Floristic composition and community variation across forest formations in Peak Wilderness Nature Reserve & Sanctuary, Sri Lanka.

Understanding floristic composition across elevational forest formations is important for identifying vegetation turnover and conservation priorities in tropical montane ecosystems. This study examined plant community composition in the Peak Wilderness Nature Reserve & Sanctuary (PWNR&S), Sri Lanka, across three major forest formations known as; Lowland Wet Evergreen Forests (LWEF), Lower Montane Evergreen Forests (LMEF), and Montane Evergreen Forests (MEF). Vegetation data were collected from 142 plots, representing 7,778 individuals, 503 species, 251 genera, and 104 families. Importance Value Index (IVI) analysis identified Palaquium rubiginosum, Psychotria nigra var. nigra, and Alpinia abundiflora as the most dominant tree, shrub, and herb species, respectively, across the sampled area of PWNR&S. Floristic dominance varied among forest formations. Lowland Wet Evergreen Forests (LWEF) were characterized by Dipterocarpaceae and other lowland-associated tree species, whereas Melicope lunu-ankenda and Palaquium rubiginosum were more strongly associated with Lower Montane Evergreen Forests (LMEF) and Montane Evergreen Forests (MEF), respectively. Several Strobilanthes species were prominent within the shrub layer of MEF. Community-level analyses further revealed clear floristic differentiation among forest formations. Non-metric multidimensional scaling ordination showed separation of plots representing LWEF, LMEF, and MEF, although partial overlap was observed. PERMANOVA confirmed significant differences in plant species composition among forest formations (F₂,₁₃₉ = 4.85, R² = 0.065, p = 0.0002), with pairwise comparisons showing significant differences among all formation pairs. Mean Bray–Curtis dissimilarity values were high among forest formations, with the greatest compositional difference between lowland and montane forests. Hierarchical clustering grouped lower montane and montane forests more closely than lowland forests. These results demonstrate strong floristic turnover along the elevational gradient and highlight the importance of conserving distinct forest formations within PWNR&S.