Shifting urban baselines limit the abundance-based resolution of a classical lichen indicator

Most urban lichen bioindication frameworks were developed under the substantially higher industrial and traffic-related pollution levels that characterised many European cities in previous decades. In many European cities, however, nitrogen dioxide (NO₂) concentrations have declined substantially over recent decades alongside ongoing climatic warming, raising the question of whether classical indicator species still track present-day pollution gradients. We evaluated the abundance-based indicator performance of the widely used lichen bioindicator Xanthoria parietina across complementary temporal (10 sites monitored from 2015 to 2025) and spatial (63 sites, 2023–2025) gradients in Vienna (Austria), combining repeated field surveys with year-specific spatially resolved NO₂ and climate data. During the study period, NO₂ concentrations declined markedly while air temperature increased. Across both datasets and modelling approaches, estimated NO₂ effects on lichen abundance were small, with confidence intervals overlapping zero and providing limited support for strong abundance responses within the observed NO₂ range. Contemporary NO₂ concentrations spanned a narrow and comparatively low range relative to historical urban conditions, suggesting that present-day exposure may lie largely below the species' effective abundance–response range. Accordingly, Xanthoria parietina provided little abundance-based resolution of current urban NO₂ gradients. These findings suggest that the diagnostic value of individual bioindicator species may decline when pollution levels fall below their effective response range, highlighting the importance of periodically reassessing indicator performance under shifting urban environmental baselines.