Taxonomic and phylogenetic perspectives reveal the community assembly of different forest strata along an altitudinal gradient

Abstract Disentangling the distribution patterns of biodiversity along altitudinal gradients and their drivers has been a hot topic in ecological research. Different forest strata show multiple distribution patterns influenced by different community assembly mechanisms. Here, we measure the dominant ecological processes affecting three forest strata (tree, shrub, and herb layers) along altitudinal gradients. We sampled plants with different forest strata in 21 plots located from 2500 to 3700 m in Lasha Mountain, northwestern Yunnan. The taxonomic alpha diversity of the herb layer showed a monotonic decreasing trend with increasing altitude, while the shrub layer was characterized by a single peak, and the tree layer showed no significant change with increasing altitude. We observed a decreasing pattern of phylogenetic diversity with increasing altitude in the herb layer, whereas phylogenetic diversity did not change in the tree and shrub layers. The phylogenetic structure of these three layers showed the same pattern: phylogenetic clustering at low altitudes and overdispersion at middle and high altitudes. The diversity and phylogenetic structure of the herb layer were driven by climate (average dry season precipitation) and topographic (slope and aspect) factors, while shrub layer diversity was mainly influenced by tree layer richness. Overall, the phylogenetic structure pattern of the three forest strata changed from clustered to overdispersed with increasing altitude, indicating that environmental filtering was the main driver at low altitudes, while interspecific competition dominated at middle and high altitudes.