Climate crisis threatening to kill off many trees that help keep cities cool

More than half of all tree species in cities might already be living outside of their comfort zone

As the planet heats up, trees could provide cities with a lot of benefits by sequestering carbon out of the atmosphere and providing needed shade during the hot summer months.

But a new study finds that cities around the world are already inhospitable to many tree species and could get even harsher as the climate crisis grows.

The study adds new weight to the challenges city planners and local officials face when it comes to keeping cities welcoming and pleasant — for both plants and people.

“We emphasize the importance of taking immediate actions in terms of the climate emergency to secure the survival and persistence of urban forests,” the study authors write.

The researchers looked at more than 3,000 species of trees in 164 different cities, from the US to South Africa to China.

Different tree species — having evolved in different corners of the world — are going to have different needs to stay healthy. Some, like trees from the rainforest, may prefer hot and wet conditions, while others, like trees from mountain regions, may prefer habitats that are cool and dry.

The team looked at what kinds of temperature and rainfall conditions each tree species prefers, and then compared that to the temperatures and rainfalls both now and in a climate-altered future. Results were published on Monday in the journal Nature Climate Change.

Currently, 56 per cent of tree species are living in areas with non-ideal temperatures for that species — and 65 per cent of species were living in areas with non-ideal amounts of precipitation.

That doesn’t mean these trees are all about to die. The study authors note that cities might be doing things like watering some trees, or the species may be a little more flexible and able to survive, even if it’s not thriving.

As the climate crisis gets worse, that flexibility may be tested even more. The researchers found that by 2050, in a scenario with a moderate amount of future greenhouse emissions, 76 per cent of urban tree species will be living outside their preferred temperatures and 70 per cent will be living outside their preferred level of precipitation.

Cities are likely going to want a lot more trees as the climate heats up, not less. Due to all the asphalt, concrete and glass in a city, urban areas can get much hotter than their surrounding areas during the summer — up to 7 degrees Fahrenheit (4 degrees Celsius) hotter, according to the US Environmental Protection Agency.

As summers get hotter and hotter, that difference is likely to become even more brutal for city-dwellers. But trees might be able to help.

Tree-shaded surfaces in cities can be up to 45F (25C) cooler than non-shaded surfaces during a hot summer day, according to the EPA.

As a result, neighbourhoods with more greenery tend to be much cooler than less-green neighbourhoods. Reporting in The New York Times in 2019 found that temperatures in some US cities on a hot day can vary by around 15F (8C) between shady and non-shady neighbourhoods.

In addition to cooling down the city, trees can also help filter stormwater runoff, absorb carbon dioxide and offer habitat to animals like birds that call the city home, the EPA notes.

To ensure cities can still reap these kinds of benefits from trees, the authors of this new study suggest that people in charge of managing “urban forests” consider what the climate might look like decades from now when deciding how to plant and manage trees.