A new study by an international team of scientists led by Western Sydney University has raised concerns that climate change is putting many trees in cities worldwide at risk.
Published in Nature Climate Change , the study found species of cherry plums, oaks, maples, poplars, elms, pines, lindens, wattles, eucalypts and chestnuts commonly found in cities around the world are among over 1,000 tree species that have been flagged at risk due to climate change.
Study lead author Dr Manuel Esperon-Rodriguez from Western Sydney University’s Hawkesbury Institute for the Environment said the research examined the impact that projected future temperatures and rainfall would have on 3,129 urban tree species in 164 cities across 78 countries by 2050 and 2070.
“A surprising finding was that about half of the tree species in each city examined are already experiencing climate changes that put them at risk,” said Dr Esperon-Rodriguez.
“By 2050 an average of 65 per cent of species in each city will be at risk.
“This study is globally important because it identifies species at risk and also indicates which species are likely to be climate resilient.”
Dr Esperon-Rodriguez said the trees and shrubs in our cities provide important benefits to 4.2 billion people – more than half of the global population.
“Trees and shrubs absorb carbon dioxide and also cool their surroundings by providing shade and by pumping water up from their roots and releasing it through their leaves. This cooling effect makes people more comfortable and reduces energy used for cooling,” said Dr Esperon-Rodriguez.
“They also purify air and water, provide habitat for native animals, and improve people’s mental health and wellbeing through connection to nature.
“With all these benefits, increasing urban forest cover is a key climate action and liveability strategy for many governments and communities around the world.”
Co-author Dr Jonathan Lenoir from the French National Centre for Scientific Research (CNRS) said when climatic conditions exceed the natural tolerance of trees it can not only lead to poor tree health, it can also reduce their cooling effect.
“During drought or heat stress, trees can stop transpiring to reduce tissue damage. This means that at a time when we most need their natural air conditioning it could switch off,” said Dr Lenoir.
“Selecting species that are resilient to future climate conditions is important to the health and survival of our urban forests, ensuring those trees and shrubs keep functioning to keep us cool.”
Senior author, Associate Professor Rachael Gallagher from Western Sydney University’s Hawkesbury Institute for the Environment, said the proportion of urban tree species in Australian cities predicted to experience stressful conditions by 2050 is very high – varying from 83 per cent in Perth to 100 per cent in Darwin.
“Common native tree species found in at least 10 Australian cities and expected to experience climate conditions beyond their natural tolerance limits for high temperatures or lack of water by 2050, include manna gum, swamp gum, yellow box, narrow-leaved peppermint, blackwood and brush box,” said Associate Professor Gallagher.
“Many well-loved, non-native urban trees are at risk too – species like jacaranda, oaks, elms, poplars and silver birch.
“Based on our analysis, reduced rainfall will be a major driver in the decline of urban trees but increasing temperatures will also be a major contributor, especially in places like Darwin.”
Dr Esperon-Rodriguez said in addition to selecting climate-resilient species, there are some simple ways that people can help urban forests survive and keep them working to keep us cool.
“Trees at risk from reduced rainfall can be helped through water-sensitive urban design, which allows rainfall to soak into the ground and reach root zones, instead of going down the drain,” said Dr Esperon-Rodriguez.
“The urban heat island effect will exacerbate increased temperatures, but more tree and shrub cover in urban areas helps to reduce this effect, so leaving large trees and shrubs in place to perform these roles is really important.”
The study was undertaken by a team of scientists from Western Sydney University, The University of Melbourne and Macquarie University in Australia and the French National Centre for Scientific Research, Université de Picardie Jules Verne and Normandie University in France.
The study received funding from the Hort Frontiers Green Cities Fund.
