Divergent structure–function coordination shapes leaf thermal tolerance in Mediterranean tree species

Increasing temperatures impair photosynthetic apparatus by damaging photosystem II (PSII) efficiency. We evaluated the relevance of two thermal indices (Tcrit and T50) as indicators of the PSII resistance to thermal stress of Quercus ilex, Acer campestre and Fraxinus ornus in the urban area of Rome. Tcrit (critical temperature) and T50, that is the temperature at which the maximum PSII efficiency (Fv/Fm) decreased by 15% and 50%, were estimated by modelling Fv/Fm response curves of leaves exposed to the temperature range of 25-60 °C. To evaluate the functional relevance, Tcrit and T50 data were integrated with leaf functional traits using a multivariate analysis approach. Q. ilex exhibited a lower Tcrit than A. campestre and F. ornus. T50 was similar between species. In Q. ilex, early Fv/Fm decline was consistent with an early photoprotective downregulation rather than reduced heat resistance, with PSII activity persisting even at extreme temperatures. A. campestre and F. ornus maintained high Fv/Fm up to Tcrit rapidly collapsing beyond this threshold. The contribution of physiological and morphological traits to thermal resistance was both metric-dependent and species-specific. Morphological and physiological traits were primarily tied to Tcrit and T50, respectively. In Q. ilex, morphological traits explained a greater variance in both thermal indices, while physiological traits dominated in A. campestre, and a shared physiological-morphological contribution prevailed in F. ornus. Thermal resistance arose from the coordinated interplay between structure and function, reflecting distinct species-specific ecological strategies.