Evaluating lichen diversity along an anthropogenic pressure gradient for environmental quality assessment in Wallonia

Urban environments strongly affect species diversity through pollution, urban heat islands, and ecological fragmentation. Assessing the presence and abundance of biological indicators in these areas provides an effective tool to evaluate environmental quality. Lichens, symbiotic associations between algae and fungi, are widely used as bioindicators, offering insights that complement physico-chemical monitoring. This study aimed to compare lichen diversity along an anthropogenic pressure gradient in Wallonia, Belgium, ranging from rural to residential and industrial areas. We selected 16 municipalities representing contrasted conditions, including dense and industry-impacted cities (e.g., Liège, Charleroi), more diffuse urban areas (e.g., Arlon, Wavre), and rural localities either exposed to agricultural pressure (e.g., Orp-Jauche) or not (e.g., Florenville). In each municipality, several sampling sites were chosen according to city extent and tree availability. The frequency of each lichen species was assessed on the trunks of four lime trees (Tilia spp.) at 100–150 cm above ground, following the European standard protocol (prEN 16413). Across the 163 sites, we identified a total of 109 lichen species. Average diversity decreased along the anthropogenic pressure gradient, with the highest values in remote rural areas and the lowest in dense industrial cities. Community composition also varied markedly, with nitrophilous species dominating agricultural landscapes and acidophilous taxa prevailing near forested areas. In addition, eight strict or facultative saxicolous species were recorded, especially in Charleroi and Liège, likely reflecting dust-related eutrophication. Finally, several rare species for Wallonia, including Anaptychia ciliaris, Parmelina pastillifera, Phaeophyscia endophoenicea, and Piccolia ochrophora, were detected in remote areas. Overall, combining lichen diversity and community ecology provides valuable insights into the impact of urbanization on biodiversity and supports the use of lichens as effective bioindicators in heterogeneous landscapes.