Saproxylic (dead-wood-associated) species of fungi are ecologically and functionally important forest-dwelling species but habitat and substrate associations of these species are still incompletely known. In particular, this applies to species that occur on less common host-tree species. We explored diversity and ecology of polypores and corticioids on the black alder (Alnus glutinosa), a little-studied substrate. In total, we collected 2528 records of 138 fungal species. We found 102 species only once or twice, and they comprised 3.8% of all the records, whereas five most numerous species comprised 59.5% of all the records. In total, 27 species were found solely on the black alder. Our results show that substrates of fungal species varied widely. Several species were restricted to specific alder substrates, while others had a remarkably wide substrate utilization, and we identified potentially at least six generalist and six specialist species. Many of the alder-associated species are rare. We conclude that also less common tree species in boreal forests can host diverse fungal assemblages. Thus, maintaining the black alder in forests enhances the diversity of saproxylic fungi.