Planting trees in the wrong place can make cities hotter

Trees offer undeniable cooling benefits during the day by providing shade and reducing heat through evaporation. However, new research reveals that at night, tree canopies can trap heat and potentially increase temperatures.

The study, led by scientists from the University of Cambridge, emphasizes the importance of strategic planning when integrating trees into urban landscapes to combat heat stress effectively.

“Trees have a crucial role to play in cooling cities down, but we need to plant them much more strategically to maximize the benefits which they can provide,” said Dr. Ronita Bardhan, an associate professor at the University of Cambridge.

Rising temperatures and urban heat stress

Urban heat stress has become a global concern that is linked to health issues, increased energy demands, social inequalities, and infrastructure challenges.

Many cities have turned to tree planting as a mitigation strategy. Yet, the study warns that the wrong species or poor placement can negate these benefits.

By analyzing 182 studies covering 17 climates and 110 cities, the researchers have conducted the first global assessment of urban tree cooling. Findings highlight the complex interplay between tree species, urban layouts, and local climates.

Day vs. night cooling dynamics

Trees cool cities during the day by blocking solar radiation, evaporating water through their leaves, and influencing airflow with their foliage.

At night, tree canopies can trap longwave radiation from the ground, leading to a warming effect.

This phenomenon, caused by aerodynamic resistance and stomatal closure (a response to heat and drought), varies significantly across climates.

Climate-specific cooling and heating effects

The analysis revealed that urban trees provide varying degrees of cooling and heating depending on the climate:

• Tropical wet and dry climates: Trees cooled cities by up to 12°C (21.6°F) but warmed them by 0.8°C (1.44°F) at night.
• Arid climates: Cooling reached over 9°C (16.2°F), while nighttime warming was 0.4°C (0.72°F).
• Temperate climates: Daytime cooling was 6°C (10.8°F), with a nighttime warming effect of 1.5°C (2.7°F).
• Tropical rainforest climates: Cooling dropped to 2°C (3.6°F), with 0.8°C (1.44°F) warming at night.

Strategic tree planting for maximum cooling

Cities with open urban layouts and mixed tree species (evergreen and deciduous) achieved better cooling results, especially in temperate and tropical climates. Mixed species provide seasonal shading and three-dimensional cooling.

In arid regions, compact urban areas (like Cairo and Dubai) with evergreen trees experienced significant cooling benefits. Open spaces in low-rise cities allowed for larger tree canopies, further enhancing cooling by 0.4°C.

“Our study provides context-specific greening guidelines for urban planners to more effectively harness tree cooling in the face of global warming,” noted Dr. Bardhan.

Matching trees to urban features

The study emphasized the importance of compatibility between tree species and urban features. Factors such as street orientation, visible sky ratio, and aspect ratio impact tree cooling efficacy.

In compact urban zones with high temperatures, excessive canopy cover can trap heat at pedestrian levels.

The researchers recommend using narrow species and sparse planting strategies in such areas.

Beyond trees: Solutions for urban cooling

While trees are valuable, the study from the University of Cambridge urges reliance on complementary strategies like solar shading and reflective materials. These solutions enhance urban cooling without the unintended effects of poorly planned tree placement.

To assist urban planners, the researchers developed an interactive map and database. This tool helps estimate cooling efficacy based on data from cities with similar climates and urban features.

“Urban planners should plan for future warmer climates by choosing resilient species which will continue to thrive and maintain cooling benefits,” said Dr. Bardhan.

In summary, strategic tree planting can significantly mitigate urban heat stress, but careful consideration is essential.

Selecting the right species, planting them in optimal locations, and incorporating complementary solutions can ensure that cities maximize cooling benefits while minimizing unintended consequences.

The cooling power of trees

Large, mature trees provide greater cooling benefits due to their expansive canopies, which block sunlight and facilitate higher evaporation rates.

Broadleaf species are especially effective, offering more shade and water evaporation compared to needleleaf trees. When planted in clusters, trees create a synergistic cooling effect by enhancing air circulation and providing widespread shade.

Proximity to water bodies further amplifies cooling effects, as the combination of tree shade and evaporation from water surfaces creates cooler microclimates.

Native species often outperform non-native ones, as they are better adapted to local climates and conditions.

Evergreen trees offer year-round cooling, particularly in regions with less distinct seasonal changes. Additionally, leaf color influences cooling, with lighter-colored leaves absorbing less heat. Tree roots also play a role, cooling the soil and supporting healthier ecosystems.

A diverse mix of tree species strengthens urban forests, making them resilient to pests and ensuring long-lasting cooling benefits.

The study is published in the journal Communications Earth & Environment.

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