Compositional Differentiation of Alnus nepalensis Communities Along a Sampled Elevational Sequence in Southeastern Xizang, China, Is Dominated by Species Turnover

Mountain studies often emphasize richness trends, yet similar richness can mask species replacement. The objective of this study was to determine whether compositional differentiation of Alnus nepalensis communities across five elevational bands (2053–2463 m) in southeastern Xizang was primarily driven by richness loss or assemblage turnover. Fifteen 20 m × 20 m plots were analyzed using presence–absence Jaccard dissimilarity, with life-form-standardized matrices as sensitivity analyses. Community composition differed markedly among bands (PERMANOVA: F = 14.15, R2 = 0.850, p = 0.0001), while multivariate dispersion did not (PERMDISP: p = 0.617); the pattern persisted with standardized life forms (R2 = 0.910). Although plot richness declined with elevation, turnover accounted for 71.0–95.3% of the mean plot-pair Jaccard dissimilarity between adjacent bands. Bootstrap intervals supported turnover dominance in all four transitions, with the greatest uncertainty at 2053–2158 m (95% CI: 54.6–87.3%). The 2158 and 2247 m bands had equal pooled richness yet showed the highest turnover proportion (95.3%, 95% CI: 93.4–97.1%). Alnus nepalensis occurred throughout, while elevation-associated shrubs and herbs contributed much of the taxonomic differentiation. These results suggest that, within this sampled sequence, species replacement—rather than richness loss alone—was the primary driver of community differentiation, although the confounding of elevation with locality prevents causal generalization.