In the last decades, several methods were proposed for assessing environmental quality — mainly air pollution — on the basis of lichen data (see chapter 4, this volume). At the end of the 80s the predictivity of 20 different methods with respect to instrumental pollution data was tested in Switzerland using multiple regression [1, 5]. The highest correlation was found with the sum of frequencies of lichen species within a sampling grid of 10 units positioned on the trunks of free-standing trees. This method was immediately and widely adopted in several other countries, esp. Italy and Germany, with some modifications, chiefly concerning the size of the sampling grid. Since 1987, hundreds of studies were carried out with this approach, which led to its standardization in the form of guidelines both in Germany [13], and in Italy [7].
The bioindication method presented here is largely based on the Swiss approach [1, 5], and on the German and the Italian guidelines [7, 13], with several important modifications which were agreed upon during a meeting of the authors in Rome (November 2000), sponsored by the Italian National Agency for the Environment (ANPA). The main modifications concern several elements of subjectivity in the sampling process which were present in both of the original guidelines. They regard mainly the locationof sampling sites, the selection of sample trees, and the positioning of the sampling grid on the trunks (see also chapter 9, this volume).
More attention is also paid to vegetational data as a source of information for interpreting lichen diversity patterns, as in the phytosociological approach largely followed in France [11].