The crustose lichen flora of fifteen areas in boreal coniferous forests from southern Sweden to Lapland was surveyed. Many of these areas are included in a national programme for monitoring long‐term environmental change, the PMK programme. In all 190 species were recorded, 129 of them occurring on bark of conifers and lignum. Some species were found exclusively in forests with a long continuity. By using ordination and hierarchical classifications further species with similar habitat requirements were identified. An Indicator Species Index of Forest Continuity (ISIFC) was designed. Only two of the twenty index species occur on deciduous trees. The ISIFC was highly correlated with forest continuity and also with occurrence of species listed as threatened. Correlations with macroclimatic variables are considered sampling artifacts, and microclimatic factors are suggested to be of primary importance for species occurring in forests with a long continuit
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