This paper reports on the distribution of lichens in the urban environment of Trieste (NE Italy) and its surroundings. The study is based on 213 phytosociological releves, taken on trees with primarily acid bark, and on pollution data from a net of deposimetric stations located within the town center. Both data sets have been submitted to multivariate analysis (classification and ordination). The results of data analysis allowed to distinguish six main zones, three defined by a peculiar lichen vegetation, the remaining three delimited on the basis of pollution data. The ecology of the releve groups obtained by classification has been studied on the basis of the tolerance ranges associated to each species by Wirth (1980). From the perifery to the center of the town there is a decrease in species, and an increase in the relative frequency of xero- photo- nitro- and neutro-basiphytic species. The distribution of lichens within the survey area cannot be explained on the only basis of air pollution. Drought in the urban environment seems to be another important factor affecting the distribution of lichens in the town of Trieste.
