Urban Lichen Diversity Reflecting Air Quality Patterns across European Cities

Air pollution represents a persistent ecological pressure in urban environments, influencing biodiversity and ecosystem functioning. Instrumental monitoring networks provide quantitative pollutant measurements but often lack fine spatial resolution and ecological integration. Epiphytic lichens are widely recognized as sensitive bioindicators of atmospheric quality due to their direct absorption of airborne substances and differential tolerance to contaminants. This study comparatively examines documented lichen diversity patterns across multiple European urban contexts, focusing on community variation along gradients of traffic intensity, urban density, and green infrastructure presence. Reported ecological indices, including species richness and atmospheric purity indicators, were evaluated in relation to environmental variables such as vehicular emissions, vegetation continuity, and urban morphology. Recurrent patterns were identified across urban areas. High-traffic zones consistently supported simplified communities dominated by pollution-tolerant crustose taxa, whereas residential and peri-urban green spaces maintained higher diversity including foliose and fruticose species sensitive to air contamination. Vegetated corridors and structurally complex green areas showed a mitigating effect, allowing greater species persistence even near emission sources. The convergence of biological responses across different geographic contexts demonstrates the robustness of lichen diversity as an indicator of urban air quality. Biological monitoring therefore represents a valuable complementary tool for spatial environmental assessment and supports its integration into sustainable urban planning and ecosystem management strategies.