The 29 degrees N latitudinal line: an important division in the Hengduan Mountains, a biodiversity hotspot in southwest China

This paper aimed to explore the division of the southern and northern Hengduan Mountains based on gradients in species similarity and richness, and to analyze species richness in each sub-region. The Hengduan Mountain region was divided into nine latitudinal belts using one degree of latitude to define the belt after which distribution of seed plants within each latitudinal belt was recorded. Latitudinal patterns of species similarity were measured using the Jaccard similarity index for each pair of adjacent latitudinal belts. Non-metric multidimentional scaling (NMDS) was also used to analyze the similarity in species composition among the nine latitudinal belts. The latitudinal pattern of species similarity and the NMDS ordination both showed a great change in species composition across the 29 degrees N latitudinal line, essentially dividing the Hengduan Mountain region into southern and northern sub-regions. Species richness, shown by the c-value of the species-area power function, and species-area ratio along a latitudinal gradient both showed a sharp decrease across the latitudinal belt from 29 degrees 0' to 29 degrees 59'N. The southern sub-region occupied 40% of the total area of the Hengduan Mountain region, but contained more than 80% of all the seed plants in the region. The higher species richness and endemism in the southern sub-region showed it to be the core of the Hengduan biodiversity hotspot, a result not unexpected because of the greater extremes of topography and wider diversity of habitats in the southern portion.