Assessing Mediterranean tree species suitability for urban environments: insights from experimental data including 23 leaf functional traits

This study investigates the responses of four Mediterranean tree species, Quercus ilex, Viburnum tinus, Acer campestre, and Fraxinus ornus, to urban-relevant abiotic stressors such as soil compaction, water deficit, and over-optimal temperature, applied singly and in combination under controlled experimental conditions. A total of 23 functional leaf traits spanning photosynthesis, water regulation, structural support, and leaf stoichiometry functions were measured. Species identity was the main driver of trait variability. Q. ilex showed the most stable performance, maintaining the net photosynthetic rate (PN), PSII maximum quantum yield, and the intrinsic water use efficiency (IWUE) across all stress treatments. Nevertheless, Q. ilex was the only species to emit Biogenic Volatile Organic Compounds (BVOCs), including camphene, which increased under stress. V. tinus was highly sensitive, especially under combined stress, with strong declines in PN and water status traits and a marked rise in IWUE. F. ornus adjusted morphologically and increased its foliar N over C allocation to face water deficit and over-optimal temperature. A. campestre activated photoprotective mechanisms but showed reduced tolerance under multi-stress conditions. Despite species-specific strategies, all species converged toward similar physiological responses under increasing stress. This functional convergence highlights the importance of selecting species based on multivariate trait syndromes. Urban tree selection in Mediterranean environments should prioritize species with stress-resilient functional syndromes, particularly traits linked to water regulation and PSII stability, to improve plant performance under multifactorial climate stress.