Loss of Lecanora conizaeoides and other fluctuations of epiphytes on oak in S.E. England over 21 years with declining SO2 concentrations

Epiphytic lichens and algae were studied on the bark of mature, free-standing Quercus robur L. at five stations along a transect extending S.S.W. for nearly 70 km into rural Sussex from central London. Percentage cover data for the epiphytes was recorded annually over the period 1979–1999 at chest height (1500 mm) and on the trunk base (450 mm). Data from the nearest pollution monitoring stations in the UK network show that SO2 concentrations have declined at all transect stations, but particularly dramatically at the three inner sites (Kensington Gardens, Putney Heath, Epsom Common). The crustose lichen Lecanora conizaeoides, initially almost ubiquitous on oak bark at Epsom Common and once frequent at all but the innermost site, had declined and is now extinct at the four outer stations. However, it had recently appeared on oak bases in Kensington Gardens, the only station to retain appreciable SO2 levels. Lepraria incana agg. had increased progressively at all sites with the decline of SO2. ‘Green alga’, probably Desmococcus viridis, had also increased with declining importance of SO2, although there was evidence that it was the most SO2-tolerant of the three epiphytes. Its abundance at the urban stations is possibly connected with their high NOx concentrations. Hypogymnia physodes was the most common macrolichen but only occurred in significant quantities at the two outer sites (Hesworth Common, Holmwood Common). It too had shown a significant decline with falling SO2 levels. No significant re-colonization of formally polluted sites by SO2-sensitive macrolichens had been noted during 21 years of study, probably because of the high acidity of oak bark and elevated NOx in inner London. The loss of L. conizaeoides supports the hypothesis that this species requires elevated SO2 inputs (or some closely related chemical factor) for healthy growth, but competitive interactions with the other epiphytes and attacks by the parasitic fungus Athelia arachnoidea may also be important. The results suggest that Lecanora conizaeoides cover on Quercus bark is a relatively sensitive bioindicator of SO2 levels in town air.