Trees alone may not be enough to cool cities, study finds

Cities around the world are planting increasing numbers of trees to reduce the impact of rising temperatures, AzerNEWS reports.

According to the information, new research suggests that trees alone are often not enough to effectively cool urban environments. In some cases, poorly designed green spaces can even reduce outdoor comfort during hot weather.

Scientists compared the results of studies conducted in Melbourne, Munich, and Hong Kong to determine how urban greenery affects the temperatures people experience outdoors.

The findings showed that multi-layered vegetation consisting of trees, shrubs, and ground cover cools cities more effectively than areas planted with trees alone. Researchers also found that the effectiveness of urban greening depends heavily on local climate conditions and street design.

Cities tend to retain heat. Roads, buildings, and asphalt absorb solar energy during the day and gradually release it back into the atmosphere, particularly at night. This phenomenon, known as the "urban heat island" effect, is becoming increasingly severe and dangerous as climate change drives temperatures higher.

To better understand how urban greenery influences heat stress, researchers conducted field studies in three cities with different climates: temperate Melbourne, cooler Munich, and humid subtropical Hong Kong.

Rather than relying solely on computer models, scientists examined real-world conditions in streets and green spaces during the summer months. They compared open urban areas without vegetation, spaces planted only with trees, and multi-layered green areas containing trees, shrubs, and ground vegetation.

In Melbourne, street trees reduced the heat radiation experienced by pedestrians by more than 18 degrees compared with treeless streets. Even where air temperatures changed only slightly, shaded streets felt noticeably cooler.

The strongest impact of multi-layered greening was observed in Munich. Streets and green spaces featuring trees, shrubs, and ground vegetation recorded heat stress levels approximately 8 degrees lower than more open areas during the afternoon.

Hong Kong also benefited from urban greenery, particularly from the shade created by dense tree canopies. However, the city's humid climate influenced the cooling process, producing different results than those seen in Melbourne and Munich.

One key conclusion emerged across all three cities: the structure of vegetation matters greatly.

The combination of trees, shrubs, and ground cover generally performed better than trees alone. However, the success of this approach depended on how green spaces interacted with local environmental conditions.

The study concludes that urban greenery can significantly reduce heat stress, but its effectiveness depends on how and where it is implemented. While Melbourne demonstrated the cooling power of street trees and Munich highlighted the benefits of multi-layered vegetation, Hong Kong showed that dense greenery can sometimes produce unintended effects in highly humid environments.