Elevational patterns of phylogenetic structure of angiosperms in a biodiversity hotspot in eastern Himalaya

Aims The tropical niche conservatism (TNC) hypothesis and the out of the tropics (OTT) hypothesis propose mechanisms generating patterns of species diversity across warm‐to‐cold thermal gradients at large spatial scales. These two hypotheses both integrate ecological and biogeography‐related evolutionary factors, but they predict opposite patterns for phylogenetic structure. Mount Namjagbarwa possesses one of the longest elevational gradients for flowering plant diversity in the world. We aim to analyse the elevational patterns of the phylogenetic structure of flowering plants in this mountain, and investigate underlying causes for the emergence of the patterns. Location Mt. Namjagbarwa located at the eastern edge of the Himalaya. Methods Species distribution data of Mt. Namjagbarwa were extracted from specimen records in online sources, literature, herbarium and our fieldwork. Bioclimatic data were extracted from the CHELSA database. Mt. Namjagbarwa was divided into 100‐m elevational belts, which were grouped into three elevation segments (100–2000 m, 2000–4000 m and 4000–5200 m) for data analysis. We calculated phylogenetic metrics for each belt. The relationships of phylogenetic metrics with elevation and climatic factors were analysed using generalized additive models and structural equation models. Results A typical hump‐shaped pattern of species richness was observed along the elevational gradient of Mt. Namjagbarwa. However, the phylogenetic structure showed a zig‐zag pattern with the three elevation segments, each of which has different formation mechanisms. Temperature tolerance played important roles throughout the entire elevational gradient in species richness and phylogenetic structure. Main conclusions Neither TNC nor OTT alone can fully explain the patterns of floristic assembly of the entire elevational gradient of Mt. Namjagbarwa. OTT and TNC are better explanation for the patterns of low and middle elevations respectively. Potential mechanisms for species assembly of different elevation segments in Mt. Namjagbarwa region alternate between niche convergence and niche conservatism. The patterns observed in this study are likely to be common across the entire Himalaya. This is important for preserving the evolutionary potential of mountain biodiversity. However, more studies are needed to determine the underlying mechanisms generating the zig‐zag pattern of the phylogenetic structure in the Himalaya.