The Sayano-Shushensky Nature Reserve is a standard primeval locus of high conservation value which remained undisturbed landscapes within the Altai-Sayan Mountain Land. In the plant cover the altitudinal belts are clearly expressed. The northern part of the Sayan-Shushensky reserve is included in moist areas of the Altai-Sayan forest vegetation zone and the southern part of it is included in insufficiently moist areas of Central Asian forest zone. The ground vegetation of the reserve is presented by high-mountain tundra, meadows and shrub thicket sites, highmountain open Pinus sibirica D u. T o u r. and Larix sibirica L e d e b. woodland, dark and light coniferous taiga, coniferous subtaiga, forest-steppe and steppe. As a whole forests occupy somewhat about 60% of the territory. Ecocenotic groups of species found in individual phytocoenoses in river basins of the northern part of the nature reserve are qualitatively different in composition and share from those ones found in the southern river basins. In the north, the high mountain ecocenotic species group enjoys the greatest abundance, and forest-meadow, taiga, and bog species are also represented substantially. Steppe and forest-steppe species are absent. Alpine, steppe, and forest-steppe species are found in equal shares in the southern part of the forest reserve. Invasive species are quite many. Taiga species contribution is three times less than in the north of the reserve. Conditions climatically optimal for the greatest species diversity are the result of a perfect balance of heat and moisture characteristics of subtaiga/forest-steppe altitudinal belt complexes (ABC). However, changes in phytochorological- and alpha-diversity of individual phytocoenoses are influenced by ecological and phytocoenotic factors combined. The biggest number of species in the reserve occurs along the forest distribution boundaries – in subtaiga and high mountain open woodland whereas species are the fewest in mountain taiga. Conversely, phytomass loading is the greatest in mountain taiga and it decreases gradually towards alpine tundra and steppes.