Two surveys of the lichen and bryophyte flora growing on oak twigs from a Welsh and a Danish locality were compared with additional data on bark pH and % nitrogen in thalli of Hypogymnia physodes. Despite differences in climate and lichen flora, both sites showed a shift in the lichen communities from nitrogen sensitive (nitrophobe) to nitrogen tolerant (nitrophile) species, which was correlated with both increasing bark pH and an increase in total nitrogen in thalli of H. physodes. The floristic survey from Wales was a repetition of a study eight years earlier (Wolseley & Pryor 1999) now showing a loss of nitrophobes in all sites and the appearance of nitrophiles in pasture sites in 2003. This study demonstrates that lichens on twigs can be used as an early warning system to detect a response to changes in land management and nitrogen deposition.