Trees are essential to cooling down cities. However, a study by Concordia researchers at the Next Generation Cities Institute and the Loyola Sustainability Research Centre shows how tree distribution influences how some residents benefit more from them than others.
In a paper published Urban Forestry & Urban Greening, the authors studied the layout of Montreal's vegetation - its trees, shrubs and grass - and compared it to daytime temperature readings on the ground, or land surface.
Using satellite imagery and laser imaging, detection, and ranging (LiDAR) technology data, the researchers found that a 10 per cent increase in tree coverage can lower land surface temperature by 1.4˚C. A similar increase in shrubs and grass lowers temperatures by about 0.8°C. They also learned that large, continuous patches of trees cool their surroundings better than small, scattered groupings.
The researchers analyzed and compared vegetation coverage using demographic information from the 2021 Canadian Census. The results revealed that neighborhoods with higher incomes, higher levels of education, and predominantly white populations tended to have access to higher quality green infrastructure. In contrast, poorer, more racially diverse areas received less cooling benefit from green infrastructure.
Underserved areas also had higher populations of vulnerable age groups, meaning those under five years old and those over 65.
"Demand for the cooling provided by urban vegetation is based on the population of vulnerable groups," says lead author Lingshan Li, a PhD candidate in the Department of Geography, Planning and Environment. "We need to care more about people who are vulnerable and most exposed to excess heat in urban areas."
Finding the cooling mismatches
The model draws on three key indicators, developed using data from several sources:
- Heat exposure - Measured using land surface temperature data from Landsat satellite imagery provided by the US Geological Survey;
- Vegetation coverage - Assessed through LiDAR and aerial imagery from the Communauté métropolitaine de Montréal's Metropolitan Canopy Index, which maps vegetation coverage across the island of Montreal
- Population data - Drawn from the 2021 Canadian Census, including statistics on age, education, income and visible minority status.
Next, the researchers created a statistical model to predict how vegetation affects surface temperatures. They used three variables: percentage of high vegetation (tree canopy), percentage of low vegetation (shrubs and grass) and a "large patch index of high vegetation," which measured how extensive and uninterrupted the main tree clusters were within each study area.
Their model explained roughly 80 per cent of the variation in surface temperatures across the island. It also showed that temperatures can be reduced by increasing vegetation coverage, and that larger, connected patches of trees amplify cooling.
Cooling supply and demand
With this information, they developed a "cooling supply index" - which assigned a value between 0 (low cooling) and 1 (high cooling) - and a "cooling demand index," which reflected the proportion of residents in vulnerable age groups. Neighbourhoods with higher numbers of these residents were determined to have higher demand for cooling.
Comparing these indices showed where mismatches occurred.
Wealthier and better educated areas like Outremont and the West Island had more tree cover and thus greater cooling, whereas Saint-Léonard, Montréal-Nord, and Anjou, which have higher proportions of visible minorities or lower average household incomes, were found to have fewer trees and more heat-vulnerable residents.
Li says this study can help planners and municipal authorities prioritize where to build parks and greenspaces so they can make their cities more equitable.
"Urban areas have limited space, so we cannot create as many green spaces as we would like," she says. "We have to better understand how to manage our urban green infrastructure to maximize its benefits."
Contributors to this study include Angela Kross, associate professor, Geography, Planning and Environment; Carly Ziter, assistant professor, Biology; and Ursula Eicker, professor, Building, Civil and Environmental Engineering.
Financial support for this study was provided by the Trottier Family Foundation and the Natural Sciences and Engineering Research Council of Canada.
Read the cited paper: "Analyzing spatial patterns of urban green infrastructure for urban cooling and social equity."
