Urban trees and forests are very vulnerable to climate change: how should we shore them up?

More than half our urban trees are at risk as global warming heats up the planet. That’s the salient news from new research which shows that by 2050, more than three quarters of urban tree species will be at risk from the changing climate

Urban trees are crucial air conditioners: the shade they provide and water they transpire can lower the temperature by several degrees.

This makes them important protections against increasingly deadly urban heat.

But, just like people, urban trees and forests are vulnerable to warmer weather.

A new study published in Nature Climate Change is aimed at policy makers.

“This paper is for governments to identify those trees that are potentially vulnerable,” says lead author Dr Manuel Esperon-Rodriguez, a researcher at Western Sydney University’s Hawkesbury Institute for the Environment.

The international team of researchers used data from the Global Urban Tree Inventory to analyse 3129 urban tree species from 164 cities in 78 different countries.

They compared the trees’ natural tolerance limits to the expected temperature and rainfall conditions for each of the cities in both 2050 and 2070.

They found 56% of urban tree species are already living in areas where the temperature range exceeds their natural preference.

Even more urban tree species – 65% – are living under abnormal rainfall levels.

Assuming that emissions continue to grow past 2050, peaking around 2060 (based on a commonly-used scenario called RCP 6.0), the researchers found that 76% of urban tree species would be at risk from temperature and 70% will be stressed by changing rainfall.

Cities closer to the equator are particularly vulnerable, as are Australian cities.

Perth can expect 83% of its urban tree species to be at risk by 2030 – and it has the most optimistic prediction. In Darwin, nearly all species will be at risk.

“Common native tree species found in at least 10 Australian cities which are expected to experience climate conditions beyond their natural tolerance limits by 2050, include manna gum, swamp gum, yellow box, narrow-leaved peppermint, blackwood and brush box,” says senior author Associate Professor Rachael Gallagher, also at also at Western Sydney University.

“Many well-loved, non-native urban trees are at risk too – species like jacaranda, oaks, elms, poplars and silver birch.”

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Read more: The importance of urban trees

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Fortunately, there are ways to plan for and remedy this risk.

“Trees can adapt, and present plasticity in the traits that can allow them to tolerate harsh conditions,” says Esperon-Rodriguez.

“When we say that those are species are at risk, we are not saying that they are going to die. We are just saying that they might experience stressful conditions based on their tolerance,” says Esperon-Rodriguez.

Vulnerable tree species can be protected with smarter watering, and management, according to Esperon-Rodriguez, while local governments and city planners can identify which trees will be more hardy when they’re planting.

“This is an option for managers and urban planners to say, ‘okay, this is how we can start thinking where we want to plant different species within the city’.”

Some city councils are already doing this – Esperon-Rodriguez highlights the City of Sydney’s urban forest strategy as an example.

Another is the City of Adelaide.

“In December 2021 the City of Adelaide adopted a corporate Climate Change Risk Adaptation Action Plan,” says Ilia Houridis, director of City Shaping at Adelaide.

“One of the actions in the current financial year is to review current species planting list and revise to ensure a more climate resilient species mix.”

Esperon-Rodriguez emphasies planting as a particularly important mitigation tactic.

“All the major benefits that are provided by urban forests are mainly delivered by the big trees,” he says.

“So if we are planting things that are going to be failing today, then there are going to be problems in the future.

“But if we make sure that what we are planting today is going to grow and survive over the next 20, 30, 40, 50 years, then we will be securing urban forests for the future generations.”