Using the response of epiphytic lichens in and around the city of Thessaloniki (Greece) to an air pollution source, the value of sampling lichens at breast height and the base of trees was compared. The data obtained were processed with detrended correspondence analysis and polythetic divisive two-way indicator species analysis, and IAP (index of atmospheric purity) values were estimated for each experimental site. The tree base IAP values were higher than those at breast height, reflecting the higher number of species present, the generally increased cover and frequency of occurrence. IAP values at the two sampling heights suggest a linear relationship and the same holds for the tree base IAP values and the first axis ordination scores. Sampling at the base resulted in better discrimination of sites, and may, therefore, offer more accurate monitoring of changes induced by air pollution.