City trees are under increased threat but research tools show that looking after them will lower temperatures, prevent flooding and reduce pollution

Standing proud at the intersection of Queen and Little Bourke streets in Melbourne’s central business district is one of the city’s beloved London plane trees. It’s in declining health and will probably need to be replaced in a decade or so. I know this not as an arborist, or even as a keen observer, but because the City of Melbourne has assiduously assessed, mapped and put online all of its more than 70,000 street and parkland trees.



“It’s quite extraordinary the amount of data that we have on our trees,” says a councillor, Cathy Oke, who chairs the city’s environment portfolio.

The Urban Forest Visual is a vital part of the city’s urban forest strategy, an ambitious management plan that will see canopy cover nearly double from its current 22% to 40% by 2040.

There’s mounting evidence that the effort will be worth it. Street trees, city parks, green roofs and gardens keep sweltering summer temperatures down, help to prevent flash flooding, reduce air and noise pollution, support native ecosystems, and ultimately make our cities more enjoyable – and healthy – places to live and work.

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So it’s little wonder that projects like this, that catalogue and track a city’s leafy assets, are blossoming – and equally unsurprising is that there is community backlash when large trees are felled.



One of the largest of these projects is Treepedia, launched this year. Developed at MIT’s Senseable City Lab in collaboration with the World Economic Forum’s Global Agenda Council on the Future of Cities and Global Shapers community, Treepedia makes use of the vast Google Street View database to map and measure the visible greenery in cities across the world.

The result is a collection of interactive maps of 16 cities – and counting – and an overall greenness rating for each, dubbed the green view index.

At the top of the green league is Singapore, with a green view index of 29.3%. Sydney and Vancouver come in equal second with scores of 25.9%, and falling at the opposite end of the spectrum – 8.8% – is Paris. But packing in a whopping 21,000 people for every square kilometre – more than 50 times that of Sydney – it is also the most densely populated of the cities.

The picture that Treepedia gives isn’t a complete one – Google’s fleet is blind to a city’s parklands, riverbanks and backyards, for instance. Nevertheless, the project does allow for broad brushstroke comparisons between cities, and even between neighbourhoods within a city.

This makes it a powerful tool for change, according to Carlo Ratti, the director of the Senseable City Lab. “Citizens themselves will use this information to compare the place where they live with other places, to put pressure on government to plant more trees,” he says.

Another tool, in the form of open-source software package i-Tree, uses the bird’s eye view of Google Earth to assess a city’s canopy cover both on public as well as private land. It also gives a dollar value to the many benefits – or “ecosystem services” – that trees provide.

As climate change brings warmer temperatures and unpredictable, drenching downpours, these benefits are taking on added significance. Studies conducted in Sydney and Melbourne have shown that a 10% increase in vegetation cover lowers land surface temperatures by more than a degree. With cities already up to 7C warmer than surrounding rural areas – a phenomenon known as the urban heat island effect – trees are an important tool councils can wield against high temperatures that drive cooling costs up and increase mortality during heatwaves.

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During storms, urban vegetation is a city’s sponge. Tree roots, green roofs and rain gardens help to sop up the deluge, easing pressure on stormwater and sewerage systems, and filtering heavy metals, nitrogen and phosphorous from the runoff that does make it into waterways.

Climate change has a sting in the tail for a city’s trees, though. In Melbourne, an extended period of drought has set many of its trees on a slow but inevitable path to the grave.

“The combination of urban heat island plus climate change means that street trees are going to be put under stress,” says Nick Williams, an urban ecologist from the University of Melbourne. “You’re suddenly five degrees outside of the natural envelope for these plants in a lot of cases, and that’s going to put them under increasing stress.”

Melbourne will need to plant 3,000 new trees a year – comprising an evolving palette of drought- and heat-tolerant species – to replace dying trees and meet its 40% canopy-cover target. Knowing the specifics of each tree in its care is a key ingredient, says Oke.

But this type ofdetail is the exception, says CSIRO’s Guy Barnett, and is usually limited to city centres. “Across large metropolitan areas like Sydney or Melbourne, our understanding is patchy,” he says.

This is especially problematic because the hottest parts of a city are often the places with more vulnerable populations, such as elderly people, who are particularly sensitive to extreme heat. “That’s where we need our urban vegetation the most,” Barnett says.