Floristic composition, phylogenetic diversity, and community assembly identify the Lutou Experimental Forest as a model subtropical hub in the tropical–temperate transition zone

Subtropical mountain ecosystems serve as dynamic interfaces between tropical and temperate biotas, hosting high biodiversity shaped by complex topography and stable historical climate. This study presents a comprehensive assessment of the Lutou Experimental Forest in the northern Mulianjiu Mountains of central subtropical China, an area situated at a key tropical–temperate transition zone. A total of 1,209 seed plant species belonging to 149 families and 595 genera were documented, showing a balanced composition of woody and herbaceous taxa as well as a coexistence of evergreen and deciduous species, typical of transitional floras. Floristic analyses demonstrated nearly equal proportions of tropical and temperate elements, complemented by exceptionally high proportions of East Asian endemic genera (18.13%) and East Asia–North America disjunct genera (10.08%). Principal component analysis (PCA) positioned Lutou centrally within the national subtropical transitional cluster, reflecting its unique biogeographic bridging role. Environmental data indicated that high annual precipitation (~ 1,900 mm), moderate annual temperature (15°C), and a pronounced elevational gradient (124–1,272 m) jointly drive strong microhabitat differentiation. Castanopsis eyrei evergreen broad-leaved forest was identified as the most mature and ecologically representative community type. Regional comparison across 16 subtropical mountain ranges in China revealed that the Mulianjiu Mountains harbor the highest narrow-endemic species richness and Weighted Endemism Index (WEI) density. We conclude that Lutou is a core floristic and evolutionary hub in the tropical–temperate transition, where climate stability, topographic heterogeneity, and ecological maturity converge to sustain both relictual persistence and ongoing evolutionary diversification.