Urban tree cooling effectiveness in Barcelona

Research conducted by the Institute of Environmental Science and Technology at the Universitat Autònoma de Barcelona indicates that extreme heat and drought are diminishing the ability of urban trees to cool cities. A study published in the journal ‘Urban Climate’ found that when soil moisture is low, plant transpiration is severely restricted, which reduces the thermal relief trees typically provide.

Under optimal soil moisture conditions, trees can reduce air temperatures by an average of 1.2 °C to 1.7 °C and reduce pedestrian thermal stress by up to 3.2 °C. However, further research in Barcelona’s Eixample district suggests that tree planting is not a universal solution. Simulations showed that during extreme heatwaves with very dry soil, streets with dense tree cover were actually 0.3 °C warmer than streets with fewer trees.

Researchers noted that excessive tree mass can hinder ventilation and trap heat radiated by buildings and asphalt in dense urban areas. Additionally, during late afternoon heatwaves, the moisture released by leaves may potentially increase thermal stress for pedestrians.