Changes in epiphytic lichen diversity along the urban-rural gradient before, during, and after the acid rain period

Abstract

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.