Trees Help Cool Down City Parks Worldwide

David Palenski
10th February, 2024

Trees Help Cool Down City Parks Worldwide

Location of the globally distributed urban green spaces included in this study, representing 265 local air temperature differentials between green spaces and urban areas (impervious surfaces) from 58 studies across 46 cities (symbol size indicates the number of studies as in the legend).

Image adapted from: Kim et al. / CC BY (Source)
Cities are getting hotter. This is due to the ‘urban heat island’ (UHI) effect, where built-up areas experience significantly warmer temperatures than surrounding rural areas. This happens because materials like concrete and asphalt absorb and retain heat, and because of reduced evaporation. Urban green spaces (UGS) – areas with vegetation like trees, grass, and green roofs – are a key strategy to combat this, offering cooling through shading and a process called evapotranspiration (where water evaporates from plant leaves, taking heat with it). However, how effective these green spaces are varies, and a recent study from Cranfield University[1] aimed to understand why. The study analysed data from 58 previously published studies, encompassing 265 local air temperature measurements taken from cities around the world. Researchers wanted to determine how much of a cooling effect different types of UGS have, and what factors influence this effect – specifically, the background climate, the type of plants used, and the characteristics of the urban environment. The findings were clear: trees are the most effective at cooling. They reduced local air temperature 2-3 times more than grass, green roofs, or green walls. This aligns with previous research highlighting the importance of green spaces in mitigating UHI effects[2], but this study goes further by identifying why trees are so effective and how this effectiveness changes depending on the climate. The researchers used a statistical model to analyse the data, revealing that background climate – specifically, mean annual temperature – and plant characteristics were the most important factors influencing cooling. Surprisingly, features of the urban environment itself (like building density or street layout) didn’t have a significant impact. The plant characteristic that mattered most was ‘specific leaf area’ (SLA), which is a measure of the total area of a leaf relative to its mass. Plants with higher SLA generally have thinner leaves and can transpire more water, leading to greater cooling. This ties into work done on how SLA varies with environmental factors in grasslands[3], demonstrating the broader ecological importance of this plant trait. The study found that trees perform best in milder climates with lower average temperatures. This suggests that planting more trees in these areas has the greatest potential to reduce the UHI effect. This also builds on earlier work that showed satellite-derived estimates of leaf area index (LAI) can be used for large-scale studies of forest structure[4], although the current study focuses on the quality of that leaf area, as measured by SLA, rather than just the quantity. The research highlights that simply adding any green space isn’t enough. The type of vegetation and the climate it’s planted in are crucial. While grass and green roofs offer some cooling, trees provide a significantly greater benefit, particularly in temperate climates. The study’s findings offer practical guidance for urban planners looking to mitigate the UHI effect and create more comfortable and healthy living environments in cities worldwide.

EnvironmentSustainabilityPlant Science

References

Main Study

1) Greater local cooling effects of trees across globally distributed urban green spaces.

Published 10th February, 2024

https://doi.org/10.1016/j.scitotenv.2023.168494

Related Studies

2) Effects of landscape composition and pattern on land surface temperature: An urban heat island study in the megacities of Southeast Asia.

https://doi.org/10.1016/j.scitotenv.2016.10.195

3) Changes in specific leaf area of dominant plants in temperate grasslands along a 2500-km transect in northern China.

https://doi.org/10.1038/s41598-017-11133-z

4) Ground and remote sensing-based measurements of leaf area index in a transitional forest and seasonal flooded forest in Brazil.

https://doi.org/10.1007/s00484-013-0713-4


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