Epiphyte recolonization of oaks along a gradient of air pollution in South-East England, 1979-1990

The abundances of epiphytes on free-standing oaks (Quercus robur) along a transect extending approximately 70 km SSW from Central London were redetermined annually from 1979 to 1990. During this period SO(2) levels in inner London fell to between one-quarter and one-seventh of levels occurring in the 1960s. The sample trees were also scored annually using Hawksworth and Rose's (1970) scale for predicting SO(2) levels from the epiphytes present. Little evidence was obtained for epiphyte recolinization of oaks at sites within the built-up area. At Epsom Common, on the urban fringe, cover of the pollution-tolerant lichen Lecanora conizaeoides fell, and in the inner suburbs, at Putney Heath, L. conizaeoides and Lepraria incana increased, while the alga Desmococcus viridis declined at this site. These changes might be due to a contraction inwards of zones of peak abundance of L. conizaeoides and D. viridis with decreasing SO(2) levels. However, it is also possible that the changes at Putney Heath resulted from scrub clearance activities and alteration to the microclimate. The cover of the moderately SO(2) tolerant foliose lichen Hypogymnia physodes remained low and static at Putney Heath during the 12-year period. Possible reasons for the poor recolinization of London's oaks by epiphytes are discussed. Particular emphasis is given to the highly acidic bark of oak in the London area (pH2.9-4.0) in comparison with other tree species and with oaks in unpolluted regions.