Fungal diversity in wood of living trees is higher in oak than in beech, maple or linden, and is affected by tree size and climate

Fungi are abundant in wood of living trees, but few studies have compared the diversity of fungi among different tree species and trees of varying age and size, aspects of importance for conservation planning. We investigated if fungal species richness and species composition in wood vary significantly among the temperate broadleaf tree species beech (Fagus sylvatica), linden (Tilia cordata), Norway maple (Acer platanoides) and pedunculate oak (Quercus robur). Each tree species was represented by four stem size classes, and the total sample included 240 trees in southern Norway. Wood cores were collected from individual trees and fungal DNA was amplified using ITS2 rRNA as a marker and subjected to high-throughput sequencing. In total, we detected 1156 fungal OTUs. Oaks had significantly higher richness of fungal OTUs than any of the other tree species and harboured unique communities. Further, oak hosted most species-specific Indicator species (39) and was the only species to host Red-Listed fungal species (five). The circumference (proxy for age) did not significantly affect neither OTU richness nor its overall composition. However, several individual Red List and Indicator species were found only in trees of the largest size class. There was a significant effect of bioclimatic section on species composition. Our results emphasize the important roles of oaks and to some extent large trees as repositories of fungal diversity, which should be considered in conservation planning.