Habitat deterioration caused by intensive silvicultural actions has resulted in the decline of saprotrophic species, including polypore fungi (Basidiomycota). Polypore assemblages in herb-rich forests remain little studied, regardless of the fact that these forests are considered as biodiversity hotspots in the boreal zone.
The aim of this study is to evaluate the significance of herb-rich forests as polypore habitats and assess the relationships between stand- and substrate-scale variables and polypore species richness, abundance and diversity. In addition, an alternative way to measure the diversity of coarse dead wood (CWD) is presented. Polypore fruit body and CWD inventories were conducted in 75 herb-rich forest stands from hemiboreal to middle boreal vegetation zones. Overall, the data covers 5097 dead wood units and 3046 observations of 101 species, including 19 herb-rich forest associated and 15 red-listed species. The largest size class of CWD units (diameter >40 cm) hosted up to 4.5 times more polypores than the smallest size class (10-14 cm). Results of GLMs and correlation analyses showed that the total polypore species richness, and the number of observations of red-listed and herb-rich forest associated species have differing responses to habitat and substrate quality. For instance, the diversity of CWD, northern coordinate and abundance of large-diameter (>30 cm) dead wood all significantly explained variation in total species richness, whereas the red-listed species responded only to the number of large dead wood units. In NMDS-ordination, the polypore assemblages were strikingly different between host-tree species, with the clearest division between coniferous and deciduous trees.
The results highlight the importance of dead wood diversity and large-diameter dead wood, especially aspen and birch, for polypore assemblages in boreal herb-rich forests. Comparing the results to previous studies, herb-rich forests are likely to host lower polypore species richness and smaller populations of red-listed species than old-growth spruce forests. However, because of a high proportion of deciduous trees in the dead wood profile, herb-rich forests can be presumed to sustain polypore assemblages divergent from those found in conifer-dominated boreal heath forests.