Fine‐Scale Diversity and Rarity Hotspots in Northern Forests

To evaluate the efficiency by which fine‐scale diversity hotspots capture threatened species in northern forests, we investigated the spatial distribution of vascular plants, bryophytes, macrolichens, and polypore fungi with 1‐ha resolution (in 0.25‐ha plots) within six approximately 2‐km 2 forest landscapes in Norway. Of 1295 recorded species (118,000 records), 62 species (842 records) were on the Norwegian Red List, of which 7 species (12 records) were new to Norway. We studied the degree of clustering of different red‐listed species among plots by comparing observed co‐occurrence of red‐listed species to a null model of random co‐occurrence. Red‐listed species were moderately clustered within study areas but less so within each forest type. Clustering was similar for areas with high and low densities of red‐listed species. However, the proportion of records captured in rarity hotspots (plots with the highest number of red‐listed species) depended on both the total number of records of red‐listed species per area and the degree of clustering of species among plots. The 5% highest‐ranking rarity hotspots captured, on average, 59% of red‐listed species and 23% of all records of red‐listed species in study areas, compared with 56% and 18% captured by hotspots selected from a random distribution of species, respectively. Hotspots based on the most species‐rich plots (richness hotspots) were significantly less effective than rarity hotspots in capturing red‐listed species and their records. Based on the observed pattern of species co‐occurrence and the high proportion of area seemingly occupied by red‐listed species in mature forest, we hypothesize that 20–25% of the populations of red‐listed species may be located within rarity hotspots constituting 5% at the scale of 1 ha. We conclude that limited spatial overlap of species constitutes an important constraint on the efficiency of a fine‐scale hotspot conservation strategy, such as the “woodland key habitats” (0.1–10 ha) currently mapped and protected in northern European forests.