11 - Studia Geobotanica. An international journal - Vol. 11 (1991)

This paper presents the results of a study on air pollution by SO2 in the Veneto Region (North Eastern Italy), using the "Calibrated Lichen lndex of Air Quality" (I.A.P.) proposed by Liebendoerfer et al. (1988). This index, based on the frequency of epiphytic lichen species within a sampling grid of 10 units, has been tested by the authors in two preliminary studies carried out in two different areas of Northern ltaly (the town of La Spezia and the northern part of the province of Vicenza), revealing a high degree of correlation with pollution by SO2, measured by automatic recording gauges. The index has been computed for 662 stations scattered throughout the Region of Veneto, whose surface area is of 18,364 km2. The average number of relevés in each station is 3.7, with a total number of 2,425 relevés. The tree genus selected for sampling is Tilia. Altogether, 80 lichen species have been recorded during the survey, with a prevalence of nitrophytic and neutro-basiphytic lichens indicating a diffuse secondary eutrophication of the bark by agricultural activities. Pollution maps for the seven provinces and for the entire region are presented. The evaluation of pollution levels is based on the correlations between the index and SO2 levels found in the preliminary studies. All maps have been drawn by programs of automatic mapping. The results show that in 93.7% of the regional territory there is a very low probability of the occurrence of SO2 concentrations higher than the thresholds established by law; this part of the region has yearly means of the 98th percentiles lower than 84 μg/m3. 25% of the region has moderate pollution levels, 23.4% a low pollution, and 44.9% a very low air pollution. The most polluted areas are located in the Southwestern part of the region. The pollution pattern agrees very well with the location of the main sources of pollution, and with the directions of the prevailing winds. The study presents also the computerized distribution maps of eight selected species (Candelaria concolor, Candelariella xanthostigma, Hypogymnia physodes, Parmelia exasperatula, Parmelia sulcata, Physcia adscendens, Physcia orbicularis, Xanthoria parietina), that are discussed in relation with the main pollution patterns. The lichens species can be subdivided into three main groups, according to their distribution patterns. The main factors affecting the distribution of lichens in the survey area are air pollution and the eutrophication of bark by agricultural activities. A monitoring strategy where bioindicators and instrumental recording of air pollution are used jointly is proposed as a good solution to air pollution mapping aver vast areas.