Systematic analysis of in the North American Rocky Mountain alpine zone.

(Russulales) is an important ectomycorrhizal fungal genus in Arctic and alpine regions where it occurs with , and , yet a complex phylogenetic analysis of the genus in these habitats is lacking. This research compared collections of from the Rocky Mountain alpine (Colorado, Montana, Wyoming) with reference specimens from Arctic and alpine habitats, mostly in Europe, using an in-depth morphological study and a phylogenetic analysis of the nuc rDNA internal transcribed spacer region ITS1-5.8S-ITS2 (ITS barcode) and the second largest subunit of the RNA polymerase II gene (). One hundred thirty-nine collections were sequenced, including type material. Ten species are reported from alpine or treeline habitats in the Rocky Mountains. This is the first formal report of . cf. , and from the Rocky Mountains and of in North America. is reported for the first time under this name with a voucher, and not as an environmental sample. Previous reports of and are molecularly confirmed. Two species are reported from subalpine habitats at treeline: with conifers and . cf. with . In this study, , and were collected in alpine habitats but have been reported below treeline in Europe; these species may also be present at lower elevations in North America. Most species have an intercontinental distribution and have been reported in other alpine or Arctic habitats. Two unidentified and potentially new species were only found in North America and are discussed. A key to the alpine Russulas of North America is provided.