University of Cordoba ranks heat tolerance in 17 urban tree species

Botanists Cristina C. Bastias and Sonia Vega from the University of Cordoba evaluated the thermal tolerance of 17 urban tree species in Cordoba, Spain, reporting that native Mediterranean species withstand extreme heat substantially better than popular subtropical imports naukawpolsce.plNauka w Polscenaukawpolsce.plOpen source ↗. The findings were reported by Nauka w Polsce, referencing a publication in the journal Urban Forestry & Urban Greening.

To identify critical stress limits, the team measured chlorophyll fluorescence in foliage to evaluate when photosynthetic apparatus efficiency begins to degrade under escalating heat. The tested selection included olive, holm oak, cork oak, ash, poplar, elm, hackberry, carob, privet, jacaranda, and orange trees, spanning native and exotic profiles from Mediterranean, temperate, and subtropical origins. The degradation thresholds define where the physiological capacity of foliage to cool surroundings via shade and transpiration diminishes.

Native Mediterranean resistance

Native Mediterranean and temperate flora demonstrated higher thermal tolerance thresholds compared to exotic subtropical plants. Native trees such as the holm oak and European olive exhibited the greatest resilience against extreme heat among all evaluated specimens. In contrast, widely cultivated subtropical ornamental trees—including orange trees, chinaberry, catalpa, and tipuana—displayed marked photosynthetic decline at elevated temperatures.

While subtropical trees naturally originate in warm climates, their native regions provide higher moisture levels and distinct atmospheric humidity compared to southern Spanish urban environments. Under urban water limitations, exotic species experience severe functional impairment that curtails environmental cooling.

Structural drivers of heat tolerance

Data revealed that high thermal thresholds strongly correlate with structural traits designed for resource conservation rather than leaf habit. Superior tolerance was coupled with higher wood density, elevated leaf mass per area, and greater leaf thickness, which are morphological signatures typical of native Mediterranean vegetation naukawpolsce.plNauka w Polscenaukawpolsce.plOpen source ↗. Conversely, whether a species is deciduous or evergreen did not determine its physiological resilience to elevated temperatures.

The researchers noted that selecting resilient species based on functional and structural adaptations is necessary to maintain green urban infrastructure amid shifting climate conditions.