Towards the adoption of an international standard for biomonitoring with lichens—Consistency of assessment performed by experts from six European countries

•Standard Operating Procedures for lichen monitoring are tested in a field exercise.•Several sources of error in the process of lichen survey have been considered.•After training an improvement of teams performance was evident.•Agreement in species identification between teams and control was weak.•It emerges the need to refer to the new CEN protocol.