A transplant experiment on the factors preventing lichen colonization of oak bark in southeast England under declining SO2 pollution

To investigate the factors responsible for limited lichen recolonization on oak with declining SO2 concentrations, Parmelia caperata (L.) Ach. and Parmelia saxatilis (L.) Ach. were transplanted to five stations along a transect running from central London (U.K.) into the surrounding countryside. Healthy thalli were transplanted onto young and mature Quercus robur L. (pedunculate oak) and Betula pendula Roth (silver birch), and some were moved with their original bark attached to investigate the importance of bark acidification. Relative growth rates and visible injury were monitored over 2 years, along with concentrations of atmospheric SO2 and NO2, measured using diffusion tube samplers and bark chemistry. SO2 concentrations were low in central London in comparison with the situation in previous decades, but they remain sufficiently high to harm the most sensitive lichens. NO2 concentrations increased sharply on approaching London, and levels were higher in winter than in summer. Bark pH was lower in mature oak than in birch or young oak at the two innermost stations, and levels of bark Mg also declined in London. Transplants of P. caperata survived on birch and young oak at all transect stations except Hyde Park, whereas P. saxatilis failed to grow over a wider range of the transect stations. We conclude that modest SO2 levels, interacting with low bark pH, still inhibit recolonization of oaks by P. caperata in central London. Parmelia saxatilis, in addition, appears to be limited by another factor such as the high NO2 concentrations that now characterize urban and suburban London, rather than through an inherently poor dispersal potential.Key words: atmospheric pollution, bark acidification, epiphytic lichens, NO2, recolonization, SO2.