Natural air conditioning under stress

Urban trees in Barcelona can reduce air temperatures by 1.2°C to 1.7°C during midday hours when soil moisture is sufficient, according to a study by the Institute of Environmental Science and Technology (ICTA) at the Universitat Autònoma de Barcelona (UAB). This transpiration effect lowers the heat stress felt by pedestrians by up to 3.2°C and can reduce radiant temperature, the heat absorbed from direct sunlight and reflected off surfaces like asphalt, by as much as 12.3°C in the most exposed parts of the Eixample district. Transpiration occurs when water absorbed by tree roots moves through the plant and evaporates from leaf pores called stomata, effectively cooling the surrounding air.

When trees shut down

However, during periods of extreme heat and prolonged drought, trees close their stomata to conserve water, drastically reducing or halting transpiration. Under these conditions, the cooling effect drops sharply: moisture release can lower ambient temperature by no more than 0.4°C, about a degree less than under optimal conditions. In this 'energy-saving' mode, trees may shed foliage to prevent dehydration. The remaining leaves provide only shade, which still blocks solar radiation and protects pedestrians and surfaces, retaining 71% to 97% of its cooling benefit for thermal comfort. But shaded areas feel significantly less refreshing than when trees are actively transpiring.

Under ideal soil moisture conditions, plant transpiration reduces air temperatures by an average of 1.2 °C to 1.7 °C during midday/mid-afternoon hours.

Preparing for a hotter future

Projections cited by ICTA suggest Barcelona could face heatwaves reaching 45°C by the end of the century. To maintain the cooling benefits of urban greenery, the study recommends two key actions. In the short term, targeted irrigation of street trees during heatwaves is essential, especially for deciduous species that keep their leaves in summer. In the long term, city planning should prioritise permeable pavements and improved urban drainage systems to help soil retain moisture during rainfall. The research, published in the journal Urban Climate, also advises selecting evergreen or drought-tolerant deciduous species that maintain canopy cover in summer months. Preserving soil moisture in tree pits and increasing subsoil water absorption are critical to ensuring trees remain effective natural coolants in a warming climate.

Sources: betevé, UAB, portalrecerca.uab.cat.