Sources of error in epiphytic lichen variables mapped as bioindicators: needs to modify the Finnish standard

A standardized lichen mapping method, to assess the environmental quality in urban and industrial areas, has been used in Finland since 1990. Comparisons of the data collected by different field workers showed large variation in both numbers of species recorded and their observed frequencies. It is recommended that a reduced number of species, which are easily and reliably identified, should be used when assessing environmental quality, and correspondingly, the standard method should be revised.