Phylogenetic structure and diversity of herbaceous communities in the conifer forests along an elevational gradient in Luya Mountain, Shanxi, China

Abstract

Aims Incorporating phylogenetic data in the studies of species diversity patterns along elevational gradients can bridge the gap between ecological and evolutionary processes, and thus shed light on the issues related to community assemblage. We aim is to explore the elevational patterns of phylogenetic relatedness and phylodiversity in the herbaceous angiosperm assemblages of alpine conifer forest within mountain ecosystem, and to quantify the relationship between the patterns and habitat factors. Methods We sampled 17 plots (20 m × 30 m) in the coniferous forest communities at ca. 50 m altitudinal intervals along the elevation gradient. In each plot, we documented all species encountered and the environmental conditions. We used the Net Relatedness Index (NRI) and Nearest Taxon Index (NTI) to quantify the phylogenetic structures of each herbaceous assemblage, and used the Mean Pairwise Distance and Mean Nearest Taxon Distance to quantify phylogenetic beta diversity. Ordinary least square regression and multiple regression on distance matrices were employed respectively to explore the elevational trends of phylogenetic structure and phylobetadiversity. We analyzed the taxonomic composition of the herbaceous assemblages within forests, and demonstrated the relationship between the clustering of clades and the significant habitat descriptors using principal coordinates of phylogenetic structure (PCPS). Important findings The result showed that the herbaceous communities tended to be more phylogenetically overdispersed at lower elevations, suggesting that intraspecific competition potentially influences the local assemblages. In contrast, species occurring at high-elevation sites tended to be more closely related, implying that these communities are structured primarily by environmental filtering. However, we found that all of the NRI (or NTI) were confined within 95% confidence intervals, suggesting strong contributions of stochastic processes on species assembly. Phylogenetic beta diversity significantly increased with the elevational distance between community pairs, also suggesting that habitat filtering probably played an important role on structuring the herbaceous communities. The first two axes of PCPS contained 55.9% of total variation in phylogeny-weighted species composition, and were both significantly related with elevation and arboreal basal area. We found that species of large families, including Asteraceae and Poaceae, were phylogenetically clustered at high elevations, whereas the others were phylogenetically overdispersed at low elevation region. Our findings suggest that exploring the patterns of phylogenetic structures across elevational gradients is important, which can provide insights into the underlying mechanisms shaping community composition within montane ecosystems.