Spatial patterns and climatic drivers of phylogenetic structure for ferns along the longest elevational gradient in the world

Abstract

Many biodiversity hotspots are located in montane regions, thus, understanding the underlying mechanisms driving species assembly along elevational gradients is of major interest in ecology and biogeography. Here, we assess spatial patterns and climatic drivers, and the effects of clade age, on patterns of phylogenetic structure of ferns along the world's longest elevational gradient in the central Himalaya. We used correlation and regression analyses to relate metrics of phylogenetic structure reflecting both shallow (tip‐weighted) and deep (basal‐weighted) evolutionary histories of ferns, and their two major groups reflecting different ages (polypods representing a young clade, all other ferns representing old clades), in fifty 100‐m vertical bands to climatic factors representing different aspects of climatic conditions (mean climate, stressful climate and climate seasonality). Variation partitioning analysis was used to determine the relative importance of each group of climatic factors on phylogenetic structure. We find that the composition of fern assemblages along the Himalayan elevational gradient in Nepal shows strong signatures of evolutionary processes. In a simplified way, species‐rich assemblages at mid‐elevations are likely the result of recent radiations in combination with low extinction rates, whereas species‐poor assemblages at low elevations are composed of numerous lineages with limited radiations, and those at high elevations of few lineages, also with limited signature of recent radiations. Variables related to temperature and climatic extremes tended to play a more important role than precipitation‐ and seasonality‐related variables, respectively, in driving fern phylogenetic structure. Combining the results of ferns and angiosperms suggests that there are a few generally consistent evolutionary processes that apply to all plant groups (e.g. niche conservatism and environmental filtering), but that the specific outcomes of these processes vary with elevation, clade age and taxon.