The species composition and quantitative indicators of fractions aboveground and underground phytomass and mortmass of tundra-steppe ecosystems of the northern macroslope of the Mongun-Taiga Mountain. Steppe communities are submitted to such mountain-steppe species as Koeleria altaica, Bupleurum multinerve, Carex ediformis, Festuca kryloviana, Potentilla sericea, Artemisia commutata, Aster alpinus. Tundra formations are high-mountain meadow and tundra species Astragalus adsurgens, Oxytropis intermedia, Stellaria petraea, Festuca altaica u Artemisia frigida. The total stock of plant matter and the main stock of green phytomass are formed by the types of dominants, such as Oxytropis intermedia, Carex rupestris u Festuca altaica. The ecological interpretation of the values of species composition in the formation of stocks of plant matter and the features of the functioning of tundra-steppe ecosystems is presented on the basis of established values of the ratio of aboveground and underground phytomass and mortmass fractions.