Lichen recolonization in London under conditions of rapidly falling sulphur dioxide levels, and the concept of zone skipping

HAWKSWORTH, D. L. & McMANUS, P. M., 1989. Lichen recolonization in London under conditions of rapidly falling sulphur dioxide levels, and the concept of zone skipping. The lichen flora on trees and wood at 50 sites in north-west London was examined in 1988 to determine what changes had taken place since 1980, a period when mean winter SO₂ levels had fallen from around 130 μg m^-3 to within the range 29–55 μg m^-3 in the study area. Forty-nine species were found. The fruticose lichens Evernia prunaslri and Ramalina farinacea now occur in many central London parks, and the yellow green foliose Parmelia caperata was discovered at 12 sites (previously only at one, Ruislip, in 1980). Comparable numbers and growth forms of species occurred throughout the area. An assemblage of zone 4–5 species (Hawksworth & Rose, 1970 Nature, 227: 145–148) tolerant of mean winter SO₂ levels of 60–70 μg m^-3 failed to penetrate central London, although more sensitive species of zones 6 and 7 had; this phenomenon, which has not previously been reported, is referred to as 'zone skipping' and is attributed to the speed at which mean sulphur dioxide levels in London have fallen. Species returning are those to be expected on the basis of previously-published field correlations with mean winter SO₂ levels. Twenty-five species not seen within 16 km of the centre of London on trees or wood during this century were found; of these, eight had not been seen during the last 200 years.