Plant species richness across the Himalaya driven by evolutionary history and current climate

The Himalaya, the world's largest mountain chain, spans a wide variety of climates. Further, different locations have historically experienced climatic perturbations to different degrees. This makes it the ideal region to assess roles of contemporary climate, diversification (speciation minus extinction), and dispersal barriers in affecting local species richness. Based on a review of all available Himalayan floras, we determined that 8765 native angiosperm species are presently documented and recorded their location and elevational distributions. We compared species richness and measures of phylogenetic structure in 100-m elevational bands for all species combined and for three major life-forms separately (trees, shrubs, and herbs) across the Himalaya. Species richness declines threefold from the east to the northwest of the Himalaya. Along elevational gradients, tree richness monotonically declines in the northwest, but peaks at ~1000 m in the east. Shrubs and herbs peak in richness at mid-elevations (~2000 m). Mean temperature and annual precipitation together explain ~60% of the variation in species richness. The general phylogenetic pattern observed in this study is that phylogenetic clustering (i.e., more closely related species on fewer long branches) increases from low to high elevations, but with a dip at mid-elevations (2000–3000 m), which may result from a mixing of distinct floras, but is not associated with exceptionally high richness. High clustering at higher elevations (3000–4500 m) and in the drier northwest suggests ongoing diversification dynamics limit richness in these harsher environments. The effects of diversification dynamics appear to be smaller than those of contemporary climate in limiting buildup of species numbers.