Spatial variations in epiphytic macrolichen richness in the city of Oslo were assessed annually 1973–2019. These observations were contrasted with earlier published data from 1930 to examine how long-term changes in species richness of functional groups track different stages of air pollution regimes. From 1930 to the 1970’s, representing the early surge and late peak of S-deposition, the lichen-deficient urban area remained largely unaltered. Epiphytic macrolichen richness in the surrounding zone declined and changed from a mix of nitrophytes and acidophytes in 1930 when agriculture was still present to a dominance of acidophytes in the 1970’s shortly after the acid rain peak. The subsequent 1980-2019-period marked by significantly lower S-emissions, and weakly decreasing N-deposition, experienced a shift from acidophytes to nitrophytes, following the successful control of acid rain. This underscores the role of pH as a contributing determinant of the strong nitrophyte recolonization. While successive pollution regimes shaped functional group-specific changes in lichen richness over the past 90 years, continuous rain in autumn 2000 led to sudden temporal lichen dieback across the urban-to-rural gradient, delaying lichen recovery after the acid rain period by approximately 5 years for nitrophytes and over 15 years for acidophytes. Epiphytic lichen richness never returned to the high levels seen in 1930, even in the outer parts of the urban-rural gradient and despite the reduction in S-deposition. Excess N impedes effective establishment of acidophytic lichens and prevents full recovery of the former diversity. Keywords: Air pollution · Dieback · Epiphytic lichens · Long-term study · N-deposition · S-deposition.