A new study reveals that, although **trees** can significantly cool cities during the day, their canopies can **trap heat** and increase temperatures at night.
This finding helps urban planners choose the best combinations of trees and planting locations to combat **urban heat stress**.
Urban Heat Stress and Mitigation Strategies
Temperatures in cities are rising, causing health problems, death, increased **energy use**, and social inequality.
Some cities have started to implement mitigation strategies, such as **tree planting**. However, a study led by the University of Cambridge warns that planting the wrong species in suboptimal locations can limit their benefits.
Impact of Trees on Temperature
Published in Communications Earth & Environment, the study found that urban trees can reduce pedestrian-level air temperature by up to 12 °C, meeting the **”thermal comfort threshold”** in 83% of the cities studied.
However, it also discovered that this cooling capacity varies significantly worldwide and is influenced by tree species, urban design, and **climatic conditions**.
Cooling Mechanisms of Trees
During the day, trees cool cities in three ways: by blocking **solar radiation**, absorbing long-wave radiation from the ground surface through **water evaporation**, and altering airflow.
At night, tree canopies can trap long-wave radiation, increasing heat.
Variability of Cooling Effect
The study found that urban trees cool cities more in warm, dry climates, and less in warm, humid climates. In humid and dry tropical climates, trees can cool cities by up to 12 °C during the day, but they can also heat them by up to 0.8 °C at night.
In temperate climates, trees can cool cities by up to 6 °C and heat them by up to 1.5 °C at night.
Importance of Tree Species Selection and Urban Design
The combined use of evergreen and deciduous trees can generate more **cooling in temperate** continental and tropical climates. In arid climates, **evergreen species** are more effective in compact urban layouts.
Open, low-height cities in dry climates allow for more tree canopies and a **greater mix of species**, improving cooling.
Ecologization Guidelines
The study provides specific guidelines for urban planners to maximize the benefits of tree cooling against global warming. It is crucial to choose resilient species and plant the right combination of trees in optimal positions for **future warmer climates**.
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