This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: The combination of heat waves and drought markedly reduces the capacity of street trees to cool the air in Mediterranean cities. The lack of soil moisture severely restricts plant transpiration, reducing the thermal relief provided by these trees during extreme heat events.
This is demonstrated by a study published in Urban Climate and conducted by the Institute of Environmental Science and Technology at the Universitat Autònoma de Barcelona (ICTA-UAB), Spain. The study analyzes the behavior of street trees in dense streets of Barcelona during intense heat episodes. The goal was to determine how soil moisture and plant canopy interact to mitigate urban heat.
Under ideal soil moisture conditions, plant transpiration reduces air temperatures by an average of 1.2 °C to 1.7 °C from midday to midafternoon. It reduces heat stress experienced by pedestrians—measured with the Universal Thermal Climate Index (UTCI)—by up to 3.2 °C and decreases mean radiant temperature (that perceived by pedestrians from solar and asphalt radiation) by up to 12.3 °C in areas most exposed to the sun. However, when extreme heat coincides with drought periods, the lack of water forces trees to close their stomata to prevent dehydration, severely impairing or eliminating evaporative air cooling.
When soil is dry, shade from tree crowns plays a key role in providing refuge from the heat. Despite the loss of evaporative air cooling, foliage blocks direct solar radiation, preventing overheating of asphalt and facades and shielding pedestrians beneath the trees from direct sun exposure. This shading retains between 71% and 97% of its protective cooling effect on pedestrian thermal comfort.
Thus, the study determines that urban greening strategies must adapt to the new climatic reality of the Mediterranean. To maximize tree effectiveness, the authors recommend prioritizing evergreen species and/or drought-tolerant deciduous species that sustain canopy structure during summer months. They also recommend ensuring strategic irrigation during heat peaks for deciduous species that retain their foliage in summer, as well as preserving soil moisture in tree pits and pervious soil.
According to the authors, "One of the main challenges facing the city is ensuring that trees maintain sufficient foliage during drought periods, and especially during heat waves, which is precisely when shade is needed the most." Ricard Segura-Barrero et al., The role of soil moisture and shading in influencing the capacity of urban street trees to reduce heat stress, Urban Climate (2026). DOI: 10.1016/j.uclim.2026.103086 Provided by Autonomous University of Barcelona BSc Life Sciences & Ecology. Microbiology lab background with pharmaceutical news experience in oil, gas, and renewable industries.
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