"We assessed the species diversity of ectomycorrhizal fungal (EMF) communities at treeline and forested sites within three Alaskan mountain ranges along a latitudinal gradient (Chugach Mountains, White Mountains, Brooks Range). The study took place during the growing seasons of 2001 and 2002 and was part of a larger study evaluating the possible mechanisms that control latitudinal and elevational limits of forest growth. In 2001, we measured EMF species richness, diversity (Simpson's Index), and similarity at and between forested and treeline sites in all three mountain ranges. In 2002, more focused studies were conducted measuring EMF abundance along a forest to treeline transect within the White Mountains. We evaluated possible species shifts spatially within the ecotone from treeline to forest as well as temporally throughout the growing season (June-August) at Twelve Mile summit within the White Mountains. Overall, we found that EMF species richness was highest in the White Mountains and lowest in the Brooks Range. There was no difference in species richness between forested and treeline sites with the exception of the Brooks Range, where species richness was higher in the forested site. Diversity of EMF at forested and treeline sites was similar, with no significant differences detected either between sites or among mountain ranges. The similarity of EMF species was higher between forested and treeline stands within a site than between sites within a range, that is, a forested site was more similar to the adjacent treeline site than it was to other forested sites within any given range. Abundance of EMF varied along the forest to treeline transect with species dominance throughout the growing season. These findings reflect the highly localized nature of EMF communities in forest and treeline habitats"--Leaves iii-iv.