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.