To clarify the role of ectomycorrhizae (ECMs) on nitrogen (N) transfer to Larix kaempferi (larch) and Salix reinii (willow) in harsh ecosystems on the volcano Mount Koma, northern Japan, N status and pathways were evaluated by N isotopes. The tephra N content was higher at the higher elevations where willow was dominant and played as an important N source. The larch varied in N-dependencies on ECMs from 0 to 90% with decreasing elevations, changing to dominant species at lower elevations, while the willow cover decreased with decreasing elevations, accompanying gradual decrease in N-dependencies on ECMs from 80% to 25%. The larch also had lower isotopic signature, i.e., higher N-dependency on ECMs, in the early growing season than the late growing season, while willow did not show seasonal variations in isotopic signature. The larch compensated the decrease of N availability, caused by decrease of willow patch with decreasing elevations and varying seasonal N demand, with flexible and strong mycorrhizal symbiosis, while willow responded unfavorably to mycorrhizal relations with elevations. The differences in N dependencies of early and late colonizers on ECMs were likely to be related to the species replacement along a successional gradient.