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Dust is among the substances usually referred to as pollutants affecting lichen growth and occurence. Dust splashed or blown on to roadside trees and shrubs usually leads to a reduction of bark acidity, thus encouraging lichen assemblages characteristic of plants with alkaline bark (i.e., Xanthorion) (Hawksworth and Rose 1976). Dust from limestone quarries and cement and lime works is also alkaline and has the same neutralizing effect on bark, promoting the development of lichen species normally confined to limestone rocks (e.g., Physcia spp., Xanthoria spp.). A number of methods are used to estimate the effect of airborne pollutants on lichens. Changes in the rate of photosynthesis, chlorophyll content and chlorophyll degradation are normally more informative than parameters such as species composition and cover (De Wit 1983).

During the last decade Athens, the capital of Greece has experienced the results of tremendous and uncontrolled growth. Almost 4 million people, 40% of the total population of the country live in Athens. Traffic jams are very common, while the quality of air has been getting worse year after year. This paper presents data on productivity and diversity of six sites in the Athens metropolitan area, selected according to their pollution load.