Floristic Diversity, Vegetation Structure, and Ontogenetic Composition of Sibiraea altaiensis (Rosaceae) Populations in the Kazakhstan Altai

The taxonomic status of the Altai populations traditionally identified as Sibiraea altaiensis remains unresolved, with international databases generally treating the name as a synonym of S. laevigata. At the same time, regional sources retain it for conservation and floristic purposes. Using S. altaiensis as an operational regional name, we established a comparative ecological baseline for six natural populations distributed across the Southern and Western Kazakhstan Altai at 814–2130 m a.s.l. Vegetation was surveyed in July 2024 and July 2025 in ten 10 m × 10 m plots per population using complete vascular plant inventories, Braun–Blanquet abundance–dominance estimates, descriptions of vertical structure, and field assessment of shrub ontogenetic states. Observed vascular plant richness ranged from approximately 35 species at Tautekeli to 78 at Lineinsky; Shubartoc and Ridder contained 76 and 72 species, respectively. Mesophytes constituted 56.3–77.0% of the population floras, demonstrating a predominantly mesic community affinity across contrasting subalpine slopes, river terraces, floodplains, and sparse conifer forest. Sibiraea altaiensis was dominant or structure-forming at every locality, although associated species composition and vegetation architecture varied substantially among habitats. Juvenile plants were not detected at five populations, whereas their occurrence at Lineinsky was insufficiently documented for comparison; this pattern is interpreted cautiously because dense midsummer vegetation and litter may reduce seedling detectability. Floristic richness did not conform to a simple Southern-versus-Western Altai division, indicating that local habitat heterogeneity and vegetation context may be more informative than regional position alone. The study provides a verified descriptive baseline and identifies permanent plot monitoring, early-season recruitment surveys, direct environmental measurements, and integrated molecular taxonomic analyses as priorities for conservation assessment.