From Exposure to Response: A Multi-Proxy Framework Linking Particulate Pollution with Plant Functioning in Celtis australis Across an Urban-Suburban Gradient

Urban vegetation helps mitigate air pollution, yet links between particulate matter (PM) exposure, particle characteristics, and plant functional responses remain poorly understood under urban conditions. This study integrated microscopic, magnetic, isotopic, and ecophysiological approaches to investigate pollution exposure and plant responses in Celtis australis. Leaves were sampled from three mature trees at each of eight sites spanning a traffic-to-suburban gradient in Florence (Italy). SEM/EDX analyses revealed pronounced spatial variability in particle load and composition. Traffic-exposed sites showed the highest PM accumulation and enrichment in Fe-rich particles, particularly in the finest PM fractions, whereas historical urban parks and the suburban site displayed lower particle loads and metal contributions. Magnetic susceptibility (χ) mirrored PM spatial patterns and was strongly associated with Fe-rich particles, supporting its use as a proxy for traffic-derived pollution. Trees growing at highly trafficked sites exhibited altered stomatal morphology, reduced photochemical efficiency, enhanced non-photochemical energy dissipation, and more negative δ¹³C values, indicating differences in carbon assimilation and photosynthetic functioning. In contrast, trees from historical parks showed lower δ¹⁵N values, suggesting differences in anthropogenic nitrogen inputs, sources, and cycling. Significant associations among PM composition, magnetic, isotopic, and ecophysiological indicators revealed links between traffic-derived pollution and both short-term physiological responses and integrated indicators of environmental exposure. The strongest associations with plant functional traits were observed for Fe-rich particles in the finest PM fraction. Overall, the results support a multi-proxy framework for assessing urban environmental gradients and identify Celtis australis as an effective bioindicator of traffic-related particulate pollution.