Scientists' Contributions

Changes in oak epiphytes monitored for 21 years along an air quality gradient into London

J. W. Bates

Department of Biology, Imperial College at Silwood Park, Ascot, Berkshire, UK

    Lichens on the trunks of Quercus robur (pedunculate oak) at five stations along a gradient of air quality from the countryside into central London have been monitored annually by Imperial College students and staff since 1979. Data from the UK network of monitoring stations show that SO₂ concentrations have declined dramatically across the transect over this period, but NO_x has increased markedly in the built-up area. Hardly any re-colonization of oak bark by fruticose and foliose lichens has accompanied the decreasing SO₂ concentrations, and even well into the 1990s the use of the Hawksworth & Rose zone scale predicted elevated SO₂ levels that had not in fact existed since the 1960s. By the late 1990s, the SO₂-tolerant crustose lichen Lecanora conizaeoides, formerly present at high cover in the outer suburban zone, had become extinct at its former strongholds, but it had established a weak foothold in central London. Marked changes in the cover of Lepraria incana agg. and 'green alga' were also observed in the built-up zone. The fluctuations in epiphyte cover will be related to changing air quality along the transect and conclusions drawn about the bioindicator value of specific lichens to SO₂. Recent transplant studies with the lichens Parmelia caperata and P. saxatilis have helped to identify reasons for the lack of re-colonization of oak bark by these foliose lichens.