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BIOMONITORING OF TRACE METALS BY LICHENS IN HIGH-RISK AREAS OF FRIULI-VENEZIA GIULIA (NE ITALY). This study is based on the concentrations of 16 metals in peripheral parts of the thalli of the lichen Xanthoria parietina, collected on trees satisfying standard conditions in 155 stations located in the lowlands of Friuli-Venezia Giulia (NE ltaly), previously selected as potential high-risk areas by Regional Authorities. The interpretation of metal concentrations is based upon two maps, showing, for each metal, its distributional pattern and the degree of deviation from background (natural) conditions. The latter has been estimated through a seven-class scale based on the percentile distributions of severa! hundreds measurements of metal concentrations carried out in foliose lichens throughout Italy during the last ten years, using similar methods. The joint occurrences of ali metals in the stations are synthetized by maps based on three indices, an index of naturality (indicating the number of metals with concentrations within normai conditions), an index of environmental alteration (indicating the number of metals strongly deviating from the norm), and an index of potential toxicity (derived from that of alteration, with each metal weighed according to its toxicity). The central-southern part of the Province of Udine is the most severely affected by metals in generai, while in severa! other stations single metals strongly deviate from normai conditions. These sites are suggested to the Regional Authorities as foca! points for instrumental monitoring of environmental pollution. The introduction includes some basic considerations about epistemologica!, methodological and terminologica( matters related to the use of biomonitoring techniques.