Aren’t planting more trees....

Scramble for shade: Why cities aren’t planting more trees

Scientists are urging cities to expand tree cover to keep cities cool, but planting trees isn’t always practical.

BY AITOR HERNÁNDEZ-MORALES

They cool streets, slash air pollution and improve urban residents' mental health, but European cities are still struggling to plant more trees.

As climate change causes temperatures to climb in cities across the bloc, local leaders are under growing pressure to find ways to keep urban landscapes cool. And leading climate scientists say trees are part of the solution.

Creating urban forests and increasing tree cover along city streets would help to "mitigate climate change directly through storing carbon, and indirectly by creating a cooling effect that reduces energy demand and reducing energy use for water treatment," the Intergovernmental Panel on Climate Change wrote in its latest report.

The document, which draws on over 18,000 pieces of research, says increasing the number of trees in cities will combat urban heat islands, cut air pollution and boost mental health.

But while the benefits of planting more trees are clear, city authorities and experts say doing so is not as easy as it looks, citing high costs and hidden challenges related to infrastructure like parking lots and underground public transport.

In Lisbon, each new tree represents an investment of around €2,000, according to Ana Luísa Soares, a landscape architect in the Portuguese capital's Ajuda Botanical Garden who has authored a number of studies on the economics of urban greening.

"Part of that is the cost of the actual tree and the rest are the maintenance expenses you'll have to spend during the next five years, when a new tree requires special attention to survive," she said. "That's why planting trees along an avenue can be a major expense for a city, and an added concern for administrations as we head into tough economic times."

And progress in greening cities is actually going backward. EU cities have been losing tree cover in recent decades, according to data compiled by the OECD. That's partially due to urban development and road expansion, said Soares, but also because the thousands of trees planted during urban beautification campaigns in the 19th and early 20th centuries, when greening streets was in vogue, are reaching the end of their lives.

To stem the decline, the European Commission is proposing that EU countries devote at least 10 percent of all cities, towns and suburbs be covered by urban canopies by 2050. The draft law, part of its bid to increase nature restoration across the bloc, would also commit countries to ensuring there's no net loss of urban green space.

But as cities look to replace and expand tree cover to address the challenges of climate change, the cost of the initial investment risks putting them off, Soares said.

The ultimate benefits far outweigh the expense, she argued: "At the basic level, trees improve air quality and lower temperature on streets, and studies show they also make city residents feel better."

Beyond their cooling effect, trees also come with other perks, she added. "Tourists flock to tree-lined streets and property values are higher on greener streets," she said. "These are wise investments."

Root of the problem

The daunting cost isn't the only obstacle to increasing tree cover in cities, where local leaders can find their hands tied as a result of decades-old infrastructure.

"The main demand we hear from all citizens is that they want more green areas and, in particular, trees," said architect José Luis Infanzón Priore, head of public spaces and infrastructure for the city of Madrid. "Unfortunately, that's not always possible because of technical issues."

That's particularly true for major squares and plazas, he said, pointing to the example of Madrid's tree-less Plaza Mayor.

Planting a large number of trees there would be nearly impossible, he explained, because of the massive parking lot underneath it — a common problem in many public squares across Europe.

Trees need "at least 1 meter of subsoil" between the concrete structures and the ground, Infanzón explained. Planting a large number of trees would require completely demolishing the underground lots and rebuilding them with greater support structures — a complicated and expensive process.

Car parks aren't the only obstacle. "Many squares ... now have transport infrastructure under them," said Infanzón, citing the Spanish capital's Puerta del Sol — which hosts both metro and regional train stations under its surface — as an example.

"Newer transport hubs, like the one in Plaza de España, were built much further underground with plenty of subsoil left for trees," he said. "But in other parts of the city they were built without taking surface conditions into account."

In Brussels, which has committed to plant some 300 trees every year until 2030, the narrowness of the city's streets also represents a challenge.

"One of the things we look at is if trees will make it difficult for ambulances to pass through or if they will complicate passage for people with reduced mobility," said Zoubida Jellab, the city's alderman for green spaces. "It's often quite difficult to guarantee these two things."'

Making space for trees can also create local tensions, she pointed out: "Sometimes the only place to plant new trees is in parts of the public space currently occupied by cars," she said. "Politically it's not always easy to scrap a parking spot in favor of a tree."

Still, despite the potential benefits for residents, some areas should remain complete no-gos for new trees, Infanzón argued.

"There are no major squares in European cities that were designed to host trees," he said, naming Saint Mark's Square in Venice, Paris' Place de la Concorde and Brussels' Grand Place as examples.

"These squares were designed to impress and to be spaces of great pedestrian access, with little to no visual obstacles," he said, adding that cultural heritage laws prohibit anything that would bloc views of the iconic façades.

But that doesn't mean there can't be major progress elsewhere in cities, Jellab argues. "A city needs to be balanced," he said. "Let's not get distracted by the few historic squares [where] we can't plant trees and instead focus on the over 90 percent of urban spaces that can and should be greened."