Urban tree planting effectiveness depends on soil moisture and density

A recent study conducted in Barcelona's Eixample district warns that urban tree planting is not a universal solution for combating heat and climate change. Research by ICTA-UAB and CIEMAT indicates that the cooling efficacy of trees depends heavily on soil moisture, tree density, and urban configuration.

In simulations comparing streets with different levels of vegetation, trees successfully reduced air temperatures by an average of 1.2 ºC and thermal stress indices by 3.2 ºC when soil was moist. However, during extreme heatwaves with very dry soil, streets with dense tree cover were actually 0.3 ºC 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 can potentially increase thermal stress for pedestrians.