Complex relationship between soil fungi and conservation value assessments in boreal forests.

Large-scale industrial forestry is a threat to biodiversity and imposes long-lasting changes to many forested biomes. Preserving forests as reserves is an important component of the strategy for safeguarding forest biodiversity. Yet, the selection of forests of high biodiversity value is usually based on proxies (i.e., subsets of aboveground habitat characteristics) rather than on direct assessments of species occurrences. This approach is based on the assumption that the diversity and community composition of all organism groups are well represented by the assessed habitat characteristics. We investigated how conservation value, assessed according to common practices based on aboveground habitat heterogeneity, corresponded to the abundance, richness, and community composition of 12 taxonomic and ecological groups of soil fungi across northern and southern Swedish forests. Overall, the assessed conservation value reflected the abundance, diversity, and community composition of deadwood-associated saprotrophs well, likely because they depend directly on the availability of the structures that the assessment is based on. However, the conservation assessment value failed to capture the overall variability for most of the soil-dwelling fungal guilds. Although the assessed value was positively associated with the diversity of ectomycorrhizal fungi, root-associated Ascomycota, and saprotrophic Basidiomycota in the southern region, no such association was evident in the northern region. Soil fertility was the best predictor of the variation in community composition in all fungal guilds. The relative abundance and diversity of most saprotrophic guilds increased as soil fertility increased, whereas root-associated guilds decreased as soil fertility increased. Current methods for assessing conservation value captured only specific subsets of soil fungi, and the predictability of capturing fungal diversity varied depending on the region. To more comprehensively preserve soil fungi, assessment methods should incorporate additional environmental parameters, especially those linked to fungal community composition, such as soil fertility.