Physiological and Biochemical Indicators of Urban Environmental Stress in Tilia, Celtis, and Platanus: A Functional Trait-Based Approach

Urban trees are exposed to multiple co-occurring stressors, including heat, drought, and pollution driven by intensified urbanization and climate change. These environmental pressures can compromise tree vitality by disrupting photosynthetic performance and oxidative balance. In this study, we assessed the structural, physiological, and biochemical responses of three common urban tree species (<i>Tilia platyphyllos</i>, <i>Celtis occidentalis</i>, and <i>Platanus × hispanica</i>) growing under urban environmental conditions in Novi Sad, Serbia. Leaf traits were measured during June and August to capture seasonal stress variation. Structural indicators (SPAD, leaf thickness, leaf temperature differential), chlorophyll fluorescence traits (<i>Φ</i>_II, <i>Φ</i>_NPQ, <i>F</i>_v'/<i>F</i>_m'), oxidative stress biomarkers (TBARS, proline, GSH), and antioxidant enzyme activities (APX, CAT) were quantified. The Tree Health Risk Index (THRI) was calculated to integrate multilevel responses. Results revealed species-specific differences, with <i>Tilia</i> exhibiting the highest sensitivity, characterized by notable photochemical declines and oxidative stress under urban conditions. <i>Celtis</i> showed moderate resilience, while <i>Platanus</i> demonstrated the most robust performance and emerged as a promising candidate for climate-resilient urban sites. Heatmap clustering and trait contribution analyses confirm oxidative stress biomarkers and chlorophyll fluorescence traits as key indicators of urban stress. This study emphasizes the importance of integrating functional trait-based approaches for assessing tree health in urban greening.