scientist and author of Mustn’t Grumble: The surprising science of everyday ailments. You can follow him @grahamlawton
