The availability of ectomycorrhizal fungi (ECMF) to conifer seedlings above the present treeline is likely related to the presence of alternate ECMF host plants, including Betula glandulosa, Arctostaphylos alpina, and Salix herbacea. The ECMF inoculum potential of soils from above treeline that either supported (host) or lacked (non-host) an alternate host plant was assessed by growing Picea mariana as ECMF bait seedlings in field-collected soils under controlled conditions. Seedlings became colonized when grown in both host and non-host soils, but ECMF percent colonization, richness, and diversity were higher for those grown in host soils. The ECMF community in Arctostaphylos host soils was most similar to the community in forest soils. Seedling growth varied among the different soil types, but was mainly influenced by percent ECMF colonization and soil nutrients. Alternate ECMF host plants will likely act as important sources of fungal inoculum, potentially improving conifer seedling establishment and growth.