The interpretation of lichen bioaccumulation data is of paramount importance in environmental forensics and decision-making processes. By implementing basic ideas underlying previous interpretative scales, new dimensionless, species-independent “bioaccumulation scales” for native and transplanted lichens are proposed. Methodologically consistent element concentration datasets were populated with data from biomonitoring studies relying on native and transplanted lichens. The scale for native lichens was built up by analyzing the distribution of ratios between element concentration data and species-specific background concentration references (B ratios), herein provided for Flavoparmelia caperata and Xanthoria parietina (foliose lichens). The scale for transplants was built up by analyzing the distribution of ratios between element concentration in exposed and unexposed samples (EU ratio) of Evernia prunastri and Pseudevernia furfuracea (fruticose lichens). Both scales consist of five percentile-based classes; namely, “Absence of”, “Low”, “Moderate”, “High”, and “Severe” bioaccumulation. A comparative analysis of extant interpretative tools showed that previous ones for native lichens suffered from the obsolescence of source data, whereas the previous expert-assessed scale for transplants failed in describing noticeable element concentration variations. The new scales, based on the concept that pollution can be quantified by dimensionless ratios between experimental and benchmark values, overcome most critical points affecting the previous scales.
